Unsteady aerodynamics, studying the effect of turbulent air flows on vehicle bodies, is beginning to draw the attention of carmakers. Unsteady aerodynamics is an attempt to introduce real world variables into wind tunnel or CFD work. It can benefit fuel economy and noise levels, but overall vehicle stability is where the gains are greatest. The most significant advances are in road holding in cross-winds and during passing manoeuvres. It could indicate modifications that minimise a vehicle's sensitivity. Motorsport was the first to take an interest in the research, but it has been slow to put it into practice. The Swiss aerodynamic firm RUAG Aerospace's approach introduces unsteadiness into the aerodynamics by quickly adjusting the position of the front and rear axles to simulate the way the car would behave as it laps a race circuit. The aerodynamic loads are measured during this motion. The data can improve a car's stability and its lap time simulation significantly. To get a feeling for the potential for road cars RUAG recently modified a race car model so it would better resemble a production car. On the other side of the Alps, Pininfarina is conducting full-scale testing. A car's back light angle is one of the most influential factors in determining its drag coefficient. Developers aim to have a shape that puts the dirty air as far behind the car as possible. But it's a lot easier to achieve in a low turbulence wind tunnel than out on the road. In reality, a turbulent flow can reattach itself over the back light. If that happens, the effect on the stability can be considerable. Other real world factors such as changes in the wind's direction due to roadside obstacles such as trees and houses can also affect the vehicle's yaw moment. But reproducing such scenarios in a modern wind tunnel is not easy. The key is introducing turbulence in a controlled, measurable way. In 2003 Pininfarina built a turbulence generation system into its wind tunnel. It works using vertical 'bat wings' that oscillate, generating turbulence in the main flow of the wind tunnel. There are still some very big questions about what the aerodynamicists should be simulating and how to deal with the results. Little or no data exists for turbulence in the road environment; meteorologists gather all their data at altitudes of 10m or more. As time goes on, Moore's Law, a bank of data and a better understanding of needs to be measured will make CFD more and more useful in unsteady aerodynamics.
Going against the flow
Automotive Engineer ; Feb ; 25-26
2007
2 Seiten, 3 Bilder
Article (Journal)
English
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