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Previous work by the authors showed the development of an aerodynamic CFD model using the Lattice Boltzmann Method for simulating vehicles inside the IVK Model-Scale Wind-Tunnel test-section. In both experiment and simulation, alternate configurations of the wind-tunnel geometry were studied to change the pressure distribution in the wind-tunnel test section, inducing a reduction in aerodynamic drag due to interference between the wind-tunnel geometry and the pressure on the surface of the vehicle. The wind-tunnel pressure distribution was modified by adding so-called “stagnation bodies” inside the collector to create blockage and to increase the pressure in the rear portion of the test section. The primary purpose of previous work was to provide a validated CFD approach for modeling wind-tunnel interference effects, so that these effects can be understood and accounted for when designing vehicles.

In automotive wind tunnels with modern road simulation installations boundary layer pre-suction is a widely-used technique for boundary layer control. The consequence of boundary layer pre-suction is an additional pressure gradient in front of the model. In order to investigate the effects of the additional pressure gradient on drag, experiments were conducted with two different models (scale 1:5) in the IVK Model Wind Tunnel. In these experiments the suction velocity of the boundary layer pre-suction served as a parameter to change the static pressure gradient along the test section and was for this purpose adjusted higher and lower than the standard suction velocity. It is shown that the total drag increment due to boundary layer pre-suction consists of at least two parts: the ground simulation increment and the static pressure gradient increment. The ground simulation increment is due to a decrease in the boundary layer thickness and the resulting modified flow beneath the model.

The influence on aerodynamic drag of a non-uniform, streamwise pressure distribution over the wake of an automobile model in both open-jet and closed-jet wind tunnels is considered in this paper. It has long been an unsolved issue in the theory of open-jet interference and is usually not important in closed-wall wind tunnels unless the model is very long. A new, semi-empirical approach is presented that is based on the observation that the drag changes due to a pressure gradient over a wake correlate with the empty-test-section pressure-coefficient difference between the base of the vehicle and the position of wake closure. A method is demonstrated that is able to remove the effect of the pressure gradient and that is not buoyancy related. This method is applied to a range of simplified and detailed automobile shapes at model scale and at full scale in various wind tunnels, as well as to normal flat plates.