Strategic Control of Transverse Jet Shear Layer Instabilities

This experimental study explores active control of the single gaseous jet in crossflow or transverse jet. Jet nozzles that are flush as well as elevated with respect to the injection wall are considered. These studies develop a strategy for control based on separate experimental findings I Megerian, S.. Davitian, J., Alves, L. S. de H., and Karagozian, A. "Transverse-Jet Shear-Layer Instabilities. Part I. Experimental Studies," Journal of Fluid Mechanics, Vol. 593, 2007. pp. 93-129]. which indicate that the jet's shear layer transitions to global instability when the jet-to-crossflow velocity ratio R lies below a critical range. The R-dependent differences in the stability characteristics suggest the necessity of a two-pronged approach to the controlled excitation of transverse jet penetration and spread, which is explored in this paper. For the transverse jets with relatively large R values, the jet shear layer exhibits convective instability, hence even low-to-moderate-level sinusoidal forcing of the jet can control penetration and spread. For the case where R is relatively low and the shear layer is globally unstable, the self-excited flow can be affected by strong sinusoidal forcing at a frequency different from the dominant mode, but the effect on the jet's actual penetration and spread is not significant. For this regime, strong periodic jet forcing with a prescribed time scale that is related to optimal vortex ring generation is required to impact visible jet penetration and spread.