The structure of a jet in cross flow at low velocity ratios

Abstract

This paper examines in detail the flow structure and associated wall pressure fluctuations caused by the injection of a round, turbulent jet into a turbulent boundary layer. The velocity ratio, r, ratio of mean jet velocity to the mean cross flow, varies from 0.5 to 2.5 and the Reynolds number based on the cross flow speed and jet diameter is 1.9×104. Particle image velocimetry is used to measure the flow and flush mounted pressure sensors installed at several locations used to determine the wall pressure. The results consist of sample instantaneous flow structures, distributions of mean velocity, vorticity and turbulence intensity, as well as wall pressure spectra. The flow structure depends strongly on the velocity ratio and there are two distinctly different regions. At low velocity ratios, namely r<2, a semicylindrical vortical layer (“shell”) forms behind the jet, enclosing a domain with slow moving reverse flow. The vorticity in this semicylindrical shell originates from the jet shear layer. Conversely, at high velocity ratios, namely r>2, the near-wall flow behind the jet resembles a Karman vortex street and the wall-normal vortical structures contain cross flow boundary layer vorticity. Autospectra of the pressure signals show that the effect of the jet is mainly in the 15–100 Hz range. At r<2, the wall pressure fluctuation levels increase with r. At r>2, the wall pressure levels reach a plateau demonstrating the diminishing effect of the jet on the near-wall flow. Consistent with the flow structure, the highest wall pressure fluctuations occur off the jet centerline for r<2 and along the jet centerline for r>2. Also, the advection speed of near-wall vortical structures increase with r at r<2, while at r>2 it is a constant.