Effect of Test Section Configuration on Aerodynamic Drag Measurements

Aerodynamic measurements in automotive wind tunnels are degraded by test section interference effects, which increase with increasing vehicle blockage ratio. The current popularity of large vehicles (i.e. trucks and sport utility vehicles) makes this a significant issue. This paper describes the results of an experimental investigation carried out in support of the Ford/Sverdrup Driveability Test Facility (DTF), which includes an aero-acoustic wind tunnel (Wind Tunnel No. 8). The objective was to quantify the aerodynamic interference associated with two candidate test section configurations for Wind Tunnel No. 8-semi-open jet and slotted wall. The experiments were carried out at 1/11-scale in Sverdrup laboratories. Four automobile shapes (MIRA models) and six Sport Utility Vehicle (SUV) shapes representing blockages from 7% to 25% were used to evaluate changes in measured aerodynamic coefficients for the two test section configurations. This blockage range is appropriate for the 200ft² nozzle in Wind Tunnel No.8. All results were obtained at zero model yaw, over a Reynolds number range of 7×10⁵ to 1×10⁶ based on model length. In addition to the open jet and slotted wall measurements, Sverdrup's sub-scale adaptive wall tunnel was used to acquire additional reference data. The results highlight the influence of test section configuration on aerodynamic measurements. Drag results are presented here, respective of their importance to automotive development. For the MIRA models, the reference results compare well to those from independent experiments. Comparison of the open jet, slotted wall, and reference results shows the open jet offers less aerodynamic interference than the slotted wall. This, coupled with the acoustic advantages of an open jet, drove the decision to strictly employ an open jet configuration in Wind Tunnel No. 8.