Near-wall Velocity Field Measurements of a Very Low Momentum Flux Transverse Jet

Abstract

Abstract : Transverse jets have been the subject of study for several decades due to their relevance in a number of flows in nature and engineering applications. The momentum flux ratio between the jet and crossflow is generally considered to be the leading independent parameter, and has been explored extensively over a nominal range from unity to several hundred. The current study considers the flow field of a transverse jet at momentum flux ratios much smaller than unity. A modulated absorption/emission thermometry technique under development at AFRL requires a narrow film barrier to maintain a clean optical access window, while limiting the transverse penetration of the jet into the crossflow to avoid corruption of the optical measurement. The current effort examined the near field behavior of very low momentum flux ratio transverse jets to elucidate the jet and crossflow interactions. Under low momentum flux ratio conditions, the crossflow has sufficient local momentum near the injection location, exceeding that of the jet, which leads to crossflow ingestion. It was expected that reduced area and consequent increase in mean jet velocity would increase the effective momentum flux ratio which would lead to increased momentum flux and penetration into the crossflow. However, the complex 3-dimensional interaction between the jet and crossflow suggests penetration is far reduced for these very low momentum flux ratio cases. Particle image velocimetry was used to investigate the jet and cross flow interactions near the jet exit over a range of low momentum flux ratios, 0.0013 or = J or = 0.075. A fully developed laminar jet was injected into a fully developed turbulent channel crossflow. The two lowest momentum flux ratio cases were found to penetrate the least into the crossflow with regard to mean behavior of the jet.