Three-dimensional pulsed disturbances in a plane mixing layer.

Abstract

The evolution of 3-D pulsed disturbances in a plane mixing layer is studied experimentally. The disturbance is effected via amplitude modulation of a spanwise uniform time-harmonic wave train which provides a clear phase reference for the phase-locked velocity measurements. The evolution of the pulsed disturbance depends critically on the time delay between the modulation pulse and the carrier wave train, the plane mixing layer is most receptive to pulsed excitation when the pulse appears in the braid region between adjacent primary vortices of the base flow. An amplitude demodulation technique is applied to decompose an isolated pulsed disturbance into a family of modal wave packets, and the evolution of the fundamental wave packet was studied in detail. The wave fronts of the wave packet in plane mixing layers are almost parallel to the span, in contrast to a boundary layer wave packet where wave fronts are bowed. The spanwise spreading speed of the wave packet is approximately equal to U(z) = 0.2U(c), while its growth in the streamwise direction is limited. The wave packet is non-dispersive, in agreement with the theoretical results. The effect of a pulse train having temporally and spatially periodic pattern is also studied. Wavelet transforms, both 1-D Morlet and 2-D Arc, are applied to study scales of the flow structures. The large-scale structure exhibits a staggered "chain-link"-like pattern in the streamwise and spanwise directions, whereas the small scale structures are initially generated half way between spanwise centers of pulses. The power spectra indicate that the energy at these small scales increases with increased x. This may suggest that pulsed disturbances may be used to enhance mixing.