Daniel Getsinger

Transition to global instability in transverse-jet shear layers

In a recent paper (Megerian et al ., J. Fluid Mech. , vol. 593, 2007, pp. 93–129), experimental exploration of the behaviour of transverse-jet near-field shear-layer instabilities suggests a significant change in the character of the instability as jet-to-crossflow velocity ratios R are reduced below a critical range. The present study provides a detailed exploration of and additional insights into this transition, with quantification of the growth of disturbances at various locations along and about the jet shear layer, frequency tracking and response of the transverse jet to very strong single-mode forcing, creating a ‘lock-in’ response in the shear layer. In all instances, there is clear evidence that the flush transverse jets near-field shear layer becomes globally unstable when R lies at or below a critical range near 3. These findings have important implications for and provide the underlying strategy by which active control of the transverse jet may be developed.

Scalar and Velocity Field Measurements in Acoustically Excited Variable Density Transverse Jets

This experimental study explores both the natural and acoustically forced behavior of variable density transverse jets. A recent study by Getsinger et al. determined that transverse jets (of Reynolds number Rej = 1800) likely transition to global instability in response to su cient lowering of the jet-to-cross ow density ratio S (below 0.45-0.40) or momentum ux ratio J (below 10). This transition is characterized by weak shear layer oscillations easily overcome by external forcing for the convectively unstable (high S and J) case, and strong pure-tone oscillations resistant to external forcing for the globally unstable (low S and J) case. Here, simultaneous PLIF and PIV measurements of jet uid concentration and velocity elds are obtained in the jet neareld, as well as standalone PLIF measurements with a larger eld of view. Neareld PLIF/PIV measurements show agreement with hotwire measurements in terms of jet structural features and the character of the shear layer instabilities. The e ect of this apparent transition on jet mixing is also examined here, with implications for control of jet behavior in practical applications.