Computational Fluid Dynamics A Practical Approach, Second Edition [2nd Edition Jiyuan Tu, Guan Heng

COMBINED SAMPLES FOR EXAM,ASSIGNMENT& PPROJCET GUIDES

Section A [30 marks] BRIEFLY answer the following questions in your answer booklet.

1. What are some of the advantages of using CFD? (List 3 of them)

2. What is the main purpose of the post-processing stage? What are some of commonly used plot methods in the post-process?

3. The Reynolds number is a ratio of two fluid properties. What are they? Based on these two fluid properties, please explain if the Reynolds number is very high, what this suggests; if it is very low, what this implies.

4. Which of the following, forward difference, backward difference and central difference, is most accurate and why? Why do we need a mesh independence test in a CFD simulation?

5. In the analysis of CFD results, what does consistency imply? (hint: using Δt, Δx to explain)

6. Give some of examples where mesh requires refinement or needs to be well designed?

Section B The marks are shown at its beginning of each question.

1.

[15 marks] Based on the following algebraic equations

a pφ p = ∑ anbφnb + b

(1)

and the condition for convergence

∑a

nb

ap

≤

1 at all nodes

(2)

where in the matrix form, ap represents the diagonal element while anb is the neighboring element, the condition for convergence stipulates that ∑ a nb a P ≤ 1, which simply means that the sum of the neighboring elements divided by the diagonal elements must be less than unity, at all grid nodal points. Analyse the following two cases, i.e. why one case will converge and the other will diverge.

Case 1:

4Φ1=3 Φ2+0.5Φ3+1.6 Φ2=0.2Φ1+0.8Φ3+3 2Φ3=1.5 Φ1+0.5Φ2+0.4

Case 2:

2Φ1=0.8Φ2+Φ3+1.4 3Φ2=1.5Φ1+2.1Φ3+9 Φ3=0.4Φ1+0.6Φ2+0.8

2. [30 marks] Analyse the case below by answering the following questions

2a. Give all names of boundary conditions you have used for your Assignment A. 2b. What advantages can a symmetry boundary condition provide and when can it be applied?

2c. What is the value of ‘P’ pressure (larger or smaller) at point C when comparing with point A? 2d. What are the values of ‘U’ velocity (positive or negative) before, after and at the re-attachment point B? 2e. What will happen to the length of Lt if the Reynolds number at inlet increased?

2f. Is the current outlet flow condition fully developed or not? If not, how to improve it?

2g. Sketch the vector profiles along the stations X1, X2 in the same way as shown in the inlet or outlet.

3.

[25 marks] Analyse the case below by answering the following questions

3a. What is the purpose of creating inflation?

3b. What is y+ and why is it important in context of near wall turbulent modelling?

3c. What is the value of ‘P’ pressure (larger or smaller) at point A when comparing with point B? Why? μ = ρC k 2 ε

μ 3d. Use the relationship between t and k to explain the reason for the different vortex predictions between k-ε and k-ω turbulence models at region C. If k-ω turbulence model under-predicts k, what would you expect for the vortex size prediction at region C and why?

Hints: In modeling turbulent flow, we effectively solve the following equation:

∂uΦ ∂vΦ ∂ ⎛ ∂Φ ⎞ ∂ ⎛ ∂Φ ⎞ ⎟ + SΦ + = ⎜ μt ⎟ + ⎜ μt ∂x ⎝ ∂x ⎠ ∂y ⎜⎝ ∂y ⎟⎠ ∂x ∂y 3e. Explain what is the influence of the spoiler on the lift and drag coefficients of the car due to the local flow field?

Computational Fluid Dynamics (CFD) Project Guideline Aim The aim of the CFD project is to provide an opportunity for students to demonstrate their understanding of the fundamentals and usage of CFD software as well as introducing them to the numerous applications within the software. Students are allowed to freely determine any topic of interest. It may be desirable that the intended CFD project topic coincides with the undertaking of your final year project or design project, keeping in mind the many constraints and complexity of the flow problem to be simulated. Students are therefore strongly encouraged to consult and discuss the project proposal with the lecturer/instructor before embarking on the next stage of their numerical study.

Objectives The following abilities should be demonstrated and explained in your project report: 1.

The ability to use the commercial CFD software packages • Mesh generation and grid quality.

• Defining the settings of a flow problem, i.e. boundary conditions and solver settings.

• Selecting appropriate CFD models, i.e. turbulence models, heat transfer or other types of simulations.

2.

The ability to use CFD as a tool for engineering design • Reduction of drag or increase of lift for flow over geometries, e.g. car body or aerofoils.

• Increase of heat transfer for cooling of a car engine, or reduction of heat transfer to prevent heat loss.

• To create desired flow control, i.e. the ability to cause flow to move within a desired region.

3.

The ability to apply CFD knowledge to analyze numerical results • Discussions of the flow patterns and behaviors (e.g. wake flow, flow separation, boundary layer and convective effects).

• Discussions of the accuracy of the CFD solution with regards to the mesh quality, flow models chosen and boundary/domain setting.

• Assumptions made on the actual model to allow for modeling simplifications to the original geometry.

Note: Not all the items listed above are applicable to all types of CFD problems. Please consult with lecturers or tutors concerning the above as well as ascertaining the suitability of the project.

Examples of past CFD project topics: • • • • • • • • • •

Effect of vehicle spacing on vehicles in convoy CFD analysis of wing in ground effect Investigation of the effect of winglet design on the lift and drag performance CFD simulation of flow over a bicycle helmet Study of the reduction of aerodynamic drag on a car-caravan combination Turbulent flow analysis over two two-dimensional wings of variable horizontal separation Investigation of flow field in areas of different hydro power plants Modeling of car air intake system and comparison of different designs Numerical investigation of trailing edge flows Comparison of fowler flap systems through CFD

Example - CFD Project proposal prepared by the student Introduction Aerofoil design plays a pivotal role in the wing and control surface performance in aerospace engineering. While wind tunnel testing can be rather time consuming and expensive, CFD provides an attractive alternative. Modelling of flow over an aerofoil is an important CFD problem. Amongst the many aerofoil design features, the flap (Figure E.6) which when deployed increases the camber of the wing to give increased lift (and drag). This phenomenon is being utilised during take off and landing in most aircraft as lift (and drag) enhancing devices (Figure E.7).

With flap CL Without flap

AOA (degrees)

Figure E.6 Plain Flap.

Figure E.7 Lift Curve Slope.

Scope The project will focus on the aerodynamic characteristics of a two-dimensional NACA 23012 aerofoil with a 20% scaled NACA 23012 plain flap. The aerofoil will be modelled at 0˚ angle of

attack with flap settings of 0˚, 10˚, 20˚, 30˚ and 40˚. The Reynolds number of 1,400,000 helps to study the turbulent characteristics of the combo. The lift and drag coefficient and flow separation will be determined using the standard, RNG k-ε and realizable k-ε turbulence models to assess the most appropriate model to be applied.

Results will be validated against benchmark experimental data of Carl Wenzinger (Carl J.Wenzinger, 1937, Pressure distribution over an NACA 23012 Airfoil with an NACA 23012 external-airfoil flap. NACA Report No.614).

Objectives • • • • • •

Create a quality mesh around the aerofoil/flap Model the airflow around the aerofoil/flap at Re = 1,400,000 and SSL conditions with flap settings varying from 0˚ to 40˚ using standard k-ε turbulence model Measure the lift and drag coefficient and pressure and velocity distribution of the aerofoil/flap Repeat with RNG k-ε and realizable k-ε turbulence models Compare and discuss the results of the three turbulence models Evaluate the accuracy of the results against experimental data

Other Topics for CFD Projects CFD Project A: CFD Simulation of Turbulent Flow over a Backward-Facing Step Background The backward-facing step is commonly used as a benchmark for validation of numerous flow characteristics, including turbulence model, multiphase flows and fundamental numerical methods. This model also has applications in industry, such as for combustion and HVAC. Objectives The aims of this simulation are to: 1.

2.

3.

4.

5.

6.

Create a backward facing step simulation appropriate for turbulence modeling.

Determine the effects of different meshing scheme.

Simulate turbulent flow and compare with the benchmark data.

Determine the effects of different turbulence models.

Understand the relevant flow characteristics.

Prepare a concise and well-written professional report.

Problem Description Dry air at 27°C flows through a 2D duct with a backward facing step at a Reynolds number, Reh = 5100, based on the step height, h. The dimensions of this flow configuration are shown below.

‘No-slip’ conditions are applied at the walls and it can be assumed they are perfectly smooth (i.e. roughness height = 0 m). The turbulence intensity of the incoming flow can be assumed to be 0.01% and thermal interaction between the walls and the fluid is assumed adiabatic (i.e. no heat transfer). For the purpose of this assignment, it is expected that no vortex shedding will occur (i.e. steady state analysis). From extremely accurate direct numerical simulations (DNS) it has been determined that re-attachment of the separated boundary layer occurs on the bottom wall at x = 16.28h.

40h

INLET

x

5h

10h

0 < h ≤ 0.1m

h

OUTLET

Required Discussions 1. Compare the following meshing schemes: (a) Uniform structured quadrilateral mesh; (b) Non-uniform structured quadrilateral mesh with mesh refinement in appropriate regions (with similar number of cells); and (c) Unstructured triangular mesh with refinement in appropriate regions (with similar number of cells). Discuss the differences and the necessity of mesh refinement.

2. Using the best mesh determined from (1), run the case using appropriate settings for (a) a standard k-ε turbulence model and (b) a realizable k-ε turbulence model. Determine which model gives the better prediction. Discuss the differences of the k-ε models, referring to their characteristic equations.

3. Using the best model determined from (1) and (2), replace the dry air with Ethylene Glycol at 37°C flowing at the same Reynolds number (Reh = 5100). Discuss any similarities or differences with the dry air simulation.

4. Discuss the relevant flow characteristics of the simulation.

CFD Project B: CFD Simulation of Pickup Trucks with Open/Closed Tubs Background CFD has been a vital tool in studying vehicle aerodynamics. These simulations aim to provide a better understanding on the flow behavior over the complex geometries as well as give insights on methods of increasing vehicle efficiency.

Objectives The aims of this simulation are to: 1. Create two-dimensional models of a pickup truck with the tub open and closed.

2. Create a mesh of sufficient quality to achieve grid independence.

3. Simulate turbulent flow and compare with benchmark data.

4. Understand and discuss the relevant flow characteristics, such as flow velocity and pressure, to design parameters, such as drag and lift forces.

5. Create a trailing pickup truck and investigate the effect of different distances on flow characteristics.

6. Prepare a concise and well-written report.

Problem Description A truck as shown below (dimensions provided in Addendum) is to be modeled (with tailgate closed). The moving truck is simulated traveling at a Reynolds number of 3.3 x 106 (based on the truck length). The standard k-ε turbulence model and standard wall functions are employed.

Students are to create a sufficiently large domain to capture the generated wake. The inlet should have a turbulence intensity of 2% and the lower boundary (parallel to the x-axis) should be set to a non-moving smooth wall, so as to simulate wind-tunnel conditions. The system is assumed to be adiabatic and time-independent. For a single pickup truck with a closed tailgate, the drag coefficient was found to be 0.44.

Tailgate

Required Discussions 1.

Determine appropriate boundary layer settings to provide a mesh of sufficient quality for this simulation, including calculations of the first layer thickness for a standard wall model turbulence case.

2. Compare the simulation drag coefficient with the benchmark data to determine mesh independence.

3.

4.

Compare and discuss the flow characteristics of a single truck with the tailgate open with one with the tailgate closed to determine if this has an effect on the overall drag of the vehicle.

Compare and discuss the effect of adding a trailing truck of variable distance (recommended you do not exceed 3 lengths of the truck) on both the leading and trailing vehicles. You may set either truck to have an open or closed tailgate for this investigation.

Plot and compare the drag and lift coefficients for both trucks against different separation lengths on a graph and discuss.

