Aerodynamic Flow Phenomena at Wheel Rotation Units in Modern 1:1 Vehicle Wind Tunnels

Modern vehicle wind tunnels, like the Aeroacoustic Wind Tunnel (AAWT) from Mercedes-Benz AG, are a fundamental tool in car aerodynamics development. The AAWT features a boundary layer conditioning system and a five-belt system consisting of one center belt and four wheel rotation units (WRUs) for road simulation. The WRUs are connected to the underfloor balance, which leads to design-related gaps to the stationary floor. The wheels are usually positioned in the center of the WRU belt. Previous measurements have shown the existence of a vehicle-dependent connection of the spacing between the tire sidewall and the edge of the belt on the measured drag coefficient. This study investigates the influence of the flow phenomena in the close area around the WRUs on the vehicle aerodynamics of a sedan. The flow was primarily analyzed by using numerical simulations. For validation purposes, experimental measurements were conducted. This involves the static surface pressure around the WRUs and the velocity field in specific planes. The results show that the low-pressure area above the longitudinal gaps, caused by the flow around the wheels, leads to a pressure gradient within the gap between the balance room and the plenum. The static pressure in the balance room always corresponds to the undisturbed plenum pressure. For this reason, a pressure-driven secondary flow occurs through the gaps. In case there is an interference between the gap leakage and the rotating wheel, the loss area of the tire wake increases. The altered wheel flow affects the rear, which results in local pressure differences. In particular, reducing the track width of the WRUs is critical, as the gap leakage directly blows onto the tire.