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International Journal of Automobile Engineering Research and Development (IJAUERD) ISSN 2277-4785 Vol. 3, Issue 2, June 2013, 73-80 © TJPRC Pvt. Ltd.

DRAG REDUCTION OF A GENERIC PASSENGER CAR USING VORTEX GENERATOR WITH DIFFERENT YAW ANGLES RAM BANSAL1 & R. B. SHARMA2 1

Research Scholar, Department of Automobile Engineering, RJIT BSF ACEDEMY Tekanpur, Madhya Pradesh, India 2

Hod of Mechanical Engineering Department, RJIT BSF ACEDEMY Tekanpur, Madhya Pradesh, India

ABSTRACT Large investments are aimed at minimizing power needed for propulsion i.e., new downsized engines with new aerodynamic devices for drag reduction. For passenger vehicles the aerodynamic drag force is the dominating resistance force at higher velocity. The vehicle body is often optimized for reducing the drag resistance. Recently, automobile industry started utilizing computer simulations like CFD (Computational Fluid Dynamics) to develop and design car body. Previously, it prepared actual car body model, and it evaluated car body aerodynamic character by running wind tunnel test. To prepare actual car body model takes time and cost. Therefore, it tended to decrease wind tunnel test and increase computer simulation. But, to re- enact actual car running had difficulty in CFD, and it had some problems like computational result accuracy. In this paper the variations of coefficient of lift and drag with and without vortex generators (VG) on the roof of a generic passenger car have been studied at varying yaw angles of VG. The yaw angles used are 10°, 15° and 20°. To measure the effect of altering the vehicle body, wind tunnel tests have been performed with 1:15 scaled model of the utility vehicle with velocities of 2.42, 3.7, 5.42 and 7.14m/s. This analysis shows that a great improvement of the aerodynamic drag force reduction can be achieved with vortex generator.

KEYWORDS: Passenger Car, External Aerodynamics, Boundary Layer, Wind Tunnel, Vortex Generator (VG) INTRODUCTION Study of the flow over car geometries can be performed by experiments by numerical simulation. Computational simulations have been extensively usedfor years in automobile aerodynamics in order to improve modern vehicle design [1]. In the past, experiments mainly involved the measurement of the aerodynamiccoefficients and flow visualization over vehicles. An aerodynamically well designed [2] car spends least power in overcoming drag exerted by air and hence exhibits higher performance- cruises faster and longer, that too on less fuel [3] (Fig.1). Numerical simulation is well integrated in the automotive industry and is now an engineering tool used in parallel withexperiments performed during the design process of road vehicles. Much fundamental research is performed on the Ahmed body,which includes most of the aerodynamic features found on a realcar In the present era, optimization of car aerodynamics, more precisely reduction of associated drag coefficient (CD), which is mainly influenced by exterior profile of car [5, 6], has been one of the major issues of automotive research centers. Air while moving past the car exerts two different forces on car surface [8]: 

Tangential forces induced by shear stresses due to viscosity and velocity gradients at boundary surface; and

Forces normal to the car surface resulting from pressure intensities varying along surface due to dynamic effects.


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An aerodynamically well-designed car spends least power in overcoming drag and hence yields higher performance-cruises faster and longer that too on less fuel. Modern streamlined designs of cars have C D values from 0.3 to 0.5 [7].

Figure 1: Fuel Economy with Reduction in Drag Coefficient (CD)

EXPERIMENTAL DETAILS Design of VG In order to find a feasible configuration, we first identify the important variables for vortex generator design. In order to reduce the degrees of freedom, most of the variables were fixed based on either analysis or recommendations of previous researchers [9]. A Single vane type delta (triangular) shaped was chosen. Due to their simplicity and widespread usage, the low drag device than any other type makes the vane type more suitable for attaching on the vehicle body. Delta shaped VG’s were most commonly used on aircraft wings [10]. In connection with the height, the thickness of the boundary layer is measured based on the assumption that the optimum height of the VG would be nearly equal to the boundary layer thickness. Figure 2 shows the velocity profile on the vehicle’s roof. From Figure 2, the boundary layer thickness at the roof end immediately in front of the separation point is found to be about 2 mm. Consequently, the optimum height for the VG is estimated to be up to approximately 2 mm. The thickness of VG was fixed at 0.5 mm uniform throughout so as to make a stiffened structure.