Note: You may need to extend the length of the domain to incorporate the extra truck.

Addendum Coordinates for the pickup truck with closed tailgate: 0.000 0.354 0.000 0.224 0.0215 0.246 0.0215 0.354 0.045 0.199 0.4226 0.246 0.4226 0.354 0.4264 0.38 0.4504 0.432 0.5458 0.432 0.5981 0.419 0.6438 0.402 0.6795 0.38 0.7185 0.35 0.7743 0.346 0.8284 0.337 0.8792 0.328 0.9263 0.199 0.9263 0.199 0.9267 0.315 0.935 0.229 0.9518 0.246 0.966 0.307 0.9771 0.285 0.9798 0.255 1.000 0.26 1.000 0.281

CFD Project C: Investigation of Cooling Electronic Components within a Computer Background CFD is a powerful tool in optimizing applications involving heat transfer. The cooling of electronic components within a computer has recently become an important issue. Particularly, increased processing power tends to generate more heat, which may cause hardware damage or failure.

Objectives The aims of this simulation are to: 1. Preparing a suitable mesh of sufficient quality to analyze the thermal and flow characteristics of air cooling within a computer.

2. Understand the requirements for cooling electronic components within the system.

3. Design the system by placing the components in optimal positions to allow operation within the safety range through the understanding of the thermal and flow behavior.

4. Determine the best positions for the electronic components.

Problem Description A system case is shown below with the dimensions listed in millimeters. There are two exhaust openings (50mm wide) and one air intake (80mm wide). Air is to enter the system at a temperature of 20˚C at a rate of 0.1m3/s with a turbulence intensity of 5%. The walls of the system are assumed to be adiabatic and smooth. Within the system, add: (a) 1 processor with dimensions 25mm x 25mm with a heat output of 800W/m2, (b) 3 RAM modules with dimensions 135mm x 30mm with a heat output of 60W/m2 and (c) 3 other components of dimensions 100mm x 50mm with a heat output of 150W/m2. Use the standard k-ε turbulence model to simulate the flow turbulence.

Exhaust 120

Power Supply 100

Disk Drives

240

100 Exhaust

420 100

200

Intake 30

450

Required Discussions Arrange the components listed above within the system casing with the given boundary conditions in at least three different designs. Additionally, analyze the fluid flow and thermal characteristics to determine the most optimum locations for your components, bearing in mind that no component surface should exceed 80˚C.

Computer Lab Guide for CFD Assignment A Learning Objectives: 1. Familiarize ANSYS Workbench environment 2. Learn to create the model geometry using ANSYS DesignModeler; 3. Learn to generate a grid/mesh with an acceptable quality using ANSYS Meshing.

4. Learn to perform a preliminary CFD analysis using ANSYS CFX, including assigning model property parameters, selecting the appropriate CFD solver, setting up the boundary and initial conditions, etc.;

5. Learn to utilise the basic post-processing techniques in ANSYS CFX Post to obtain the basic CFD data, such as, pressure distribution, velocity variation in the entire computational domain, as well as at a specified region of the domain.

Procedure Start ANSYS Workbench: (1) From Windows Start menu, select ANSYS 13.0 > Workbench (2) When Workbench opens, select File > Save, then save the project as Assign1 into the folder of your choice on the computer D:\ drive.

(3) Expand the Analysis Systems toolbox, then drag and drop a Fluid Flow (CFX ) object into the top left area of the Project Schematic window as shown in Figure 1.

(4) Specify the units system as “Metric ( kg, m, s, ºC, A, N, V)”, and choose to “Display Values in Project Units”.

Figure 1. ANSYS Workbench window

Stage 1.

Create the model geometry

Double click on the Geometry cell to launch DesignModeler. Select m as the unit of measure for the session when prompted.

Under Tree Outline, click on XY Plane, then click on New Sketch icon is to insert Sketch1 in XY Plane.

Click on Look At icon

on the top menu. This

to orient the sketch plane in the normal direction.

Click on the Sketching tab to switch to sketch mode.

Sketching: Settings > Grid Toggle on Show in 2D and Snap to Grid Click on Major Grid Spacing and set it to 1.0 m Click on Minor-Steps per Major and set it to 2 Zoom in on the model view, so that the area displayed is placed around the origin of the coordinate with 10 major steps in the positive x- direction and 5 major steps in the negative xdirection.

Figure 2 Setup the Grid for creating the 2D geometry

Sketching: Draw > Polyline -

Draw the outline of the 2D backward-facing step model by single LMB clicking on the points with the given coordinates sequentially, as shown in Figure 1 of Assignment A, on the sketch, starting from the origin (0, 0). When come back to the origin point (0, 0), RMB and then select Close End from the pop up menu.

The 2D dimensions of model are defined now by drawing within the grid with the given coordinates of the model.

However, in the next step, we will still set up the dimensions of the model, as shown in Figure 3.

This is to parameterize the model dimensions for the possible model geometry modifications in the future.

Sketching: Dimensions > Horizontal

(1) Single LMB click on the line labelled 1 (refer to Figure 3) to select the first line, and then single LMB click on the line labelled 2 to select the second line. A pencil will appear on the screen. Move the pencil by dragging the mouse to an appropriate location, and then single LMB click to define the dimension H1 (note: the number on your drawing may differ due to the sequence of your setting up the dimensions).

(2) Repeat the same procedure to select the lines labelled 2 and 3 to define the dimension H2.

Sketching: Dimensions > Vertical (3) Single LMB click on the line labelled 4 (refer to Figure 3) to select the first line, and then single LMB click on the line labelled 5 to select the second line. A pencil will appear on the screen. Move the pencil by dragging the mouse to an appropriate location, and then single LMB click to define the dimension V3 (note: the number on your drawing may differ due to the sequence of your setting up the dimensions).

(4) Repeat the same procedure to select the lines labelled 5 and 6 to define the dimension V4.

Figure 3 Setup the four dimensions of the model

On the top menu bar, click on the Display Plane icon icon to switch the graph to isometric view.

Click on the Extrude icon , on the top menu bar, to bring up the Details View for the Extrude 3D operation (the Tree outline will consequently switch to the Modeling phase).

In the Details View (refer to Figure 4), select Reversed for Direction, and then enter 0.01 for , on the top menu bar, to complete the 3D Extrusion.

FD 1 Depth (>0). Then click

to turn off the grid. Then click on the

Figure 4 Details View for the 3D Extrude operation settings We have now created the model geometry of the backward-facing step.

Save the Project and exit DesignModeler to return to the Workbench and ready for Meshing.

Stage 2

Create the Mesh

In ANSYS Workbench, double click on Mesh cell to bring up ANSYS Mesh (Refer to Figure 5).

(1) Click on Mesh under the Outline tree view to bring up the Details of Mesh panel at the leftbottom of the screen. In this panel: (2) Set Physics Preference to CFD; Set Solver Reference: CFX (3) Expand Sizing and set the Relevance Centre to Medium.

(4) In the Outline tree view, RMB click on Mesh > Generate Mesh to create the mesh with a medium density, as shown in Figure 5.

Figure 5 Create a structured mesh with a medium density.

Define the boundaries of the fluid domain by creating the Named Selections for all the faces that enclose the fluid domain: inlet, outlet, top wall, bottom wall, step wall. The front and back faces of the model are also need to be defined as the symmetry planes.

(1) Refer to Figure 6 to define the inlet boundary through the following steps: - Ensure that the Face Selection Filter is active, single LMB click to select the Inlet face; - RMB > Create Named Selection ; - In the pup window, enter a name for the selected face, e.g. inlet, then OK. (2) Repeat the same procedure to create the Named Selections for the outlet, the two symmetry planes (name them the way you like, e.g. symm1 and symm2), and the top, bottom and step walls of the channel (e.g. wall_top, wall_bottom, wall_step).

Figure 6 Create a Named Selection for inlet face

Click File > Save Project … to save the project file.

Click File > Close Meshing to exit ANSYS Mesh and return to the Workbench Project page.

In the Workbench Project Schematic window, there should be a green tick next to the Mesh cell.

If not, RMB on the Mesh cell, in the pup up menu select Update. Wait until the green tick appears. Then proceed to Stage 3.

Note: To create a finer mesh as required in Q3 of Assignment A, the simplest way is to set (under Sizing) the Relevance Centre to Fine, then click on Update on the top menu bar to generate the mesh with the fine density.

If you wish to further refine the mesh in a unified manner, you can decrease the values for Min Size and Max Face size under Sizing. To ensure the accuracy of the 2D model solution, the limit for the Min Size is the Extrusion depth, which in this case is 0.01 m.

Stage 3 Specify Boundary Types, Materials, Fluid flow models and Initial values In Workbench Project Schematic window, double click on the Setup cell to open CFX Pre If you wish to change the default domain name to a meaningful name, then (1) RMB on Default Domain under the Outline tree (2) Select Rename, and then change the name to Backward Step.

Specify a new material (refer to Figure 7) (1)

RMB on Materials > Insert > Material, in the pup up window click OK to accept the default material name: Material 1 (you also can change the name by replacing it to a different name).

(2)

In Details of Material 1, click on Material Properties tab. Under Equation of State, enter the value for Density (refer to Assignment A paper for the fluid density value).

(3)

Expand Transport Properties and tick Dynamic Viscosity, then enter the value for Dynamic Viscosity (refer to Assignment A paper for the fluid viscosity value)

(4)

Click on Apply to save the change, and then OK to exit Details of Material 1.

(a)

(b)

Figure 7 Specify a new fluid material property: (a) Insert Material 1; (b) Enter the new material property.

Under Flow Analysis 1, double click on the fluid domain re-named Backward Step to open the Details of Backward Step in Flow Analysis 1, which contains three tabs named Basic Settings, Fluid Models and Initialisation. Go through each of them as follows: (1) In the Basic Settings tab, select Material 1 for the fluid material; leave other fields to the default settings (2) Click on the Fluid Models tab, select None for Heat transfer; select (None) Laminar for Turbulence.

(3) Click on Initialisation tab, tick the Domain Initialisation to view the default settings.

Accept all the settings by clicking on Apply, then OK to exit the panel.

Specify boundary conditions for the flow domain. In the Outline tree: (1) RMB on Backward Step > Insert > Boundary. In the pup up window, type: inlet, and then OK. This brings up Details of inlet in Backward Step in Flow Analysis 1: (a)

Select Inlet for Boundary Type; select inlet for Location.

(b)

Click on Boundary Details tab, enter the value for Normal Speed (refer to Assignment A paper for the inlet speed value in x-direction), and keep other fields with the default settings.

(c)

Click Apply to finalise the setting, and then OK to exit the Details tab.

(2) Repeat the same procedure to define the other boundaries of the fluid domain named Backward Step. Refer to Table 1 for the corresponding settings.

Table 1: Boundary settings for the flow domain named Backward Step Name

Boundary Type

Location

Boundary Details

outlet

Outlet

outlet

Average Static Pressure = 0 Pa

symm

Symmetry

symm1, symm2 (Ctrl + LMB click for the multiple selection)

wall

Wall

wall_top, wall_bottom, wall_step (Ctrl + LMB click for the multiple selection)

No Slip Wall

Under Flow Analysis 1, double click on Solver Control to open the tab named: Details of Solver Control in Flow Analysis 1.

Under the Basic Settings tab, as shown in Figure 8, set: - Max. Iterations: 200.

- Residual Target: 1e-06 (or 0.000001), - Apply to save the change.

- OK to exit the Details tab

File > Save Project

File > Quit to exit CFX-Pre and return to the Workbench Project window.

Figure 8. Solver Control tab

Stage 3. Solving for a solution

Save the Workbench Project

In Workbench Project Schematic, LMB double click on Solution cell to launch the CFX Solver Manager

The CFX Solver Manager will start with the simulation ready to run.

Click on Start Run button to begin the solution process.

