Forces and flow structures evolution on a car body in a sudden crosswind

• Yaw moment unsteady coefficient can be 16% higher than corresponding steady values.
• Transient flow variations are mostly located at the rear of the leeward flank.
• Rear geometry only affects established lateral coefficients.
• Found correlation between lateral coefficient and leeward vortex circulation.

A vehicle driver is commonly exposed to strong side air flows, for example when passing through a wind gust. The aerodynamic efforts generated in these situations may induce undesired lateral deviations, which can lead to dramatic effects, if the driver is surprised. In order to simulate a sudden yaw angle change on a moving vehicle, a double wind tunnel facility, adapted from the one of Ryan, Dominy, 2000. Wake Surveys Behind a Passenger Car Subjected to a Transient Cross-wind Gust. SAE Technical Paper No. 2000-01-0874 is developed. Two Windsor car body models, differing from their rear geometry, are analysed. The transient evolution of the side force and yaw moment aerodynamic coefficients are interpreted in connection with the unsteady development of the flow, based on TR-PIV and stereoscopic PIV measurements. Our analysis shows that the region which is most sensitive to crosswind is located at the rear part of the leeward flank. However, changes in the rear geometry (from squareback to fastback body) only affect the established lateral coefficients values while transient duration and the force overshoots appear not to be significantly modified. Furthermore, the circulation of the most energetic leeward vortex appears to be correlated with the lateral coefficients transient evolutions.