Simulation of Transient On-Road Conditions in a Closed Test Section Wind Tunnel Using a Wing System with Active Flaps

Typical automotive research in wind tunnels is conducted under idealized, stationary, low turbulence flow conditions. This does not necessarily reflect the actual situation in traffic. Thus, there is a considerable interest to simulate the actual flow conditions. Because of this, a system for the simulation of the turbulence intensity I, the integral linear scale L and the transient angle of incidence β measured in full-scale tests in the inflow of a test vehicle was developed and installed in a closed-loop, closed test section wind tunnel. The system consists of four airfoils with movable flaps and is installed in the beginning of the test section. Time-series of the flow velocity vector are measured in the empty test section to analyze the system’s envelope in terms of the turbulence intensity and the integral length scales. It is shown that the length scales in spanwise and in driving (streamwise) direction can be varied from 0.15 m to 7.9 m and from 0.15 m to 2.5 m, respectively, depending on the frequency of the flap movement. The maximum obtained turbulence intensity in the driving direction x is 3% and in the spanwise direction y 9.8%, depending on the flap’s amplitude. It is further shown that the turbulence intensity in driving direction can be increased to 5.6% with passive turbulence generators. Additionally, a model for predicting the flap movement reproducing the transient angle of incidence β measured in the on-road tests during an overtaking maneuver was developed. Measurements of the forces acting on the vehicle revealed an influence of the non-stationary flow on the non-stationary force coefficients. Finally, changes of up to 0.002 in Δc_d and 0.157 in Δc_s were measured.