Planar Velocimetry of a Fin Trailing Vortex in Subsonic Compressible Flow

Abstract

A subscale experiment has studied the trailing vortex shed from a tapered fin installed on a wind-tunnel wall to represent missile configurations. Stereoscopic particle image velocimetry data have been acquired for several locations downstream of the fin and at different fin angles of attack. The vortex's tangential velocity decays with downstream distance while its radius increases, but the vortex core circulation remains constant. Circulation and tangential velocity rise greatly for increased fin angle of attack, whereas the radius remains approximately constant or slightly decreasing. The vortex axial velocity is always a deficit, whose magnitude diminishes with downstream distance and smaller angle of attack. No variation with Mach number can be discerned in the normalized velocity data. Vortex roll up is largely complete by about four root chord lengths downstream of the fin trailing edge; before this point, the vortex is asymmetric in the tangential velocity but the core radius stays nearly uniform. Vortical rotation draws low-speed turbulent fluid from the wind-tunnel wall boundary layer and the fin wake toward the vortex core, which appears to hasten vortex decay and produce a larger axial velocity deficit than expected. Self-similarity of the vortex is established even while it is still rolling up.