Aerodynamic analysis of automotive vehicles will typically focus on the straight-line condition. A reason is that wind tunnels remain the primary design tool, and they can only produce flow in straight-lines. However, there are some circumstances where aerodynamic performance through corners becomes more important. Particularly in motorsport, the downforce generated through corners permits the vehicle to sustain higher speeds and increases performance. Aerodynamic effects can also become significant for passenger vehicles travelling through corners at high speeds.
Although this condition can be of high importance, it remains difficult to test. The relative curvature of the flow introduces a velocity gradient and changes the angle of the flow along a body, while constant static pressure is maintained. The condition introduces a more complicated flow-field, resulting in forces and moments acting in all directions and asymmetrical flow structures.
Fortunately Computational Fluid Dynamics (CFD) is not limited in the same way as wind tunnels. Using a rotational reference frame and modified domain, producing the correct flow conditions is achievable. Adopting higher order modelling techniques such as Large Eddy Simulation (LES) allows in-depth analysis of even small-scale flow effects and the time-dependent features of the flow.
The two bodies being analysed are both representative of geometries used in the automotive industry, although they produce a very different flow fields:
• The Ahmed Body is a bluff body that is representative of a simple car geometry. It produces highly separated and unsteady flow at the rear of the body, due to the blunt trailing face. Flow features demonstrate low frequency (<50Hz) unsteadiness, and both lift and drag are closely related to vortices. The body is longer than it is wide, making it more susceptible to effects caused by the change in flow angle along the length.
• The T026 inverted front wing geometry experiences mainly attached flow. The wing produces two strong vortices where the wing surfaces meet the endplate. In a practical situation a front wing will likely be experiencing a yawed condition through a corner, as well as flow curvature, due to its position on a vehicle. The lift produced by the wing is due to a pressure gradient created across the pressure and suction surfaces of the wing. As the span is greater than the chord length, the wing is more affected by the velocity gradient.
Results are demonstrating that the two bodies are affected differently due to the cornering condition. This will help understand the types of flow most sensitive to the cornering condition and the effects experienced in both cases.
Why can’t you use experiments too?On the road or track we are capable of testing aerodynamics through corners. However, it isn’t a controlled environment so it makes it difficult to come to definitive conclusions due to variability. Within industry and research there have been attempts to experimentally represent the condition. Currently the use of curved test sections and bent models are recognised as the best methods:
• Curved test sections use the adaptive wall technology typically found in modern wind tunnels to force curved flow in the test section. Adaptive wall technology is typically used to overcome blockage effects due to the presence of the wind tunnel model. The desired outcome is to create a constant static pressure along the wind tunnel walls. Forcing curved flow inevitably leads to a significant pressure gradient and results in a flow velocity profile which is the reverse of the true cornering condition. While the curved flow is achieved, the flow quality is compromised.
• Bent models will curve the wind tunnel model itself to create the correct relative flow angles with the freestream flow. Unfortunately these models are difficult to produce accurately, and again are unable to represent the true cornering condition. Inevitably the bent model has higher local Reynolds numbers on the ‘inboard’ side closest to the centre of rotation, opposing what occurs in reality.
Ultimately, to gain meaningful and consistent results, the true cornering condition requires the correct flow conditions in a controlled environment. I have an idea how that might be possible. The first prototype is on its way…
Keogh, J., Barber, T.J., Diasinos, S., Doig, G.
The Aerodynamic Effects on a Cornering Ahmed Body
Journal of Wind Engineering & Industrial Aerodynamics, 154, pp. 34-46
Keogh, J., Diasinos, S., Barber, T.J., Doig, G.,
A New Type of Wind Tunnel for the Evaluation of Cornering Motion.
AIAA SciTech 2016, San Diego, CA.
Keogh, J., Doig, G., Diasinos, S., Barber, T.J.
The influence of cornering on the vortical wake structures of an inverted wing.
Proc. IMechE. Part D: Journal of Automobile Engineering, vol. 229 no. 13. pp 1817-1829.
Keogh, J., Diasinos, S., Barber, T.J., Doig, G.
Techniques for Aerodynamic Analysis of Cornering Vehicles.
18th Asia Pacific Automotive Engineering Conference (APAC18), 10-12 March, Melbourne, Australia.
Keogh, J., Diasinos, S., Doig, G.
Flow compressibility effects around an open-wheel racing car.
The Aeronautical Journal, Vol. 118 (1210).
Keogh, J., Doig, G., Barber, T. J., Diasinos, S.
The Aerodynamics of a Cornering Inverted Wing in Ground Effect
Applied Mechanics and Materials Vol. 553 pp 205-210.
Keogh, J., Doig, G., Diasinos, S., Barber, T.J.
Detached Eddy Simulation of the Cornering Aerodynamics of the Ahmed Reference Model
In FISITA 2014 World Automotive Congress-2-6 June 2014, Maastricht, The Netherlands.
Keogh, J., Doig, G., Barber, T.J., Diasinos, S.
The Aerodynamics of an Inverted Wing in Ground-Effect Through a Corner.
1st Australasian Conference on Computational Mechanics, October 2013.
Keogh, J., Doig, G., Diasinos, S.
The Influence of Compressibility Effects in Correlation Issues for Aerodynamic Development of Racing Cars.
18th Australiasian Fluid Mechanics Conference, Launceston, 3-7 December 2012.