Experimental Measurement of the Velocity Field of Round Wall Jet in Counterflow

Abstract

Experimental investigations showing the mean and fluctuating velocity fields of a three-dimensional round wall jet in counterflow are presented. The ratios of jet to counterflow velocity ranged from 2.5 to 25 and the jet Reynolds numbers were from 1,000 to 10,000. The jet penetration length, which is the axial distance that the wall jet penetrates into the counterflow before stopping, was measured and scaled with a power-law relation. The decay of the jet velocity during its penetration into the oncoming flow was analyzed with proper scaling parameters. The results show that up to 70% of the wall jet penetration length, the velocity decay, and the spreading rate are effectively identical to those of a wall jet in quiescent ambient flow. Above this distance, the velocity decay is linear and the jet spreading rate increases sharply. The mean axial velocity profiles show self-similarity between 25 and 80% of the penetration distance and are similar to the case with no counterflow. The vortical structures from the jet and the opposing turbulent boundary layer interact in the stagnation area to provide a large recirculation region with stochastic oscillations that enhance the mixing of the wall jet and counterflow. These interactions result in a region of high turbulence intensity near the upstream extents of the wall jet. The profiles of Reynolds stresses are presented and discussed. It was observed that the normalized velocity fluctuation profiles show self-similarity for the region within 30 and 70% of the wall jet penetration length, which is shorter than the region for which the axial velocity profiles are self-similar.