The solution process will stop either the residual target is met or the maximum Iteration number is reached, whichever comes first. Steadily decreased Residual curves indicate a convergent solution.

If the maximum iteration number is reached but the Residual targets have not met, we will need to continue the iterations: LMB click on File > Define Run > Start Run, until all the Residual targets are met.

Exit CFX Solver Manager to return to Workbench Project window.

Stage 5: Post-processing the result in CFD-Post

In Workbench Project Schematic window, save the project, and wait for the Solution cell getting a green tick.

LMB double-click the Result cell to launch the CFX CFD-Post.

Create a pressure contour plot on the symmetry plane from the top menu bar.

-

Click on the Contour icon

-

In the pup up window, accept the default name Contour 1 by clicking on OK.

-

In the Details of Contour 1, set the Locations to symm

-

Set the Variable to Pressure,

-

Increase the # of Contours to 20.

-

Click Apply to generate the pressure contour plot (see Figure 9).

Figure 9 Pressure contour plot Study the pressure contour plot. Has the flow reached to the fully developed stage before prior to the step?

Insert the pressure contour plot into the Report (refer to Figure 10): (1)

under the Outline tree, RMB on Report > Insert > Figure,

(2)

in the pup up window, change the name Figure 1 to Pressure Contour, and then OK.

To create another contour plot, e.g. velocity contour, simply click on Contour 1 under the Outline tree, and then change the Variable to Velocity, and then Apply to view the plot.

Similarly, we can also insert the velocity contour plot into the Report. With the velocity vector plot in the graphics window, RMB on Report > Insert > Figure, then given a name and OK. Figure 10 Procedure for Inserting a graph into the Report

Create a velocity vector plot on the outlet: -

Click on the Vector icon

-

In the pup up window, accept the default name Vector 1 by clicking on OK.

-

In Details of Vector 1, set the Locations to outlet

-

Set the Variable to Velocity.

Click on Symbol tab, and then enter 2 for Symbol Size

-

Click Apply to generate the velocity vector plot on the outlet (see Figure 11)

from the top menu bar.

Figure 11 Velocity vectors on outlet Has the flow reached to a fully developed stage prior to the outlet?

You may follow the same procedure to plot the velocity vectors on one of the symmetry planes, which may be similar as the graph in Figure 12.

Figure 12 Velocity vectors on outlet

Create a streamline plot on the symmetry plane (see Figure 13): from the top menu bar.

-

Click on the Streamline icon

-

In the pup up window, accept the default name, Streamline 1, then OK

-

In Details of Streamline 1, set (a) Type: 3D Streamline; (b) Start from: symm, inlet; (c) Sampling: Equally Spaced;

(e) # of Points: 30, (f) Direction: Forward and Backward -

Click Apply to generate the streamline plot as shown in Figure 13.

Figure 13 Procedure for creating the streamline plot

Create a line that is very close to the bottom wall from the step to the outlet (see Figure 14): -

Expend the Location icon from the top menu bar, and then select Line

-

In the pup up window, accept the default name, Line 1, then OK

-

In Details of Line 1, set (a) Domain: All Domains (b) Enter the coordinates of Point 1 and Point 2 (refer to Figure 14); (c) Tick Sample for Line Type (e) Enter 30 for Samples; (f) Click Apply to create Line 1 (the yellow line in the Graphics window that almost overlaps with the bottom wall of the step) as shown in Figure 14.

Figure 14 Procedure for creating Line 1 (the yellow line)

Create a chart plotting x-velocity along the newly created Line 1 (see Figure 15): -

Click on the Insert Chart icon

-

In the pup up window, accept the default name, Chart 1, then OK

-

In Details of Chart 1, there are six tabs. we need to set, at least, the first four tabs to produce the chart:

from the top menu bar.

(1) Under the General tab: (1a) for Type select xy; (1b) for Title, entre the title name of your own choice, e.g. x-velocity along the step bottom wall.

(2) Click on Data Series tab: (2a) highlight: Series 1(line1); (2b) tick Location, and then select Line 1.

(3) Click on X Axis tab: (3a) select X for Variable; (3b) tick Conservative.

(4) Click on Y Axis tab: (4a) Variables: select Velocity u; (4b) tick Conservative; (4c) Click Apply to generate the Chart 1 as shown in Figure 15(e).

(5) Extract the numerical data of Chart 1 by clicking on Export and save the data into a spreadsheet.

(a)

(b)

(c)

(d)

(e) Figure 15

Procedure for creating the chart of x-velocity versus Line 1: (a) General tab settings; (b) Data Series tab settings; (c) X Axis tab settings; (d) Y Axis tab settings; and (e) Chart 1 on the Graphics screen.

Publish the report: -

RMB on Report, select Refresh Preview

-

RMB on Report, select Publish

-

In the pop up Publish Report window, note the location of the report files and then OK.

Exit CFD-Post and return to the Workbench Project page.

Save the project

Exit ANSYS Workbench.

Figure 16 Report preview and publish

CFD Assignment A: Sample Graphs 1. Mesh

1(a) Mesh 1

1(b) Mesh 2

1(c) Mesh 3

1(d) Mesh 4

1(e) Mesh 5 Figure 1 Uniformly structured meshes with increased mesh fineness from 1(a) to 1(e).

Note that the higher number of the mesh ID indicates the higher level of mesh fineness.

2. Flow phenomenon for Re = 100 case

Figure 2 Pressure contour

Figure 3 Velocity contour

Figure 4 Streamlines

Figure 5 Velocity vector plot

Figure 6 Zoomed view on the velocity vectors showing flow reversal and reattachment location after the step

Figure 7 Velocity profiles at different cross sections before and after the step.

Note the parabola velocity profile before the step indicating that the flow has reached its fully developed state before the step.

Figure 8 Parabola velocity profile at the outlet showing the flow has reached its fully developed state

3. Mesh independence test Pressure Gradient_X along the Bottom Wall after Step 0.08 0.07

Mesh 1 Mesh 2

0.06 Pressure Gradient X

Mesh 3 0.05

Mesh 4 Mesh 5

0.04 0.03 0.02 0.01 0 -0.01

0

1

2

3

4

5

6

7

8

9

10

-0.02 x-distance from the step

Figure 9 Pressure gradient_X along the bottom wall after the step for the five meshes with increased mesh fineness for the Re = 100 case.

Table 1 The critical section of the data series in Figure 9, revealing the x location where the Pressure Gradient_X changes from positive to negative for the five different meshes, respectively. x (m) 0 0.20408 ----4.08163 4.28571 4.48979 4.69388 4.89796 5.10204 5.30612 5.5102 5.71428 5.91837 6.12245 6.32653 6.53061 6.73469 ----9.79591 10

Note:

Mesh 1 -0.00206 -0.00206 ----0.025823 0.015853 0.006401 -0.00123 -0.00752 -0.01184 -0.01429 -0.01547 -0.01521 -0.01453 -0.01347 -0.01244 -0.01165 -0.01098 -----0.01042 -0.01042

Mesh 2 0.00128 0.00128 ----0.04656 0.03629 0.02649 0.01764 0.01001 0.00387 -0.0008 -0.0042 -0.0064 -0.0077 -0.0082 -0.0084 -0.0083 -0.008 -----0.0084 -0.0084

Mesh 3 -0.00154 -0.00122 ----0.065776 0.057948 0.049129 0.039957 0.030888 0.022384 0.014813 0.008409 0.003243 -0.0007 -0.00359 -0.00555 -0.00681 -0.00756 -----0.00857 -0.00849

Mesh 4 -0.00216 -0.00277 ----0.068358 0.061734 0.053747 0.045011 0.036045 0.027408 0.019497 0.01259 0.006825 0.00224 -0.00127 -0.00383 -0.00561 -0.00677 -----0.00901 -0.00855

Mesh 5 -0.00127 -0.00352 ----0.069683 0.063177 0.055274 0.046497 0.037442 0.028643 0.020537 0.013429 0.007468 0.002693 -0.00098 -0.00367 -0.00555 -0.00679 -----0.00914 -0.00793

The location of the re-attachment point is the value of x when the Pressure Gradient_X = 0.

Using the linear interpolation, we can estimate the x values for the five meshes for the Re = 100 case, as shown in Table 2.

Table 2 Calculated re-attachment length (for Re = 100) using the five meshes, respectively.

Mesh ID Re-attachment length

1

2

3

4

5

4.660908

5.271289

5.882098

6.048679

6.06816

Re-attachment length vs fineness of mesh for the Re=100 case 7

Re-attachment length

6 5 4 3 2 1 0 0

1

2

3 Mesh ID

4

5

6

Figure 10 The calculated re-attachment length (for Re=100 case) using the five meshes, respectively, showing the convergence of the re-attachment length after the level of mesh fineness reaches Mesh 4.

Note for students: (1)

Based on your mesh independence test results, you are expected to comment on which mesh is the optimal mesh that would be used in a real life investigation.

(2)

HOWEVER, in order to SAVE time in the assignment exercise, students are advised to use a mesh with the fineness of one or two levels coarser than the optimal mesh for Q4 and Q5.

4. Reynolds number versus the location of the re-attachment point •

Check whether the flow is fully developed at the outlet. If not, the model length after the step should be increased until a fully developed flow at outlet is achieved.

(a)

(b) Figure 11 Flow profiles at the outlet for the Re=200 case, (a) with the original model of 10 units length after the step; (b) with the modified model, of which the model length after step is extended to 20 units.

Table 3 Calculated re-attachment length for Re = 50, 100, 150 and 200, respectively.

Reynolds Number Re-attachment length

50

100

150

200

3.624688

6.048679

7.957252

14.31134

Re-attachment Length vs Reynolds Number 16

Re-attachment length

14 12 10 8 6 4 2 0 0

50

100 Reynolds Number

150

200

Figure 12 Re-attachment lengths versus Reynolds number, showing that the re-attachment length increases more rapidly when Reynolds number increases from 150 to 200.

5 The model with the free shear boundary condition on the top wall

The free shear wall boundary condition means that a symmetry boundary is applied on the geometry, instead. The model effectively becomes a flow of Re = 400 case with only half of the geometry is modelled.

Be aware that when the same geometry as for the previous Re=200 case is used (which is 20 units model length after the step), the flow is far from the fully developed stage at both regions: (a) before the step; (b) the outlet, as shown in Figure 13.

(a)

(b)

Figure 13 Velocity profile at the regions (a) before the step, and (b) the outlet, with the model length of 20 units after the step, is far from a fully developed stage.

Thus, for a meaningful solution the model length would have to be extended much further in both sections before and after the step, until a fully developed flow (a half parabola profile in a cross section) is achieved, as shown in Figure 14.

The model used for the Q5 results, shown in Figures 14 and 15, has been extended up to 80 units after the step and 20 units before the step.

(b)

(b)

Figure 14 After extending model lengths in both sections before and after the step, the velocity profiles at the regions (a) before the step, and (b) the outlet, show a fully developed flow phenomenon.

(a)

(b)

(c) Figure 15 Velocity contour plot for the top wall free shear model (a) full model view; (b) zoomed view at the section around the step, (c) zoomed view at the section near the outlet.

Computer Lab Guide for CFD Assignment B (Part 1) Learning Objectives: 1. Learn to create 3D model geometry in ANSYS DesignModeler (Part 1) 2. Learn to generate an inflated boundary layer mesh around a 3D wall in ANSYS Meshing (Part 1) 3. Learn to generate a 3D grid/mesh with an acceptable quality using ANSYS Meshing (Part 1) 4. Learn to perform the CFD analysis for external turbulent flows using ANSYS CFX, including assigning model property parameters, selecting the appropriate CFD solver, setting up the boundary and initial conditions et cetera. (Part 1) 5. Learn to write expressions in ANSYS CFX Post to calculate the required CFD data, for example the lift and drag coefficients (Part 2) 6. Learn to utilise the various visualisation tools in ANSYS CFX Post to present the 3D turbulent flow characteristics around the car body (Part 2)

Procedure Start ANSYS Workbench: (1) From Windows Start menu, select ANSYS 13.0 > Workbench (2) When Workbench opens, select File > Save, then save the project as Assign2 into the folder under the D:\ drive on the computer, e.g. D:\S3000000\AssignB (3) In the Analysis Systems, drag and drop a Fluid Flow (CFX) system into the window of Project Schematic.

