Experimental Investigation into Unsteadiness and Instability in Passenger Car Aerodynamics

Flow unsteadiness has been investigated experimentally for two idealised model geometries including the Ahmed form. Several techniques were used including twin hot-wire probes located at different positions in the wake and a frequency domain correction method for pneumatic tubing. Levels of periodicity in the wakes and on the surfaces of the models have been examined using spectral analysis techniques. Unsteadiness was found to originate from movement of the closed separation bubble at the end of the large radii at the front of the models and from vortex shedding when large radius curved rear surfaces are present.