Study of subsonic base cavity flowfield structure using particle image velocimetry

A new particle image velocimetry system has been used to study the near-wake structure of a two-dimensional base in subsonic flow to determine the fluid dynamic mechanisms of observed base drag reduction in the presence of a base cavity. Experiments were done over a range of freestream Mach numbers up to 0.8, including local flowfield velocities over 300 m/s. Effects of the base cavity on the von Karman vortex street wake were found to be related to the expansion and diffusion of vortices near the cavity, although the effects are of small magnitude and no significant change in the vortex formation location or path was observed. The base cavity effects are also less significant at higher freestream velocities due to the formation of vortices further downstream from the base. The base cavity drag reduction was found to be mainly due to the displacement of the base surface to a location upstream of the low-pressure wake vortices, with only a slight modification in the vortex street itself.