Turbulence Manipulation to Increase Effective Reynolds Numbers in Vehicle Aerodynamics

In the testing of small-scale models of road vehicles Reynolds number requirements usually are violated. Instead of increasing the freestream velocity (Mach number problems), the approach in the present paper makes use of Taylor's theory on the interdependence between the intensity and structure of freestream turbulence and the Reynolds number corresponding to a defined location of the point of transition. Two differently scaled principle models of a passenger car were tested in the wind tunnel. Reynolds numbers (based on overall length) varied from 1.7 x 10 to 5.2 X 10. The resulting drag coefficients (CD) decreased from 0.40 to 0.28, and the overall flow structure changed significantly, which can be seen from the pressure distributions. When a screen of adequate mesh and bar size in the nozzle outlet is used, pressure distributions corresponding to different Reynolds numbers nearly fall onto a single curve that is equivalent to an increase of the effective Reynolds number. Analogous to the critical Reynolds number of the sphere, a Reynolds number corresponding to a fixed value of CD can be defined.