Stage 1

3D Model Geometry Creation

Double click on the Geometry cell to launch DesignModeler. Select m as the unit of measure for the session when prompted.

Under Tree Outline, click on XY Plane, then click on New Sketch icon top menu. This is to insert Sketch1 in XY Plane.

Click on Look At icon

Click on the Sketching tab to switch to the sketch mode.

on the

to orient the sketch plane in the normal direction.

We first create the computational fluid domain:

In Sketching: click on Draw > Rectangle, then draw a 2D rectangle box as shown in Figure 1. Make sure that the bottom line of the box overlaps with the x-axis.

In Sketching: Click on Dimensions, then set up the dimension parameters as shown in Figure 1 (note: your dimension numbers may be different).

Figure 1 The 2D rectangle box of the computational domain with dimension parameters

In the Details for Dimension window, modify the values of the dimension parameters as shown in Table 1.

Table 1 The 2D rectangle dimension parameter values H1

H2

V3

8

18

3

Click on Extrude icon on the top menu bar above the model viewer window to bring up the Details View for the Extrude 3D operation (the view will consequently switch to the Modeling view).

Edit the Details of Extrude 1 page: (1) select Add Frozen for Operation; (2) icon select Reversed for Direction; (3) set FD1 Depth to 3. Click on to complete the Extrude operation.

We then create a plane parallel with the XY plane:

Ensure the XY Plane is active, and then click on the New Plane icon insert a plane that parallel with the XY Plane.

In Details of Plane4, set Transform 1 to Offset Z and FD1 to 0 (as shown in Figure 2).

Figure 2 Insetting and editing the new plane (Plane4)

Click on

icon on the top menu bar to complete the operation.

to

We finally create the car body (refer to Figure 3 below)

Highlight the newly created Plane 4 by clicking on Plane 4 under the Tree Outline

Click on

In the Details of Point 1,

icon on the Top bar menu

(1) set Definition to be “From Coordinates File”; (2) At Coordinate File, click on

to browse;

(3) Browse necessary to locate the file “car-profile”, and Open it; (4) Click on

icon to complete the operation.

(5) Zoom in to get a detailed view of the points, as shown in Figure 3.

Figure 3 Import the car profile data points into DesignModerler

Click on Sketching tab > Draw > Spline

In the Graphics window, click on the top 8 points sequentially from left to the right, then RMB click to bring up a menu, and select Open End to complete the spline as the top profile of the car.

Repeat the same procedure to create a spline though the 7 points along the bottom for the base profile of the car.

Click on Line, and then in the Graphics window link the remaining points along the front and back sections of the car with straight lines (see Figure 4).

Figure 4 Sketch the car profile using Spline and Line operations

Click on the icon on the top menu bar to bring up the Details View for the Extrude 3D operation.

In the Details of Exdrude2, as shown in Figure 5, (1) select Slice Material for Operation; (2) select Reverse for Direction, and (3) type 0.17 for FD 1 Depth (>0).

Click on icon on the top menu bar to complete the Extrude operation.

Under the Tree Outline,

Figure 5

Details of Extrude2

(1) Expend the 2 Parts 2 Bodies ; (2) Rename the two Solid objects to Fluid Domain and Car, respectively; (3) RMB click on the Car, and then select Suppress Body, as shown in Figure 6

The 3D model geometry creation is now completed as shown in Figure 7.

Figure 6 Suppress the car body

.

Figure 7 The completed 3D car model geometry, with the names of boundary faces that enclose the fluid domain.

Create Named Selections for the boundaries of the flow domain: (1) Ensure that the Face Selection Filter is active, click on the inlet face to highlight the face; (2) RMB > Create Named Selection ; (3) In the Details View panel, next to the Geometry cell, click on Apply; (4) Click on the Named Selection cell, and change the name to inlet; (5) Then click on the creation.

icon on the top menu bar to complete the

Repeat the same procedure to create the Named Selections for outlet, symmplane, sky, sideplane, and ground.

Ctrl_click to select all the faces around the car body (total 12 faces), and then RMB > Create Named Selection and re-name the selection as carbody, then click on icon to complete the creation.

Save the Project and exit DesignModeler to return to the Workbench and ready for Meshing.

Stage 2

Create the Mesh

In the Workbench, RMB on the Geometry cell.

(1) In the pop up menu select Properties.

(2) Ensure that the Named Selection cell is ticked ON. (3) Delete the value: NS for the Named Selection Key. This is to ensure that the Named Selections are transferred to the next operation.

Double click on Mesh cell of the Fluid Flow (CFX) system block to bring up ANSYS Mesh.

Click on Mesh under the Outline view to bring up the Details of Mesh panel, set the Physics Preference to CFD, Solver Reference: CFX.

Expand Sizing and set the Relevance Centre to Fine.

Insert an Inflation to create the boundary layers on the faces of the car body

RMB on Mesh under the Outline view, select Insert > Inflation (see Figure 8(a)).

Note that a panel named Details of “Inflation” is now appeared on the left bottom corner of the screen.

Note that the cell next to the Geometry cell under Scope in the Details of “Inflation” is highlighted. This means that the Geometry requires definition.

Click on the highlighted cell next to Geometry to bring up Apply and Cancel buttons, and then move the curser to the Graphics window to select the fluid domain by clicking on it, then click on the Apply button.

Under Definition, click on the highlighted cell next to Boundary to bring up Apply and Cancel buttons, and then move the curser to the Graphics window to select the12 faces of the car body by Ctrl_clicking on them, then click on the Apply button.

Set Inflation Options to First Layer Thickness.

Type 0.0013 m for First Layer Height Note: Refer to Appendix 1 for the calculation of the 1st layer height.

Type 16 for Maximum Layers.

Keep the default value1.2 for Growth Rate.

Insert a sizing function

Ensure that Face selection on the top menu bar is active.

RMB on Mesh under the Outline view, select Insert > Sizing (see Figure 8(b))

Note that a panel named Details of “Face Sizing” is now appeared on the left bottom corner of the screen

In the Details of “Face Sizing” panel, select Named Selection for Scoping Method.

Select carbody for Named Selection.

Enter 0.01 for Element Size.

(a)

(b)

Figure 8 Mesh settings for (a) Inflation; (b) Sizing Preview the inflated boundary layer mesh

RMB on Mesh under Outline view, select Preview > Inflation.

Wait for a couple of seconds for the boundary layer mesh to appear (see Figure 9).

Figure 9 Preview inflation

Generate the body mesh

In Outline view, RMB click on Mesh, select Generate Mesh.

Wait for the body mesh to be completed (see Figure 10).

(a)

(b)

Figure 10 3D body mesh (a) over the computational domain; and (b) around the car body.

File > Save Project

File > Close Meshing to exit ANSYS Mesh and return to the Workbench Project page.

RMB click on the Mesh cell > Update. Wait until a green tick appears for the Mesh cell.

Stage 3 Specify boundary types, materials, fluid flow models and initial values Note: Though the flow phenomenon in this assignment is, by nature, transient, we are seeking steady state solutions. This is because the main objective of the assignment is to obtain the drag coefficient, for which the steady-state turbulence modelling in most CFD software can provide good solutions with much lower computational cost.

In Workbench, double click on Setup cell to open CFX Pre Change the default domain name to a meaningful name. RMB on Default Domain in the Outline tree and select Rename, then change Default Domain to Airflow.

Under Flow Analysis 1, double click on Airflow to open the panel named Details of Airflow in Flow Analysis 1, which contains three tabs named Basic Settings, Fluid Models and Initialisation. Go through each of them as follows:

-

-

In the Basic Settings, note the default material is Air at 25 C, which is the material we are using in this analysis. Leave other fields to the default settings.

-

Click on the Fluid Models tab, select None for Heat transfer; select KEpsilon for Turbulence. Leave other fields to the default settings. Accept all the settings by clicking on Apply.

Click on Initialisation tab, tick the Domain Initialisation to view the default settings. Accept these settings by clicking on Apply, then OK to exit the Details panel.

Under Materials, double click on Air at 25 C to open the panel named Details of Air at 25 C. Click on Material Properties tab, find the values of Density under Equation of State, and Dynamic Viscosity under Transport Properties, as shown below. These values will be used to calculate: (1) Inlet velocity; (2) inlet turbulent kinetic energy k; (3) inlet turbulent dissipation rate ε; (4) lift and drag coefficients.

Density: ρ = 1.185 [kg m^-3]; Dynamic Viscosity: μ = 1.831E-05 [kg m^-1 s^-1].

Click on Close to exit the panel.

Specify the boundary conditions of the flow domain (1) RMB on Airflow > Insert > Boundary, type: inlet in the pup up window, then OK This brings up Details of inlet in Airflow in Flow Analysis 1

(a) Select Inlet for Boundary Type; select inlet for Location. (b) Click on Boundary Details tab, see Figure 11 below, enter 27.78 for Normal Speed (c) Under Turbulence, select Medium (Inrensity = 5%) for Option. (d) Click Apply to finalise the setting, then OK to exit the Details of Inlet panel.

Figure 11

The inlet condition settings

(2) RMB on Airflow > Insert > Boundary, type: ground in the pup up window, then OK This brings up Details of ground in Airflow in Flow Analysis 1. Refer to Figure 12: (a) Select Wall for Boundary Type; select ground for Location. (b) Click on Boundary Details tab. Tick Wall Velocity to open the cell, and then enter 27.78 for Wall U, 0 for Wall V and Wall W. Leave rest settings as the default settings. (c) Click Apply to finalise the settings, then OK to exit the Details of Ground panel.

Figure 12 The ground condition settings

(3) RMB on Airflow > Insert > Boundary, type: farfield in the pup up window, then OK This brings up Details of farfield in Airflow in Flow Analysis 1 (a) Select Opening for Boundary Type; select sky and backplane for Location, refer to Figure 13(a). (b) Click on Boundary Details tab. In Mass and Momentum cell, select Entrainment for Option, and enter 0 for Relative pressure.

In Turbulence cell select Zero Gradient for Option. Keep the default Subsonic for Option in the Flow Regime cell (see Figure 13(b)).

(4) Click Apply to finalise the settings, then OK to exit the Details panel.

(5) Repeat the same procedure to define the remaining boundaries of the fluid domain named Airflow. Refer to the following Table 1 for the corresponding settings.

(a) Figure 13

(b) The sky and backplane condition settings

Table 1: Some boundary settings of the Airflow domain

Name

Boundary Type

Location

Boundary Details

outlet

Outlet

outlet

Average Static Pressure = 0 Pa

symm

Symmetry

symmplane

carbody

Wall

carbody

No Slip Wall

Under Flow Analysis 1, double click on Solver Control to open the panel named: Details of Solver Control in Flow Analysis 1.

Accept the default Basic Settings by clicking on Apply (as shown in Figure 14), then OK to exit the Details of Solver Control panel.

Under Flow Analysis 1, double click on Output Control to open the panel named: Details of Output Control in Flow Analysis 1.

Click on the Monitor tab, then refer to Figure 15, (1) expand Monitor Points and Expressions; (2) add a monitor point: DragCoef; (3) select Expression for Option; (4) type the following drag coefficient expression for the Expression Value: force_x()@carbody*2/1.185[kg m^-3]/27.78[m s^-1]^2/0.252[m]/0.17[m]

Click on Apply, to save the settings, then OK to exit the Details of Output panel.

