Aerodynamics of Bluff Bodies

AERODYNAMICS OF BLUFF BODIES DISASTER RESILIENT BUILDINGS (ARC324)

Hima Karthik. S 1190100870 B.Arch. SEM VI SPA Vijayawada

What is Aerodynamics?

It is the study of the motion of air, particularly when affected by a solid object.

Aerodynamics is the way air moves around things.

In Aerodynamics, the forces of interest are the fundamental forces of flight: lift, drag, thrust and weight. Of these, lift and drag are aerodynamic forces, i.e. forces due to air flow over a solid body.

• Thrust is the force produced by the engine. It is directed forward along the axis of the engine.

• Drag is a component of the aerodynamic force, namely the projection onto the direction parallel to the relative wind.

• Lift is another component of the aerodynamic force, namely the projection onto the two directions perpendicular to the relative wind.

• Weight is the force of gravity. It is equal to the mass of the airplane times the local gravitational acceleration.

What is a Bluff body?

A bluff body is defined as that body whose surface is not aligned with the stream-lines, when placed in the flow. It has diameters that are fairly large in relation to their length.

A bluff body, when kept in fluid flow, the fluid does not touch the whole boundary of the object, creating larger wake regions.

Flow over a bluff body Venturi Effect

Building Aerodynamics The following factors need to be determined to ensure the integrity, and stability of a building: •Wind-induced pressure of facades •Influence on pedestrian level environment •Influence on wind conditions of surrounding built environment •Natural ventilation availability and feasibility •Thermal comfort within the building Schematic of wind-induced pressures on a building

Source: https://www.archdaily.com/269195/joint-office-building-and-passenger-cargo-terminal-proposal-de-architekten-cie

WIND/ BUILDING INTERACTIONS

Roof uplift caused by suction

Schematic of internal pressure condition when the dominant opening is in the windward wall

Schematic of internal pressure condition when the dominant opening is in the leeward wall

Economic Considerations to Building Aerodynamics Economic considerations encompass both the initial overheads, material, and construction costs based on the building’s design as well as the projected economic success of the structure’s planned function. No matter what the buildings’ intended use is, a pleasant wind environment from a pedestrian level is key to its economic success.

Urban Landscape Considerations: The landscape of the urban environment must be accounted for in addition to the built environment and climate. For example, hilly terrains will create more uncertainty and fluctuation in weather patterns than flat surface areas. uses CFD to analyze the wind comfort of a city characterized by very hilly terrain. The case study explains how you can evaluate gathered results from velocity and pressure plots, using different cutting planes to locate high-pressure stagnation points upstream of the city and even low-pressure areas on the back sides of buildings. some buildings cause significant recirculation and vortices to develop. Pedestrian comfort would be particularly undesired in these areas.

Static and Dynamic Building Wind Loads: While static loads on a building are defined as being independent of time and therefore constant such as the force of gravity or the weight of the structure itself, dynamic loads are time-dependent where the load force can be accelerating or deaccelerating. Earthquakes, snow loads, and wind loads are all classified as dynamic. Static loads play less of a role in the aerodynamics of a building as they only have fixed solutions.

Dynamic loads, such as wind loads, are more complex as they can have a multitude of solutions and consequences to the aerodynamics of a building.

Building Ventilation Building ventilation is necessary to interchange old and fresh air in order to control temperatures, reduce moisture and odor buildup, and create airflow that ultimately dictates thermal comfort for inhabitants.

If the aerodynamics of the structure allows for natural ventilation via vents and windows facilitating the movement of air, this is an often preferred and energy efficient solution. However, in some circumstances, mechanical ventilation is required.

Case Study: The District “La Défense” in Paris, France We will use the district of La Défense, in Paris, as an example to assess the surrounding pedestrian wind comfort. La Défense’s layout is characterized by tall buildings that are very close in proximity, creating interesting flow phenomena.

Wind Effects Generated by Buildings There are three main wind effects created by buildings: Downwashing, channeling, and corner acceleration. We will use the example of La Défense to illustrate each one of them.

Downwashing The downwash effect happens when there is a tall building surrounded by lower buildings. The tall building captures the wind at higher levels and then redirects it towards the ground, causing a threedimensional flow moving downwards. In turn, this effect often creates a large recirculation at the ground level, causing greater wind activity.

Downdraught effect:

When wind reaches the wall of a building, it gets deflected in all directions However, a significant portion of the air is deflected downward along the building wall, causing draughtiness and turbulence at ground level.

Effects of downwashing at the ground or pedestrian level

Channeling Channeling (also known as Venturi) effect happens when buildings are located in close proximity to one another. The effect is perpetuated by a reduction of wind pressure, resulting in wind acceleration through the tight channel created between the buildings.

This effect can create high winds that can even be unsafe for pedestrians.

Wind funnel effect

Venturi wind effect can be seen

Corner Acceleration Corner acceleration happens when the wind reaches the vertical edge of a tall building.

This effect is mainly created from sharp building shapes, causing acceleration around the corners.

These corners of tall buildings are the regions that can create the highest discomfort in a built environment.

To reduce the impact of these wind effects, some measures can be taken dependent on the development stage of the buildings, i.e., if they are in the development process or pre-exisitng.

For example, canopies can be installed to counter the downwash effect; they will shield the wind and keep it from entering the building.

Also, entrances can be positioned away from corners, to avoid the corner acceleration effect.

Cornering wind effect can be seen

Aerodynamics of High-Rise buildings The free form of architecture being expressed in the modern construction of tall buildings today do not only have the advantage of reflecting the architects idea, but also have thee advantage of reducing the effects of wind on the structure due to aerodynamic effects. Some aerodynamic modifications in architectural design are one of the effective design approaches which can significantly reduce the effect of the lateral wind force and thus, the building motion.

Some aerodynamic modifications in tall buildings

Absolute towers, Mississauga, Canada

Kingdom centre, Riyadh, Saudi Arabia

SOURCES • http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-3-Physics-Vol-1/Aerodynamics-Real-life-app lications.html • https://www.simscale.com/blog/2019/05/building-aerodynamics/ • https://medium.com/@simscale.cae/how-to-assess-building-aerodynamics-wind-effects-on-pedestrians-686cf6b33 463 • https://www.simscale.com/blog/2020/02/building-aerodynamics-and-wind-effects/ • https://www.archdaily.com/269195/joint-office-building-and-passenger-cargo-terminal-proposal-de-architekten-ci e • https://www.wbdg.org/resources/wind-safety-building-envelope • https://structville.com/2020/10/aerodynamics-of-high-rise-buildings.html