Transition to Self-Excited Oscillations in Low Density Transverse Jet Shear Layers

摘要

The instabilities forming in the near-field of the variable density transverse jet shear layer have been characterized experimentally. Jets composed of helium and nitrogen mixtures were injected from a converging nozzle mounted flush with an injection wall, issuing into air crossflows. The jet-to-crossflow density ratio S was varied between 1.00, the equidensity case, and 0.14, by changing the proportions of nitrogen and helium. Jet-to-crossflow momentum flux ratio J was varied in the range oo > J > 5 at each value of S. The results of single-component hotwire measurements in the jet shear layer indicate that a transition from convective to absolute instability likely occurs as the jet-to-crossflow momentum flux ratio J is brought below approximately 10, and/or as the jet-to-crossflow density ratio is brought below approximately 0.45-0.40. This transition is characterized by several clear spectral features, including strong, pure-tone spectral peaks and resistance to low level acoustic forcing for the globally unstable (self-excited) case, and broadband oscillations with high receptivity to applied forcing for the convectively unstable case. This transition has implications for the control of transverse jet behavior in practical applications.