Near-exit flow physics of a moderately overpressured jet

Abstract

The developing region of high-speed jets is studied using particle image velocimetry methods. Ensemble-averaged and fluctuating velocity profiles were measured at a range of exit pressures, from a subsonic pressure-balanced case to an overpressured condition 3.2 times atmospheric. When pressure-balanced, the mean flow structure showed gradual development to a bell-shaped profile at approximately 8 diameters downstream, but the turbulent Reynolds stresses were far below self-similar levels. When overpressured, the mean structure displayed a series of compressions and expansions, including a normal shock, and the flow was far from Gaussian after 8 downstream diameters. The turbulent stresses were more suppressed than in the pressure-balanced jet, with little change exhibited in the jet core except very near the normal shock. Instantaneous vorticity contours also showed that the shear layer was divided into two bands at overpressure. This suggests that the turbulent eddies driving entrainment in the near-exit region were substantially weaker than in the self-similar region, which would result in lower mass flow from the ambient.