Investigations of Flow Conditions in an Automotive Wind Tunnel

Licentiate thesis, 2017

As vehicle manufacturers seek to shorten the development time of new models, wind tunnel testing of prototype vehicles needs to be partially replaced by virtual simulations. However, this requires thorough validation of the virtual methods, which is done by comparison to wind tunnel tests. Such comparisons are not straightforward, since a number of interference effects occur between the wind tunnel and the tested vehicle. The objective of this thesis is to improve the understanding of two aspects of interference effects in the Volvo Cars slotted wall aerodynamic wind tunnel. The first aspect that is studied is the influence of six different boundary conditions, for the inlet and the moving ground system, on the longitudinal pressure gradient in numerical simulations of the tunnel. This is done with both steady-state and unsteady methods using a design of experiments approach. The results of the steady-state study show that the boundary layer scoop suction influences the pressure gradient throughout the whole test section, and that a smaller contribution to the pressure variations can be attributed to the two distributed suction systems. Changing the flow angularity on the inlet or varying the speed of the moving belts have shown no significant effect on the pressure distribution. Furthermore, the unsteady method provides better simulation accuracy than the steady-state procedure in the downstream region of the test section. This is attributed to a better representation of the shear layer in the open slots in the slotted walls. The second aspect under consideration is the influence of the tangential blowing system that is part of the ground simulation in the wind tunnel. Using physical measurements, it is shown that the displacement thickness of the boundary layer is reduced by the blowing, and that non-uniformities in the thickness profile are present at interfaces between moving belts and stationary floor. Furthermore, it is shown that the force differences measured between different configurations of a vehicle can be significantly affected by the tangential blowing. The results indicate that vehicles with a larger base area are more sensitive to this phenomenon, and that configurations altering the underbody flow are more affected than those acting to change the flow in the base wake only. Furthermore, numerical simulations are used to trace the force differences between tangential blowing on and off to the rear of the vehicle. It is also demonstrated that the overall behaviour of the boundary layer downstream of the tangential blowers can be well represented in the simulations by using a simplified modeling approach for the blowing slots.