Figure 2: Velocity Profile on Roof


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Length was taken in proportion of the height of the VG. In this experimental work L/H ratio are taken as 2 with the Interval to height ratio of 6. Based on this ratio, a single row of VG was positioned on the roof with 11 numbers of VG as shown in Figure 3 and the arrangement of VG is shown in Figure 4.

Figure 3: Dimensions of VG

Figure 4: Arrangement of VG in Row This parameter describes the spacing between VG in a row. One row of VG was fixed at 5 mm from the roof end. This point was fixed, based on the boundary layer measurements and separation point of the stream line on the roof. The number of row was limited to one in order to minimize weight and potential manufacturing cost. The delta shaped VG’s is installed at varying yaw angle of 10°, 15° and 20° to the airflow direction. But the airflow direction was found to be different between sideways positions on the roof. The airflow is aligned directly with the backward direction at center of a vehicle, but it increasingly deviates toward the center as the measurement point shifts away from the central position. Scale Model and Experimental Setup The test model uses the generic passenger car with a scale ratio of 1:15. The length, breadth and height of the scaled model are was 0.432 m, 0.98m and 0.086m respectively. Thickness of the sheet metal used was 0.5mm. The Vortex generators were cut into pieces from the sheet metal and they were fixed on to a base plate by gas welding process. The base plate with VG was fastened to the roof of scaled model by means of bolt and nut. To measure the static pressure on the body, 0.2 mm diameter holes were drilled on the center line of the vehicle body starting from the front end along the roof to the rear end of the vehicle. 15 pressure tapping’s are used. Out of which five of them are on the roof, three on the rear end and remaining seven are on the front end of the vehicle. Pressure tubes are fixed from inside of the holes. Pressure tappings are connected to micro manometer using pressure tubes.


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Figure 5: Scale Model An open circuit wind tunnel with a test section of 0.09 m2 was used. The schematic of the wind tunnel is shown in Figure 6. The total length of the wind tunnel was 6m and the test section length was 1 m. A 25 HP electric motor was used for suction. The pressure data were not corrected for horizontal buoyancy as the static pressure gradient in the wind tunnel was deemed negligible. The wind tunnel tests were conducted at positive and negative yaw angles between ±15°. The frontal area of the scale model of the vehicle is 0.0108 m2. The blockage ratio is calculated to be about 9.2%. The relative air speed was measured by using micro manometer in wind tunnel test section. This relative air speed was measured to calculate the dynamic pressure variations along the centre line of vehicle body. A micro manometer has an accuracy of ±0.5%. Velocity uniformity is ±0.96% which is 1% as given in SAE Wind Tunnel Test procedure [11].

Figure 6: Experimental Setup Experimental Procedure The experiment was done with an objective of measurement of drag force, pressure variations and relative speed with varying speeds along the center line of the vehicle under straight wind conditions. The pressure points are observed on the front, the roof and the rear. The pressure tubes are connected from the model to 20-Way single Selection box and then to the Digital Manometer and the pressure difference is observed. For calculating Drag and lift force load cell directly attached to platform on which vehicle model is fixed was used as transducer which changes the variation of position due to force in equivalent change in resistance, that change in resistance is converted in numerals by means of display unit.

DATA REDUCTION Coefficient of Drag The drag force is the component of the resultant force parallel and opposite to the flow. The drag coefficient (C D) is obtained experimentally through the vehicle geometry or form, and it allows the results do not depend on the real dimensions of the vehicle. The CD represents the relation between drag force and the force of the relative fluid, being expressed by the Equation.


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Coefficient of Lift The lift force is the component of the resultant force perpendicular and opposite to the flow. The lift coefficient (CL) is obtained experimentally through the vehicle geometry or form, and it allows the results do not depend on the real dimensions of the vehicle. The CL represents the relation between lift force and the force of therelative fluid, being expressed by the Equation.

