Turbulence Reduction in the Initial Region of a Plane Jet Using Passive Control

Abstract

The introduction of a thin cylinder in each mixing layer of a plane jet has been found to reduce both longitudinal and cross-stream rms velocity fluctuations (u’ and v’) by 47% and 50% respectively on the jet centre-line at x = 3H, where H is the nozzle width. The spread rate of the mixing layers was decreased by 27%. Normalised distributions of the mean velocity U and Reynolds stresses exhibit a higher degree of self-preservation than in the undisturbed jet. Such an improvement seems to be caused by an attenuation in the modified jet of the formation of organised motion in the mixing layer. Spectra of u support this observation as peaks associated with the natural roll-up frequency and pairing events are absent in the disturbed flow. There remains some organisation in the mixing layers since v spectra show a maximum at the natural instability frequency and a flat peak at the sub-harmonic frequency suggesting the possibility of vortex pairing.