Control of Organized Structures in Round Jets at High Reynolds Numbers

Abstract

Researchers have studied jets for decades and have observed very early their sensitivity to sound. Dual-mode dual-frequency acoustic excitation can be applied to round air jets to alter dramatically their structure [3,4]. The axial excitation locks the formation of the shear-layer vortex rings on the excitation frequency and therefore controls the vortex spacing, while the helical excitation controls the eccentricity of the rings. The development of the jet depends on the ratio R _f of the axial excitation frequency f _a to the helical frequency f _h . When this ratio is a non-integer between 1.6 and 3.2 the jet is said to bloom [3] as the vortex rings are sent in all directions. When this ratio is equal to 2 the jet bifurcates into two distinct branches. As the axial excitation frequency is increased, the vortex spacing is reduced, leading to a stronger mutual interaction and jet spreading angle. The Strouhal number St _a of 0.55 based on the axial frequency, mean exit velocity and exit diameter was found to produce the largest spreading angles [3]. If the spacing is too small however, the rings tangle and the spreading effect disappears at once. The judicious combination of longitudinal and azimuthal excitation has been shown to enhance jet entrainment drastically [2].