Aerodynamic drag improvements on a square-back vehicle at yaw using a tapered cavity and asymmetric flaps

Emissions of greenhouse gasses from passenger vehicles is a concern globally. One of the factors that influence the vehicles energy consumption is the aerodynamic drag, continuing to be an active topic of interest. This work investigates the vehicle wake in relation to aerodynamic drag in steady crosswind conditions. The vehicle used is a modified version of the generic Windsor geometry with wheels and a rearward-facing base cavity with nine angled surfaces, or flaps, distributed at the trailing edge of the cavity along the roof and sides. A surrogate model-based optimisation algorithm was used to minimise the drag coefficient by optimising the angle of each flap individually. The experiments were performed in the Loughborough University Large Wind Tunnel. The time-averaged and unsteady results of both the base pressures and tomographic Particle Image Velocimetry indicate that the optimised flap angles improve drag primarily by altering the wake balance. This is achieved by reducing the strength of a large leeward side vortex, reducing the crossflow within the wake.