Induced Drag of Ground Vehicles and Its Interaction with Ground Simulation

For the aerodynamic development of an aircraft the induced drag is an important quantity and it has a significant impact on the design of the wing. The induced drag corresponds to the power requirement of the wing to generate the necessary lift. In many cases this is the dominant source of drag for aircraft. In ground vehicle aerodynamics the concept of induced drag up to now has attracted much less attention. This is partly due to the fact, that vehicle aerodynamicists usually optimize the vehicles to generate little or no lift. The second reason is that it is much more difficult for a ground vehicle to separate the total drag into the different contributions. During wind tunnel tests of vehicles with and without ground simulation some astonishing results were found, especially when comparing results for different rear end shapes. Notchback vehicles typically displayed lower drag results when measured with ground simulation, whereas wagon backs showed higher drag figures compared with the case without ground simulation. To explain these surprising results, the contribution of induced drag to the total drag was analyzed in detail. The proportion of induced drag was determined from measured polar diagrams. Different lift levels of the vehicles were created using an adjustable rear spoiler, whereas the trim level of the vehicle was kept constant. Notchbacks typically generate rear lift and wagon backs rear downforce. The two rear end types therefore are located on different branches of the parabola describing the induced drag. By improvement of the ground simulation typically the lift is reduced and thus the induced drag is modified. Due to the difference in the basic lift level the drag is reduced for the notchback and increased for the wagon back. Induced drag can of course describe only a part of the complex influence of improved ground simulation. Notch-back vehicles do not generally show lower drag results when using ground simulation. Nevertheless induced drag can explain some significant influences found in recent test results.