Cd = 1 2

Figure 14 Solver Control settings

Fd

ρU ∞2 ⋅ A

Figure 15 output Control settings

Click on the global Initialization icon on the top menu bar to open the panel named Details of Global Initialization in Flow Analysis 1. Refer to Figure 16: (a) Tick the Velocity Scale to open the cell, and then (b) Enter 27.78 for Value. Leave other settings as the default. (c) Click on Apply to save the settings, then OK to exit the panel.

Figure 16 Global Initialization settings

Exit CFX Pre and return to the Workbench Project page.

Stage 4 Solving for a solution

Save the Workbench Project

In Workbench double-click Solution cell to launch the CFX Solver Manager

The CFX Solver Manager will start with the simulation ready to run.

Click on Start Run to begin the solution process.

The solution process will stop either the residual target is met or the Max. Iteration number is reached, whichever comes first. Steadily decreased residual curves indicate a convergent solution. It will take approximately 8 mins to obtain the solution)

Exit CFX Solver Manager to return to Workbench Project page.

File > Save.

Click File > Exit … to quit ANSYS Workbench.

Appendix 1 In ANSYS CFX, hybrid methods are provided for turbulence models (automatic nearwall treatment) involving boundary layer simulation. The value of y+ < 300 is recommended. Furthermore, a minimum 10 grid nodes inside the boundary layers are also required.

To save time in meshing, it is useful to estimate the distance between the wall and the first grid node, y1. ANSYS CFX recommends using the following formula to estimate the y1 value based on the desired y+ value:

y1 = L ⋅ y + 74 Re −L13 / 14 where

L = 1 is the length scale (i.e. length of the car); and

Re L - is the Reynolds number based on the length scale.

Based on the recommendations of ANSYS CFX for the max y+ value, in this guide I took a third of the maximum y+ value, i.e. y+ =100, to estimate the 1st inflation layer height y1 as: y1 = 0.0013 for the inflation settings.

Lab Guide for Assignment B (Part 1) Learning Objectives: 1. Learn to create 3D model geometry in ANSYS DesignModeler (Part 1) 2. Learn to generate an inflated boundary layer mesh around a 3D wall in ANSYS Meshing (Part 1) 3. Learn to generate 3D grids with acceptable mesh qualities using ANSYS Meshing (Part 1) 4. Learn to perform the CFD analysis for external turbulent flows using ANSYS CFX, including assigning model property parameters, selecting the appropriate CFD solver, setting up the boundary and initial conditions et cetera. (Part 1) 5. Learn to write expressions in ANSYS CFX Post to calculate the required CFD data, for example the lift and drag coefficients (Part 2) 6. Learn to utilise the various visualisation tools in ANSYS CFX Post to present the 3D turbulent flow characteristics around the car body (Part 2)

Procedure Start ANSYS Workbench: (1) From Windows Start menu, select ANSYS 13.0 > Workbench (2) When Workbench opens, select File > Save, then save the project as Assign2 into the secured folder on the hard disk of the computer by the following path: Computer > Windows(C:) > scratch > Folder named by your student number (e.g. s3012345)

(3) In the Analysis Systems, drag and drop a Fluid Flow (CFX) system into the window of Project Schematic.

Stage 1

3D Model Geometry Creation

Double click on the Geometry cell to launch DesignModeler. Select m as the unit of measure for the session when prompted.

Under Tree Outline, click on XY Plane, then click on New Sketch icon top menu. This is to insert Sketch1 in XY Plane.

Click on Look At icon

Click on the Sketching tab to switch to the sketch mode.

on the

to orient the sketch plane in the normal direction.

We first create the computational fluid domain:

In Sketching: click on Draw > Rectangle, then draw a 2D rectangle box as shown in Figure 1. Make sure that the bottom line of the box overlaps with the x-axis.

Page 1 of 14

In Sketching: Click on Dimensions, then set up the dimension parameters as shown in Figure 1 (note: your dimension numbers may be different).

Figure 1 The 2D rectangle box of the computational domain with dimension parameters

In the Details for Dimension window, modify the values of the dimension parameters as shown in Table 1.

Table 1 The 2D rectangle dimension parameter values H1

H2

V3

8

18

3

Click on Extrude icon on the top menu bar above the model viewer window to bring up the Details View for the Extrude 3D operation (the view will consequently switch to the Modeling view).

Edit the Details of Extrude 1 page: (1) select Add Frozen for Operation; (2) icon select Reversed for Direction; (3) set FD1 Depth to 3. Click on to complete the Extrude operation.

We then create a plane parallel with the XY plane: to

Ensure the XY Plane is active, and then click on the New Plane icon insert a plane that parallel with the XY Plane.

In Details of Plane4, set Transform 1 to Offset Z and FD1 to 0 (as shown in Figure 2).

Page 2 of 14

Figure 2 Insetting and editing the new plane (Plane4)

Click on

icon on the top menu bar to complete the operation.

We finally create the car body (refer to Figure 3 below)

Highlight the newly created Plane 4 by clicking on Plane 4 under the Tree Outline

Click on

In the Details of Point 1,

icon on the Top bar menu

(1) set Definition to be “From Coordinates File”; (2) At Coordinate File, click on

to browse;

(3) Browse necessary to locate the file “car-profile”, and Open it; (4) Click on

icon to complete the operation.

(5) Zoom in to get a detailed view of the points, as shown in Figure 3.

Figure 3 Import the car profile data points into DesignModerler Page 3 of 14

Click on Sketching tab > Draw > Spline

In the Graphics window, click on the top 8 points sequentially from left to the right, then RMB click to bring up a menu, and select Open End to complete the spline as the top profile of the car.

Repeat the same procedure to create a spline though the 7 points along the bottom for the base profile of the car.

Click on Line, and then in the Graphics window link the remaining points along the front and back sections of the car with straight lines (see Figure 4).

Figure 4 Sketch the car profile using Spline and Line operations

Click on the icon on the top menu bar to bring up the Details View for the Extrude 3D operation.

In the Details of Exdrude2, as shown in Figure 5, (1) select Slice Material for Operation; (2) select Reverse for Direction, and

(3) type 0.17 for FD 1 Depth (>0).

Click on icon on the top menu bar to complete the Extrude operation.

Under the Tree Outline,

Figure 5

Details of Extrude2

(1) Expend the 2 Parts 2 Bodies ; (2) Rename the two Solid objects to Fluid Domain and Car, respectively; (3) RMB click on the Car, and then select Suppress Body, as shown in Figure 6

The 3D model geometry creation is now completed as shown in Figure 7.

Figure 6 Suppress the car body

Page 4 of 14

Figure 7 The completed 3D car model geometry, with the names of boundary faces that enclose the fluid domain.

Create Named Selections for the boundaries of the flow domain: (1) Ensure that the Face Selection Filter is active, click on the inlet face to highlight the face; (2) RMB > Create Named Selection ; (3) In the Details View panel, next to the Geometry cell, click on Apply; (4) Click on the Named Selection cell, and change the name to inlet; (5) Then click on the creation.

icon on the top menu bar to complete the

Repeat the same procedure to create the Named Selections for outlet, symmplane, sky, sideplane, and ground.

Ctrl_click to select all the faces around the car body (total 12 faces), and then RMB > Create Named Selection and re-name the selection icon to complete the creation. as carbody, then click on

Save the Project and exit DesignModeler to return to the Workbench and ready for Meshing.

Page 5 of 14

Stage 2

Create the Mesh

In the Workbench, RMB on the Geometry cell.

(1) In the pop up menu select Properties.

(2) Ensure that the Named Selection cell is ticked ON. (3) Delete the value: NS for the Named Selection Key. This is to ensure that the Named Selections are transferred to the next operation.

Double click on Mesh cell of the Fluid Flow (CFX) system block to bring up ANSYS Mesh.

Click on Mesh under the Outline view to bring up the Details of Mesh panel, set the Physics Preference to CFD, Solver Reference: CFX.

Expand Sizing and set the Relevance Centre to Fine.

Insert an Inflation to create the boundary layers on the faces of the car body

RMB on Mesh under the Outline view, select Insert > Inflation (see Figure 8(a)).

Note that a panel named Details of “Inflation” is now appeared on the left bottom corner of the screen.

Note that the cell next to the Geometry cell under Scope in the Details of “Inflation” is highlighted. This means that the Geometry requires definition.

Click on the highlighted cell next to Geometry to bring up Apply and Cancel buttons, and then move the cursor to the Graphics window to select the fluid domain by clicking on it, then click on the Apply button.

Under Definition, click on the highlighted cell next to Boundary to bring up Apply and Cancel buttons, and then move the cursor to the Graphics window to select the12 faces of the car body by Ctrl_clicking on them, then click on the Apply button.

Set Inflation Options to First Layer Thickness.

Type 0.0016 m for First Layer Height Note: Refer to Appendix 1 for the calculation of the 1st layer height.

Type 10 for Maximum Layers.

Keep the default value1.15 for Growth Rate.

Insert a sizing function

Ensure that Face selection on the top menu bar is active.

RMB on Mesh under the Outline view, select Insert > Sizing (see Figure 8(b))

Note that a panel named Details of “Face Sizing” is now appeared on the left bottom corner of the screen

In the Details of “Face Sizing” panel, select Named Selection for Scoping Method.

Select carbody for Named Selection.

Enter 0.01 for Element Size.

Page 6 of 14

(a)

(b)

Figure 8 Mesh settings for (a) Inflation; (b) Sizing Preview the inflated boundary layer mesh

RMB on Mesh under Outline view, select Preview > Inflation.

Wait for a couple of seconds for the boundary layer mesh to appear (see Figure 9).

Figure 9 Preview inflation Generate the body mesh

In Outline view, RMB click on Mesh, select Generate Mesh.

Wait for the body mesh to be completed (see Figure 10).

(a)

(b)

Figure 10 3D body mesh (a) over the computational domain; and (b) around the car body.

Check the mesh quality Page 7 of 14

Click on Mesh under the Outline view to bring up the Details of Mesh panel.

In the Details of Mesh panel, expand Statistics, and then set the Mesh Merits to Skewness.

Check the skewness values for the Max., Average and Standard Deviation (see Figure 11).

Note: Refer to Appendix 2 for the definition for skewness.

Figure 11 3D mesh skewness statistics

File > Save Project

File > Close Meshing to exit ANSYS Mesh and return to the Workbench Project page.

RMB click on the Mesh cell > Update. Wait until a green tick appears for the Mesh cell.

Stage 3 Specify boundary types, materials, fluid flow models and initial values Note: Though the flow phenomenon in this assignment is, by nature, transient, we are seeking steady state solutions. This is because the main objective of the assignment is to obtain the drag coefficient, for which the steady-state turbulence modelling in most CFD software can provide good solutions with much lower computational cost.

In Workbench, double click on Setup cell to open CFX Pre Change the default domain name to a meaningful name. RMB on Default Domain in the Outline tree and select Rename, then change Default Domain to Airflow.

Under Flow Analysis 1, double click on Airflow to open the panel named Details of Airflow in Flow Analysis 1, which contains three tabs named Basic Settings, Fluid Models and Initialisation. Go through each of them as follows:

Page 8 of 14

-

-

In the Basic Settings, note the default material is Air at 25 C, which is the material we are using in this analysis. Leave other fields to the default settings.

-

Click on the Fluid Models tab, select None for Heat transfer; select KEpsilon for Turbulence. Leave other fields to the default settings. Accept all the settings by clicking on Apply.

Click on Initialisation tab, tick the Domain Initialisation to view the default settings. Accept these settings by clicking on Apply, then OK to exit the Details panel.

Under Materials, double click on Air at 25 C to open the panel named Details of Air at 25 C. Click on Material Properties tab, find the values of Density under Equation of State, and Dynamic Viscosity under Transport Properties, as shown below: Density: ρ = 1.185 [kg m^-3]; Dynamic Viscosity: μ = 1.831E-05 [kg m^-1 s^-1].

These values are used to calculate: (1) Inlet velocity; (2) lift and drag coefficients.

Click on Close to exit the panel.