RESULTS AND DISCUSSIONS Coefficient of Drag It is clear from the figure 7 that the value of CD decreases due to the addition of VG. This can be attributed due to the avoidance of flow separation with the help of VG. For instance at a velocity of 2.42 m/s the coefficient of drag is reduced by a maximum of 35% when VG with a yaw angle of 15° is used when compared to the values obtained without VG. Similarly at same velocity a minimum of 20% reduction in drag is obtained for VG with a yaw angle of 10°. For varying value of yaw angle, the CD remains constant for increase in velocity. However when angle yaw is increased the CD values varies with increase in velocity. Hence, it is observed that VG with a yaw angle of 15° will be useful at lower velocity.

Figure 7: Variation of CD for Different Values of Yaw Coefficient of Lift It is clear from the figure 8 that the valueof CL decreases due to the addition of VG. This can beattributed due to the avoidance of flow separation with thehelp of VG. For instance at a velocity of 2.42 m/s thecoefficient of lift is reduced by a maximum of 85% whenVG with a yaw angle of 15° is used when compared to thevalues obtained without VG. Similarly at same velocity aminimum of 50% reduction in lift is obtained for VG witha yaw angle of 10°. However the value of CL decreaseswith increase in velocity with and without VG. The resultsrevealed that at higher velocity the value of CL remainsconstant for VG of all yaw angles.


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Figure 8: Variation of CL for Different Values of Yaw

CONCLUSIONS From the experimental investigation on the measurement of the variation of pressure coefficient and dynamic pressure on the roof of a utility vehicle with and without vortex generators (VG), the following conclusions were made: 

The value of CD is reduced by 35% with the addition of VG at a velocity of 2.42 m/s and a minimum of 20% reduction in drag is obtained for VG with a yaw angle of 10°.

It is observed that VG with a yaw angle of 15° will be useful at lower velocity.

The value of CL decreases with increase in velocity with and without VG and the results revealed that at higher velocity the value of CL remains constant for VG with varying yaw angles.

REFERENCES 1

Hans_Hermann B & Ulrich S, Handbook of Automotive Engineering (SAE international, Warrendale, PennsylvaniaUSA) 2005, 5-27.

2

Aider J L, Dubuc L, Hulin G & Elena L, Experimental and numerical investigation of the flow over a simplified vehicleshape, in Proc 3rd MIRA Int Vehicle AerodynConf (Rugby, UK)2000.

3

Stapleford W R, Aerodynamic improvements to the body and cooling system of a typical small saloon car, J Wind EngIndAerodyn, 9 (1981) 63-75.

4

McCallen R, Browand F, Leonard A, & Rutledge W, Systematic approach to analyzing and reducing aerodynamic drag of heavyvehicles, Annu Auto Tech Dev Customers’ Coord Meeting(Dearborn, Michigan) 27-30 October 1997.

5

Frederique Muyl, Laurent Dumas & Vincent Herbert, Hybrid method for aerodynamic shape optimization in automotive industry, Comp & Fluid, 33 (2004) 849-858.

6

Hucho W H, Aerodynamics of Road Vehicles (Butterworth, London) 1997.

7

Katz J, Race Car Aerodynamics (Robert Bentley Publishing) 1995.

8

Daily James W &Harleman Donald R F, Fluid Dynamics (Addison-WesleyPublications) 1966.

9

Image:

http://www.carbodydesign.com/archive/2009/05/14-

Testing-1-lg.jpg.

volkswagen-polo/VW-New-Polo-Wind-Tunnel-


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10 Masaru koike, TsunehisaNagayoshi and Naoki Hamamoto. 2004. Mitsubishi Motors, Technical Review, No. 16. 11 SAE Wind Tunnel Test Procedure for Trucks and Buses, Recommended Practice-July 1981- SAE J 1252. APPENDICES D

Drag Force

L

Lift force

CD

Drag coefficient

CL

Lift coefficient

ď ˛

Density of air

A

frontal area of vehicle

V

Velocity of vehicle



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