Unsteady phenomena have been identified at the rear of all three vehicles. The described flow structure found for sedan and fastback affects the pressure distribution on the side of the vehicle. The delayed reaction of this flow structure to the change of yaw angle results in a significant increase of unsteady peak yaw moments compared to quasi-steady approximation. A different flow structure at the top of the wake region was identified for the station wagon which, however, shows a similar delayed behaviour during the gust. While the lateral flow structures of sedan and fastback have an impact on side force and yaw moment the flow structure in the wake of the station wagon influences the lift force at the rear of the vehicle but not to the same extent. With respect to cross-wind sensitivity a variation of lift force may have an indirect effect on the vehicle motion compared to the direct effect of side force and yaw moment. In general, station-wagon type vehicles are perceived as being less sensitive to cross-wind than sedan and fastback type vehicles. Therefore, the unsteady behaviour and peak values for the yaw moment could explain discrepancies between perceived cross-wind sensitivity in real world and constant-yaw wind tunnel measurements. As mentioned in the introduction, quasi-steady approximations are applied in the development process today. In order to further improve prediction accuracy of current processes unsteady aerodynamic effects will need to be taken into account in the future. To this end however, prediction accuracy and turnaround times still need to be improved.
Unsteady aerodynamic phenomena under time-dependent flow conditions for different vehicle shapes
Instationäre aerodynamische Phänomene unter zeitabhängigen Strömungsbedingungen für verschiedene Fahrzeugformen
2010
12 Seiten, 7 Bilder, 1 Tabelle, 17 Quellen
Aufsatz (Konferenz)
Englisch
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