Specify the boundary conditions of the flow domain (1) RMB on Airflow > Insert > Boundary, type: inlet in the pup up window, then OK This brings up Details of inlet in Airflow in Flow Analysis 1 (a) Select Inlet for Boundary Type; select inlet for Location. (b) Click on Boundary Details tab, see Figure 12 below, enter 27.78 for Normal Speed (c) Under Turbulence, select Medium (Inrensity = 5%) for Option. (d) Click Apply to finalise the setting, then OK to exit the Details of Inlet panel.

Figure 12

The inlet condition settings

(2) RMB on Airflow > Insert > Boundary, type: ground in the pup up window, then OK Page 9 of 14

This brings up Details of ground in Airflow in Flow Analysis 1. Refer to Figure 13: (a) Select Wall for Boundary Type; select ground for Location. (b) Click on Boundary Details tab. Tick Wall Velocity to open the cell, and then enter 27.78 for Wall U, 0 for Wall V and Wall W. Leave rest settings as the default settings. (c) Click Apply to finalise the settings, then OK to exit the Details of Ground panel.

Figure 13 The ground condition settings (3) RMB on Airflow > Insert > Boundary, type: farfield in the pup up window, then OK This brings up Details of farfield in Airflow in Flow Analysis 1 (a) Select Opening for Boundary Type; select sky and backplane for Location, refer to Figure 14(a). (b) Click on Boundary Details tab. In Mass and Momentum cell, select Entrainment for Option, and enter 0 for Relative pressure.

In Turbulence cell select Zero Gradient for Option. Keep the default Subsonic for Option in the Flow Regime cell (see Figure 14(b)).

(4) Click Apply to finalise the settings, then OK to exit the Details panel.

(5) Repeat the same procedure to define the remaining boundaries of the fluid domain named Airflow. Refer to the following Table 1 for the corresponding settings.

Page 10 of 14

(a)

(b)

Figure 14 The sky and backplane condition settings

Table 1: Some boundary settings of the Airflow domain

Name

Boundary Type

Location

Boundary Details

outlet

Outlet

outlet

Average Static Pressure = 0 Pa

symm

Symmetry

symmplane

carbody

Wall

carbody

No Slip Wall

Under Flow Analysis 1, double click on Solver Control to open the panel named: Details of Solver Control in Flow Analysis 1.

Set Max. Iterations to 50 and accept the default Basic Settings for the rest by clicking on Apply (as shown in Figure 15), then OK to exit the Details of Solver Control panel.

Under Flow Analysis 1, double click on Output Control to open the panel named: Details of Output Control in Flow Analysis 1.

Click on the Monitor tab to open the Monitor Options, and then refer to Figure 16 to define the expression that calculates the drag coefficient: (1) Expand Monitor Points and Expressions; (2) Add a monitor point: DragCoef; (3) Select Expression for Option; (4) Type the following drag coefficient expression for the Expression Value: force_x()@carbody*2/1.185[kg m^-3]/27.78[m s^-1]^2/0.252[m]/0.17[m] (5) Repeat the above procedure (Step 1to Step 4) to define the expression for the lift coefficient: LiftCoef: force_y()@carbody*2/1.185[kg m^-3]/27.78[m s^-1]^2/0.252[m]/0.17[m]

Click on Apply, to save the settings, and then

Click on OK to exit the Details of Output panel.

Page 11 of 14

Cd = 1 2

Figure 15 Solver Control settings

Fd

ρU ∞2 ⋅ A

Figure 16 output Control settings

on the top menu bar to open the Click on the global Initialization icon panel named Details of Global Initialization in Flow Analysis 1. Refer to Figure 17 to do the following: (a) Tick the Velocity Scale to open the cell, and then (b) Enter 27.78 for Value. Leave other settings as the default. (c) Click on Apply to save the settings. (d) Click on OK to exit the panel.

Figure 17 Global Initialization settings

Exit CFX Pre and return to the Workbench Project page.

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Stage 4 Solving for the k-epsilon turbulence model solution

Save the Workbench Project

In Workbench double-click Solution cell to launch the CFX Solver Manager

The CFX Solver Manager will start with the simulation ready to run.

Click on Start Run to begin the solution process.

The solution process will stop either the residual target is met or the Max. Iteration number is reached, whichever comes first. Steadily decreased residual curves indicate a convergent solution (Note: it may take approximately 2 minutes to obtain the k-epsilon model solution)

Exit CFX Solver Manager to return to Workbench Project page.

File > Save.

Click File > Exit … to quit ANSYS Workbench; OR

Click on the Results cell to commence Stage 5 CFX Postprocessing (Referring to part 2 of the lab guide).

Appendix 1 Estimation for y1 value In ANSYS CFX, hybrid methods are provided for turbulence models (automatic nearwall treatment) involving boundary layer simulation.

To save time in meshing, it is useful to estimate the distance between the wall and the first grid node, y1. ANSYS CFX recommends using the following formula to estimate the y1 value based on the desired y+ value:

y1 = L ⋅ y + 74 Re −L13 / 14 where

L = 1 is the length scale (i.e. length of the car); and

Re L - is the Reynolds number calculated based on the length scale.

If the k-epsilon turbulence model is used, ANSYS CFX requires: (y+)max < 300 In this guide a conservative value: y+ =120 was used as the desired y+ value to estimate the 1st inflation layer height y1 : y1 = 0.0016 for the inflation settings (refer to Figure 8a on Page 7).

For the k-omega turbulence model, ANSYS CFX requires: (y+)max < 2 Based on the same principle, if we choose a conservative value: y+ = 0.8 as the desired y+ value, the 1st inflation layer height y1 would then be estimated as: y1 = 1.0 x 10-5 Page 13 of 14

Note: Refer to Part 2 of the Lab Guide for the mesh and solver control settings for the k-omega turbulence model.

Appendix 2 Skewness Skewness is defined (in general) as

− ϕ e ϕ e − ϕ min ⎤ ⎡ϕ , Skewness = max ⎢ max ⎥ ϕe ⎣ 180 − ϕ e ⎦ where φmax = the largest angle in the face or cell. φmin = the smallest angle in the face or cell. φe = angle for an equiangular face/cell (e.g., 60 for a triangle, 90 for a square).

According to the definition, the value of skewness varies from 0 to 1, with skewness = 0 as the ideal element; and skewness = 1 the worst element (no area/volume exists).

For a pyramid, the cell skewness is the maximum skewness computed for any face. An ideal pyramid (skewness = 0) is the one in which the 4 triangular faces are equilateral (and equiangular) and the quadrilateral base face is a square.

Page 14 of 14

Lab Guide for Assignment B (Part 2) Learning Objectives: 1. Learn to create 3D model geometry in ANSYS DesignModeler (Part 1) 2. Learn to generate an inflated boundary layer mesh around a 3D wall in ANSYS Meshing (Part 1) 3. Learn to generate 3D grids with acceptable mesh qualities using ANSYS Meshing (Part 1) 4. Learn to perform the CFD analysis for external turbulent flows using ANSYS CFX, including assigning model property parameters, selecting the appropriate CFD solver, setting up the boundary and initial conditions et cetera. (Part 1) 5. Learn to write expressions in ANSYS CFX Post to calculate the required CFD data, for example, the lift and drag coefficients (Part 2) 6. Learn to utilise the various visualisation tools in ANSYS CFX Post to present the 3D turbulent flow characteristics around the car body (Part 2)

Procedure Preparation: •

Transfer all your ANSYS files (the Workbench file, e.g. Assign2.wbpj and the associated folder that contains the model data base, e.g. Assign2_files) produced in Part 1 into the secured folder named by your student number on the hard disk of the lab computer, e.g.: Computer > Windows(C:) > scratch > Folder named by your student number (e.g. s3012345)

•

Start ANSYS 13.0 > Workbench -

File > Open

-

Change the path to the secured folder of yours specified above

-

Click to open the Workbench project file, e.g. Assign2.wbpj.

Note: this Workbench project file had been created in the previous lab session, during which we completed Stages 1 - 4 of the project (refer to the Lab Guide Part 1)

Stage 5: Post-processing the results in ANSYS CFD-Post

In the Workbench make sure that the cells 1 – 5 have a green tick.

In Workbench Project Schematic page, double-click Results cell to launch CFX Post.

Create the y+ contour plot around the car body (refer to Figure 1): -

Click on the Contour icon

from the top menu bar.

-

In the pup up window, enter a name, e.g. YplusCarbody, then OK.

-

In the Details of YplusCarbody, set the Locations to carbody

Page 1 of 13

-

Set the Variable to Yplus (click on icon to open the Variable Selector and scroll down to find the Yplus, then click OK).

-

Set the Range to Local.

-

Click Apply to generate the y+ contour plot.

-

Zoom in to observe y+ variations around the car body surface (see Figure 2).

Note the maximum y+ value. Does it satisfy the basic requirement for the kepsilon turbulence model?

-

To include the graph into the report, RMB on Report branch under the Outline tree, and then select Insert > Figure, provide a meaningful name for the Figure, e.g. YplusOnCarbody, then OK.

Figure 1 Create the y+ contour graph on the car body

Page 2 of 13

Figure 2

The y+ contour plot on the car body surfaces

Create a pressure contour plot on the symmetry plane: -

In the Outline tree under User Locations and Plots, un-tick YplusCarbody contour (this is to clear the Graphics screen and prepare for the next figure you are going to plot).

-

Click on the Contour icon

-

In the pup up window, enter a name, e.g. PressOnSymm, then OK.

-

In the Details of PressOnSymm, set the Locations to symm

-

Set the Variable to Pressure,

-

Click Apply to generate the pressure contour plot

-

Re-orientate the model view and zoom in to observe pressure variations around the car body on the symmetry plane (see Figure 3).

-

Include the plot in Report (refer to the last step of the previous procedure for plotting the Y+ figure).

from the top menu bar.

Figure 3 Pressure contour on the symmetry plane

Page 3 of 13

Create a velocity vector plot on the symmetry plane: -

Clear the Graphic window by ticking off all the previous plots.

-

Click on the Vector icon

-

In the pup up window, enter a name, e.g. VectorVelSymm, then OK.

-

In Details of VectorVelSymm, set the Locations to symm

-

Set the Variable to Velocity.

Click on Symbol tab and set the Symbol Size to be 0.01.

-

Click Apply to generate the velocity vector plot on the symmetry plane.

-

Zoom in to observe the flow phenomenon at back of the car (see Figure 4).

-

Include the figure in Report, if you’d like.

from the top menu bar.

Figure 4 Velocity vectors in the wake of the car

Create a 3D streamline plot: -

Clear the Graphic window by ticking off all the previous plots.

-

Click on the Streamline icon

-

In the pup up window, click OK to keep the default name: Streamline 1.

-

In Details of Streamline 1, set Start From to Airflow and inlet

-

Set Reduction to Max Number of Points.

-

Enter 100 for Max Points

-

Set Forward and Backward to Direction

-

Click Apply to generate the Streamline plot.

-

Zoom in to the car to observe the 3D flow phenomenon around the car body surfaces (see Figure 5)

-

Include the figure in Report, if you’d like.

from the top menu bar.

Page 4 of 13

Figure 5 The 3D Streamline plot

Create a turbulence kinetic energy contour plot on the symmetry plane: -

Clear the Graphic window by ticking off all the previous plots.

-

Click on the Contour icon

-

In the pup up window, enter a name, e.g. TKEonSymm, then OK.

-

In the Details of TKEonSymm, set the Locations to symm

-

Set the Variable to Turbulence Kinetic Energy,

-

Set the Range to Local

-

Increase the # of Contours to 30.

-

Click Apply to generate the TKE contour plot

-

Re-orientate the model view and zoom in to observe TKE variations particularly in the wake (see Figure 6).

from the top menu bar.

