Influence of Wheel Drive Unit Belt Width on the Aerodynamics of Passenger Vehicles

<div class="section abstract"><div class="htmlview paragraph">Wind tunnels are an essential tool in vehicle development. To simulate the relative velocity between the vehicle and the ground, wind tunnels are typically equipped with moving ground and boundary layer control systems. For passenger vehicles, wind tunnels with five-belt systems are commonly used as a trade-off between accurate replication of the road conditions and uncertainty of the force measurements. To allow different tyre sizes, the wheel drive units (WDUs) can often be fitted with belts of various widths. Using wider belts, the moving ground simulation area increases at the negative cost of larger parasitic lift forces, caused by the connection between the WDUs and the balance.</div><div class="htmlview paragraph">In this work, a crossover SUV was tested with 280 and 360mm wide belts, capturing forces, surface pressures and flow fields. For further insights, numerical simulations were also used. It was found that the belt width can substantially alter the flow field, mainly locally around the wheels but also away from the belts. The drag coefficient increased by approximately 0.010 <i>C_D</i> for the narrow belts, mainly due to differences in the vortex structures downstream of the front wheels, resulting in larger low-energy regions for the narrow belts. Numerical simulations showed that the change in vortex structures was caused by leakage flows from the gaps between the WDUs and the stationary floor. Comparing two rim designs, it was found that the belt width did not only affect the absolute force measurements but also the drag delta between them.</div><div class="htmlview paragraph">Although the parasitic lift forces increase with the wider belts, a correction is necessary regardless. Hence, given the fact that the interfering leakage flows are further away from the tyres with the wider belts, they should be the preferred choice, allowing more accurate investigations of wheel aerodynamics.</div></div>