Spanwise-nonuniform excitation of a plane mixing layer.

Abstract

The formation and evolution of secondary vortical structures in a plane mixing layer subjected to spanwise-nonuniform excitation has been studied in a closed-return water facility. It is shown that secondary vortices may result from spanwise-nonuniformities in the nominally two-dimensional vorticity layer close to the flow partition, or from spanwise core deformations of the primary vortices further downstream. These distinctly different mechanisms are excited by time-harmonic wavetrains with spanwise amplitude or phase variations, respectively, synthesized by a mosaic of surface film heaters flush-mounted on the flow partition. The appearance of the secondary vortical structures is accompanied by significant distortions in transverse distributions of the streamwise velocity component. Inflection points, which are not present in corresponding velocity distributions of the unforced flow, suggest the formation of locally unstable regions of large shear in which broadband perturbations, already present in the base flow, undergo rapid amplification. This amplification is followed by breakdown to turbulence thus producing the small-scale motion necessary for mixing transition. The present investigation further shows that the flow is extremely receptive to spanwise-periodic amplitude excitation at any wavelength synthesizable by the heater mosaic. Spanwise-periodic phase excitation leads to substantial deformations of the primary vortices, although the receptivity of the flow appears to have a short wavelength cutoff. Spanwise-nonuniform amplitude and phase excitations at a subharmonic frequency of the Kelvin-Helmholtz instability result in complex pairing interactions of the primary vortices.