Figure 6 Turbulence kinetic energy contour on the symmetry plane

Page 5 of 13

Create turbulence kinetic energy contour plot on a series planes in the wake of the car and perpendicular to the car’s travelling direction (3D visualisation on turbulence kinetic energy): -

Clear the Graphic window by ticking off all the previous plots.

-

Creating a series planes in the wake of the car and perpendicular to the car’s travelling direction: a. Click on the Location icon the list, then select Plane.

from the top menu bar to expand

b. In the pup up window, enter a name, e.g. WakePlane 1, then OK (see Figure 7(a)). c. In the Details of WakePlane 1, set the Method to YZ Plane (see Figure 7(b)). d. Set the X to 1.07 [m], then click OK to create the plane. e. Repeat the procedure to create another three planes at various distances away from the car of your own choices (e.g. set the X to 1.02 [m], 1.12 [m], and 1.17 [m], respectively).

-

Click on the Contour icon

-

In the pup up window, enter a name, e.g. TKEonWakePlane, then OK.

-

In the Details of TKEonWakePlane, set the Locations to WakePlane 1, WakePlane 2, WakePlane 3, WakePlane 4,carbody.

-

Set the Variable to Turbulence Kinetic Energy, and the Range to User Specified, then entre a fixed value for the maximum e.g. 25 (see Figure 8)

-

Click Apply to generate the TKE contour plot on these planes and carbody surfaces.

(a)

from the top menu bar.

(b)

Figure 7 Create a YZ plane in the wake of the car.

Page 6 of 13

Figure 8 Turbulence kinetic energy on the car body and in the wake

Create an expression that calculates the drag force on the car body: -

Click on the Expressions tab to open the Expression Tree (refer to Figure 9a).

-

RMB on Expressions, select New. In the pup up window, enter a name, e.g.

DragForce, click on OK to open the panel named Details of DragForce

-

In the Details of DragForce panel (refer to Figure 9b): (1) RMB > Functions > CFD Post > force_x (2) RMB >Locations > carbody (3) You will see that the following expression has been entered for the Definition of DragForce (see Figure 9c) force_x()@carbody

-

Click Apply to evaluate the expression, and note the answer is given for the Value of the drag force (see Figure 9c).

(a)

Page 7 of 13

(b)

(c)

Figure 9 Create an expression: (a) inset a new expression, (b) define the expression (c) calculate the value of the expression

Repeat the same procedure to insert and define the following expressions: DynamicPressure, LiftForce, FrontalArea, DragCoef and LiftCoef, as shown in Figure 10 below:

Figure 10

List of the user defined expressions (within the green rectangle)

Note the values for drag and lift coefficients.

File > Save Project.

File > Close CFD Post to return to ANSYS Workbench.

K-Omega Turbulence Model Settings

In Workbench, duplicate the system block for the single car. Modify the name of the new system block to indicate the difference in the turbulence model used.

Double click on the Mesh cell of the 2nd system block to launch CFX Mesh.

Modify the inflation settings as shown in Figure 11.

Click on Update button to re-generate the mesh.

Double click on the Setup cell of the 2nd system block to launch the CFX Pre. Page 8 of 13

Figure 11 Inflation settings for k-Omega turbulence model

When CFX Pre opens, double_click on AirFlow to open the Details of AirFlow in Flow Analysis 1 panel

Click on the Fluid Models tab, and then select k-Omega for Turbulence Options. (see Figure 12a)

Click on Apply and then OK to exit the Fluid models.

Click on Solver Control to open the Details of Solver Control in Flow Analysis 1 panel

Select High Resolution for Turbulence Numerics Option (see Figure 12b), and accept all other settings.

Click on Apply button, and then OK to exit the panel.

Return to Workbench and run the Solution cell.

(a)

(b)

Figure 12 K-Omega turbulence model settings: (a) settings for Fluid Models, (b) settings for Solver Control.

In CFX Post, observe the phenomenon and calculate Cd and Cl. Page 9 of 13

Save the project and return to ANSYS Workbench.

Create the car-spoiler model

In Workbench, duplicate the system block for the single car k-epsilon model. Modify the name to a meaningful one, e.g. car-spoiler.

Double click on the Geometry cell of the copied system block to launch DesignModeler.

We now create the spoiler in DesignModeler: - Under Tree Outline activate Plane 4 by clicking on it - Click on

icon on the Top bar menu

- In the Details of Point 2 (refer to Figure 13) (1) set Definition to be “From Coordinates File”; (2) At Coordinate File, click on

to browse;

(3) Browse necessary to locate the file “spoiler-profile”, and Open it; (4) Click on

icon to complete the operation.

- Zoom in to have a detailed view of the points (see Figure 13)

Figure 13 Read in the spoiler profile data points You may like to add some construction points along with the spoiler profile points where you see they fit: - Sketching > Draw > Construction Points Click on the graphic screen to create some construction points (see Figure 14)

Page 10 of 13

Figure 14 Draw some construction points - Sketching > Draw > Spline (1) Link the top 4 points; and then (2) RMB > Open End to draw the up-spline.

Repeat the same process to link the down 4 points to draw the down-spline, and the remaining points for the head-spline (see Figure 15).

Figure 15 Draw three splines that outline the spoiler profile - Click on the Extrude 3D operation

icon on the top menu bar to bring up the Details View for the

- In the Details of Exdrude3 (1) select Slice Material for Operation; (2) select Reverse for Direction, and then (3) type 0.17 for FD 1 Depth (>0) - Click on Figure 16)

icon on the top menu bar to complete the Extrude operation (see

Page 11 of 13

Figure 16 The car-spoiler model geometry

Suppress the solid of the spoiler.

Select all the surfaces of the spoiler, and then create a Named Selection and give a name, e.g. spoiler

File > Save Project.

File > Close DesignModeler to retune to Workbench.

In Workbench, update the Geometry cell in the car-spoiler system block.

Double click on the Mesh cell of the car-spoiler system block to launch the Mesh.

Insert an Inflation for the spoiler: Refer to the instructions in the Lab Guide Part 1 for inserting Inflation for the carbody, but use the following recommended settings, instead: (a) 0.00016 m for First Layer Height (b) 10 for Maximum Layers. (c) 1.2 for Growth Rate

Insert a Sizing for the spoiler Refer to the instructions in the Lab Guide Part 1 for inserting Sizing for the carbody, but use the following recommended setting, instead: 0.001 for Element Size

RMB on Mesh under Outline view, select Preview > Inflation

In Outline view, RMB click on Mesh, select Generate Mesh.

Save Project. Quit the Mesh and return to Workbench

Double click on the Set Up cell to launch CFX Pre

In ANSYS Pre, add the spoiler surfaces as a wall boundary

Solve for the solution

In CFX Post, observe the phenomenon and calculate Cd and Cl.

Save the project and exit ANSYS Workbench.

Page 12 of 13

Appendix Note for calculation of Cd and Cl for the car-spoiler model For this car-spoiler model, since the spoiler is positioned within the height and the length of the car, the frontal area of the car-spoiler model is the same as that for the single car model.

This means that the same frontal area of the single car model can be used for the car-spoiler model to calculate Cd and Cl. The only changes are the drag force and lift force, which are: Drag force = drag force of the car + drag force of the spoiler Lift force = lift force of the car + lift force of the spoiler.

Cd =

Cl =

F d _ car + F d _ spoiler 1 2

ρU ∞2 ⋅ A

Fl _ car + Fl _ spoiler 1 2

ρU ∞2 ⋅ A

Page 13 of 13

Assignment B Sample Graphs 1

Mesh issues

(a)

(b) Figure 1

(a) Boundary layer mesh around the car body (inflation); (b) Mesh on the computational domain

Page 1 of 10

Figure 2a Y+ contour around car body (k-epsilon)

Figure 2b Y+ contour around car body (k-omega)

Page 2 of 10

2

Single car model (comparison of k-epsilon and k-Omega model solutions)

Figure 3a Pressure contour (k-epsilon)

Figure 3b Pressure contour (k-Omega)

Page 3 of 10

Figure 4a Streamlines (k-epsilon)

Figure 4b Streamlines (k-Omega)

Page 4 of 10

Figure 5a Velocity vectors (k-epsilon)

Figure 5b Velocity vectors (k-Omega)

Page 5 of 10

Figure 6a Turbulence kinetic energy (k-epsilon)

Figure 6b Turbulence kinetic energy (k-Omega)

Page 6 of 10

Figure 7a Turbulence kinetic energy around car body and at wake (k-epsilon)

Figure 7b Turbulence kinetic energy around car body and at wake (k-Omega)

Page 7 of 10

3

Car-spoiler model (k-epsilon model solution only)

Figure 12 Pressure contour on the symmetry plane

Figure 13 Y+ contour around the car body and the spoiler

Page 8 of 10

Figure 14 Velocity vector plot

Figure 15 Velocity vectors at rear section of the car

Page 9 of 10

Figure 17 Turbulence Kinetic energy on the symmetry plane

Figure 18 Turbulence Kinetic energy around the car, spoiler and on 4 sliced planes in the wake region

Page 10 of 10

Lab Guide for Assignment C Learning Objectives: 1. Learn to create the model geometry using ANSYS DesignModeler; 2. Learn to generate a grid/mesh with an acceptable quality using ANSYS Meshing.

3. Learn to perform a preliminary CFD analysis using ANSYS CFX, including assigning model property parameters, selecting the appropriate CFD solver, setting up the boundary and initial conditions, etc.; 4. Learn to utilise the basic post-processing techniques in ANSYS CFX Post to obtain the basic CFD data, such as, pressure distribution, velocity variation in the entire computational domain, as well as at a specified region of the domain.

Procedure Start ANSYS Workbench: (1) From Windows Start menu, select ANSYS 13.0 > Workbench (see Figure 1(a)) (2) After Workbench opens, specify the units system as “Metric (kg, m, s, ºC, A, N, V)”, and choose to “Display Values in Project Units” (see Figure 1(b)).

Note: on the top left corner of the Workbench window, the default Workbench project name is: Unsaved Project.

(a)

(b) Figure 1: Launch ANSYS Workbench

(3) Save the project: - On the Workbench top menu bar, click on Save As icon (refer to Figure 2 below) - On the pop-up window, navigate to your secured folder on the hard disk of the computer by the following path: Computer > Windows(C:) > scratch > Folder named by your student number (e.g. s3012345)

- Name the Unsaved project, for example: R-Bend, and then click on Save.

Note: ANSYS automatically appends the .wbpj extension for the file (see Figure 2).

Figure 2: Save the Workbench project in your secured folder on the hard disk of the computer

(4) Expand the Analysis Systems toolbox, then drag and drop a Fluid Flow (CFX) object into the Project Schematic window as shown in Figure 3 below.

Note: the Workbench project name has now been changed to a meaningful name: R-Bend.

Figure 3: Create a Fluid Flow (CFX) system on the Workbench Project Schematic

Stage 1.

Create the model geometry

Double click on the Geometry cell to launch DesignModeler. Select m as the unit of measure for the session when prompted.

Under Tree Outline, click on XY Plane, then, click on New Sketch icon menu. This is to insert Sketch1 in XY Plane.

Click on Look At icon Figure 4).

Click on the Sketching tab to switch to sketch mode.

Sketching: Settings > Grid

Toggle on Show in 2D and Snap to Grid

Click on Major Grid Spacing and set it to 1.0m

Click on Minor-Steps per Major and set it to 2

on the top

to orient the sketch plane in the normal direction (refer to

Figure 4: Setup the Grids for creating the geometry

Zoom in on the model view, so that the area displayed is centred about the origin with several major steps in the x- and y- directions.

Sketching: Draw > Polyline

-

-

Draw the outline of the horizontal section of the model by sequentially clicking on the 4 corner points (in order points 1, 2, 3, and 4, see Figure 5). Point 1 has the coordinates (0, 1). Do not close the section, after clicking on point 4, RMB and select Open End from the pop up menu.

-

Do the same to define the outline of the vertical section of the model using Points 5, 6, 7, and 8, starting from Point 5.

