Structural Variation in Convectively and Absolutely Unstable Jets in Crossflow

This experimental study explores the behavior of both unforced and acoustically forced variable density transverse jets, via acetone PLIF and stereo PIV measurements. Jets composed of mixtures of helium and nitrogen are injected normally from a converging nozzle into an air crossflow, for a range of jet-to-crossflow momentum flux ratios $J$ and density ratios $S$. A recent study\footnote{Getsinger, et al., {\bf Expts in Fluids}, 2012} determined, based on hotwire-based spectral characteristics and excitation response, that transverse jet shear layers transition to global instability in response to sufficient lowering of $S$ (below 0.45-0.40) and/or sufficient lowering of $J$ (below 10). The changes in flow structure during such transitions are documented in the present study, where alterations in the transverse jet's vorticity field, cross-sectional symmetry or asymmetry, and dynamic flow features are affected. Both absolutely unstable transverse jets and forced transverse jets are observed to have more symmetric cross-sections than those for convectively unstable jets at higher $J$ values, the latter of which also can exhibit the presence of tertiary vortex structures first identified by Kuzo.\footnote{Kuzo, D., Ph.D. thesis, Caltech, 1995}