Cooling Drag of Ground Vehicles and Its Interaction with Ground Simulation

Cooling drag is the increase in the total drag due to the internal flow in the cooling system. Because of the high flow resistance in the heat exchanger the momentum of the fluid needed for engine cooling usually is dissipated nearly completely. The resulting drag penalty can be approximated by the so called ram drag. For ground vehicles the cooling drag is typically lower than this approximation due to positive interference of the cooling flow with the general flow around the vehicle.

Different mechanisms for the positive interference have been described in the literature. Inlet interference as well as outlet interference can result in significant reduction of the share of the cooling drag. Positive outlet interference is obtained, when the remaining kinetic energy of the cooling flow contributes significant thrust to the overall momentum balance. Positive inlet interference is found for configurations, where the flow situation without cooling flow is not properly optimized and separation occurs due to the blocked inlet. A second possible positive inlet interference is due to “inlet spillage drag”, the additional drag of the vehicle body due to the increased surface flow velocity with blocked inlet. A further, newly identified source of interference is associated with the vehicle wheels. The mechanism of this cooling drag reducing interference and the effect of ground simulation will be treated in this paper.

With ground simulation the positive interference is much smaller compared to the case with stationary ground. The resulting cooling drag therefore is bigger with ground simulation. As will be shown in this paper the main effect of ground simulation is produced by the rotating wheels. Without any wheels the positive interference vanishes completely and the cooling drag attains a maximum. By additional experiments it is shown that the interference is not limited to the front or the rear wheels. The results presented highlight again that with and without ground simulation different optimum vehicle configurations are found.