Sketching: Draw > Arc by Centre - Draw the inner arc of the R-bend: on the Graphics window click on the origin of the coordinate system, (0,0) to define the centre point of the inner arc, and then click on the two end points of the arc, Points 1 and 5, to define the arc.

- Do the same to define the outer arc of the R-bend. Use the point (0, -1) as the centre point of the outer arc, and Points 4 and 8 to define the arc.

The dimensions of the channel cross section are defined now by drawing within the grid with the given coordinates of the model geometry (refer to Figure 5).

However, we will still set up the dimensions of the model, as shown in Figure 5. This is to parameterize the model dimensions for possible geometry modifications when required.

Sketching: Dimensions > Horizontal (1) Click on Point 3 and Point 4 (refer to Figure 5). A pencil will appear on the screen. Move the pencil up by dragging the mouse to an appropriate location, and then click to define the horizontal channel length H7 (note: the number on your drawing may differ due to the sequence of setting up the dimensions).

(2) Repeat the similar procedure to define the outlet dimension H9.

Sketching: Dimensions > Vertical (3) Click on Point 5 and Point 6 (refer to Figure 5). A pencil will appear on the screen. Move the pencil to the left by dragging the mouse to an appropriate location, and then click to define the vertical channel length V10 (note: the number on your drawing may differ due to the sequence of setting up the dimensions) (4) Repeat this procedure to define the inlet dimension V8.

Sketching: Dimension > Radius, then click on the inner arc and drag the mouse away from the arc to an appropriate location, release the mouse to define the bend radius R12 (note: the number on your drawing may differ due to the sequence of setting up the dimensions).

NOTE: the information for these dimensions can be found on the Details of Sketch1 window at the left-bottom of the screen (see Figure 5).

Figure 5: Create the model geometry and define the dimensions.

Click on the Extrude icon on the top menu bar to bring up the Details View panel, at the left-bottom of the screen, for the Extrude operation (the view will consequently switch to the Modeling view).

In the Details View, select Reverse for Direction and type 0.01 m for FD 1 Depth (>0), as shown in Figure 6, then click on in the top menu bar.

Figure 6: Details View for the Extrude operation settings

We have now created the R-bend 2D cross-section geometry.

Save the Project and exit DesignModeler to return to Workbench. We are now ready to create the Mesh.

Stage 2

Create the mesh and set up flow boundaries

In ANSYS Workbench, double click on Mesh cell to bring up ANSYS Mesh. (Refer to Figure 7) (1) Click on Mesh under the Outline view to bring up the Details of Mesh panel at the left-bottom of the screen. In this panel: (2) Set Physics Preference to CFD (3) Set Solver Reference: CFX. (4) Expand Sizing, and then (a) set Relevance centre to Fine; (b) set Min Size to 0.001 and Max Face Size to 0.2.

(5) In the Outline tree view, RMB click on Mesh > Generate Mesh to create the mesh similar to the mesh shown in Figure 7.

(6) Note: To create a finer mesh, you can simply reduce the value for the Max Face size, then, click on Update on the top menu bar.

Figure 7: Discretise the model by creating a mesh (quadrilateral elements).

Create Named Selections for the boundaries of the fluid domain (refer to Figure 8): -

Ensure that the Face Selection Filter is active Select the inlet face by clicking on it RMB > Create Named Selection In the pup window, enter a name, e.g. inlet, for the selection, then OK.

Figure 8: Create a Named Selection for the inlet face boundary

- Repeat this procedure to create the Named Selections for outlet, the two symmetry planes (name them the way you like, e.g. symm1 and symm2), and the inside and outside walls of the channel (also name them the way you like, e.g. wall1 wall2).

Click File > Save Project … to save the project file.

Click File > Close Meshing to exit ANSYS Mesh and return to the Workbench Project Schematic page.

In the Workbench Project Schematic page, you will see a yellow lightning bolt against the Mesh cell. This means the cell needs updating.

- RMB on the Mesh cell -> select Update. Wail until the yellow lightning bolt is replaced by a green tick.

Stage 3

Specify Boundary Types, Materials, Fluid flow models and Initial values

In Workbench, double click on Setup cell to open CFX Pre. Change the default domain name to a meaningful name: (1) RMB on Default Domain in the Outline tree (2) Select Rename, and then change the name to Rbend.

Specify a new material (refer to Figure 9) (1)

RMB on Materials > Insert > Material, in the pup up window click OK to accept the default material name: Material 1 (you can change the name the way you like).

(2)

In Details of Material 1, click on Material Properties tab. Under Equation of State, enter the value for Density (refer to the assignment paper) .

(3)

Expand Transport Properties and tick Dynamic Viscosity, then enter the value for the Dynamic Viscosity (refer to the assignment paper).

(4)

Click on Apply to save the changes, then, OK to exit Details of Material 1.

(a)

(b)

Figure 9 Specify a new fluid material properties: (a) Insert Material 1; (b) Enter the new material properties.

Under Flow Analysis 1, double click on the fluid domain re-named Rbend to open Details of Rbend in Flow Analysis 1. Flow Analysis 1 contains three tabs named Basic Settings, Fluid Models and Initialisation. Go through each of them as follows:

-

In the Basic Settings, select Material 1 as the fluid material; leave other fields to the default settings.

-

Click on the Fluid Models tab, select None for Heat transfer; select (none)Laminar for Turbulence Click on the Initialisation tab, tick the Domain Initialisation to view the default settings. Accept all the settings by clicking on Apply, then OK to exit the page.

Specify boundary conditions for the flow domain (1) RMB on Rbend > Insert > Boundary, type: inlet in the pup up window, then OK - This brings up Details of inlet in Rbend in Flow Analysis 1: (a) Select Inlet for Boundary Type; select inlet for Location. (b) Click on Boundary Details tab, enter the inlet velocity value for Normal Speed (refer to the assignment paper for the value), and keep other fields with the default settings. (c) Click Apply to finalise the setting, then OK to exit the Details tab.

(2) Repeat this procedure to define the other boundaries of the fluid domain named Rbend. Refer to Table 1 for the corresponding settings.

Table 1 Boundary settings of the flow domain named Rbend Name

Boundary Type

Location

Boundary Details

outlet

Outlet

outlet

symm

Symmetry

symm1, symm2

wall

Wall

wall1, wall2

For Mass and Momentum Option, choose Average Static Pressure ; For Relative pressure = 0 [Pa].

No Slip Wall

Under Flow Analysis 1, double click on Solver Control to open the tab named: Details of Solver Control in Flow Analysis 1 (refer to Figure 10).

Under the Basic Settings tab, as shown in Figure 10: (1) Increase: Max. Iterations to 300; (2) Reduce Residual Target to 1e-6 (or 0.000001), (3) Click Apply to save the change, then, OK to exit the Details of Solver Control in Flow Analysis 1 tab.

Exit CFX-Pre and return to the Workbench Project page.

Figure 10: Solver Control tab

Stage 4 Solving for the solution

Save the Workbench Project

In Workbench double-click on Solution cell to launch the CFX Solver Manager

The CFX Solver Manager will start with the simulation ready to run.

Click on Start Run to begin the solution process.

The solution process will stop either when the residual target is met or the Max. Iterations number is reached, whichever comes first. (A decreasing residual indicates a converging solution.)

Exit CFX Solver Manager to return to Workbench Project page.

Stage 5

Post-processing in CFD-Post

Update the Workbench Project, and wait for every cell to get a green tick.

In Workbench Project Schematic page, double-click Result cell to launch the CFDPost.

Create a pressure contour plot on the symmetry plane (Refer to Figure 11):

-

Click on the Contour icon

from the top menu bar.

-

In the pup up window, enter a name, e.g. PressOnSymm, then OK.

-

In the Details of PressOnSymm, set the Locations to symm

-

Set the Variable to Pressure,

-

Set # of Contours to 20.

-

Click Apply to generate a pressure contour plot similar to that shown in Figure 11.

Figure 11: Pressure contour plot Study the pressure plot and comment on whether the flow has reached to a fully developed stage before approaching the 900 bend and at the outlet.

Create a velocity vector plot on the symmetry plane: -

Click on the Vector icon

from the top menu bar.

-

In the pup up window, enter a name, e.g. VelOnSymm, then OK.

-

In Details of VelOnSymm, set (a) Domain to Rbend; (b) Locations to symm; (c) Sampling to Vertex (d) Click Apply to generate a velocity vector plot similar to that shown in Figure 12.

-

Zoom in on the flow reversal region to observe the velocity vectors (see Figure 13(a))

-

Zoom in on the outlet boundary to observe the velocity vectors (see Figure 13(b))

Figure 12: Creating flow velocity plot on the symmetry plane

(a)

(b)

Figure 13: Flow behaviour at (a) reversal region; (b) outlet boundary.

Create a streamline plot on the symmetry plane (see Figure 14): -

Click on the Streamline icon

-

In the pup up window, accept the default name, Streamline 1, then OK

-

In Details of Streamline 1, set

from the top menu bar.

(a) Type to Surface Streamline; (b) Surfaces to symm; (c) Start From to Vertices; (e) Reduction to Maximum Number of Points; (f) Max Points to 100.

-

Leave remaining settings as the default values. Click Apply to generate a streamline plot similar to that shown in Figure 14.

Figure 14: Streamline on the symmetry plane

Create a line that overlaps with the inside vertical wall (see Figure 15): -

Expand the Location icon from the top menu bar, and then select Line

-

In the pup up window, accept the default name, Line 1, then OK

-

In Details of Line 1, set (a) Domain to Rbend (b) Enter the coordinates of Point 1 and Point 2; (c) Tick Sample for Line Type (e) Enter 30 for Samples; (f) Click Apply to create Line 1 (the yellow line in the Graphics window that overlaps with the inside vertical wall) as shown in Figure 15.

Figure 15: Procedure for create a Line object

Create a chart plotting Pressure Gradient Y component along the newly created Line 1 (see Figure 16): -

Click on the Insert Chart icon

from the top menu bar.

-

In the pup up window, accept the default name, Chart 1, then OK

-

In Details of Chart 1, there are six tabs. we need to set, at least, the first four tabs to produce the chart: (1) Under the General tab: (1a) for Type select xy; (1b) for Title, enter the title name of your own choice, e.g. Pressure Gradient_Y along the inside wall vs. Y (see Figure 16(a)).

(2) Click on Data Series tab: (2a) for Name, select Series 1; (2b) tick Location, and then select Line 1(see Figure 16(b)).

(3) Click on X Axis tab: (3a) select Y for Variable; (3b) tick Conservative; (3c) tick Invert axis (see Figure 16(c)).

(4) Click on Y Axis tab: (4a) select Pressure Gradient Y for Variables (See icon next to the Variable to expand Figure 16(d). Note: click on the the variable list, and then choose the variable from the list); (4b) tick Conservative; (4c) click Apply to generate the Chart 1 as shown in Figure 16(e).

(5) Extract the numerical data of Chart 1 by clicking on Export (see item 4(d) in Figure 16(d)), and then save the data into a spreadsheet file.

(a)

(b)

(c)

(d)

(e) Figure 16: Procedure for creating a chart of pressure gradient along Line 1: (a) General tab settings; (b) Data Series tab settings; (c) X Axis tab settings; (d) Y Axis tab settings; and (e) Chart 1 on the Graphics screen.

Include the pressure figure into the report: -

Tick the PressureOnSymm plot under User Locations and Plots in the Outline tree;

-

RMB on Report, Insert > Figure, and then Click on OK in the pop up window

-

Repeat the same procedure to include the velocity vector plot and streamline plot into the report ,

Publish the report (see Figure 17): -

RMB on Report, select Refresh Preview

-

RMB on Report, select Publish

-

In the pop up Publish Report window, note the location of the report files and then OK.

Figure 17: Report preview and publish

Exit CFD-Post and return to the Workbench Project page.

Save the project

Exit ANSYS Workbench.