Experimental study of the coherent structure dynamics and control of an isolated jet in cross flow

Abstract

We explore the dynamics and control of largescale, organized structures via sub-scale experiments in an isolated, incompressible circular jet issuing into a uniform cross-flow having a velocity/blowing ratio (between the jet and the crossflow) of 6. Surveys of the unsteady flow field were conducted using a single sensor hot-film probe. A spinning mechanical valve was used to provide single frequency unsteady forcing to the flow. The flow receptivity to high frequencies in the jet near field (associated with the jet instabilities) and to low frequencies farther downstream (associated with the counter-rotating vortex pair dynamics) was observed. Prior studies have focused on mixing enhancement solutions associated with excitation of the jet instabilities (namely, Kelvin Helmholtz instabilities), which tend to be organized in higher frequency bands. Here, low frequency, open-loop forcing in a jet Strouhal number range near 0.1 < StD < 0.2 was recognized as an effective means to excite organized unsteadiness in the flow field farther downstream where enhanced entrainment and mixing are desired. Measurements of the time-averaged velocity field as well as scalar field (using Mie scattering) demonstrate increased entrainment and concomitant mixing enhancement as a result of low-frequency forcing.