THE EFFECT OF PHASE VARIATIONS AND CROSS-SHEAR ON VORTICAL STRUCTURESIN A PLANE MIXING LAYER

The evolution of spanwise phase variations of nominally two-dimensiona l instability modes in a plane shear layer is studied in a closed-retu rn water facility using time-harmonic excitation having spanwise-non-u niform phase or frequency distributions. The excitation waveform is sy nthesized by a linear array of 32 surface film heaters flush-mounted o n the flow partition. A spanwise-linear phase distribution leads to th e excitation of oblique waves and to the rollup of oblique primary vor tices. When the prescribed phase distribution is piecewise-constant an d spanwise-periodic, the flow is excited with a linear combination of a two-dimensional wavetrain and pairs of equal and opposite oblique wa ves, the amplitudes of which depend on the magnitude of the phase vari ation Delta Phi. As a result of the excitation, the primary vortices u ndergo spanwise-non-uniform rollup and develop spanwise-periodic defor mations that induce cross-shear and secondary vortices in the braid re gion. The amplitude of the deformations of the primary vortices and th e shape and strength of the secondary vortices depend on the magnitude of Delta Phi. When Delta Phi is small, the secondary vortices are cou nter-rotating vortex pairs. As Delta Phi increases, cross-shear induce d by oblique segments of the primary vortices in the braid region resu lts in the formation of single secondary vortex strands. The flow is n ot receptive to spanwise phase variations with wavelengths shorter tha n the streamwise wavelength of the Kelvin-Helmholtz instability. When the phase variation is Delta Phi = pi, the flow is excited with pairs of oblique waves only and undergoes a double rollup, resulting in the formation of spanwise-deformed vortices at twice the excitation freque ncy. Measurements of the streamwise velocity component show that the e xcitation leads to a substantial increase in the cross-stream spreadin g of the shear layer and that distortions of transverse velocity profi les are accompanied by an increase in the high-frequency content of ve locity power spectra. Detailed schlieren visualizations shed light on the nature of 'vortex dislocations' previously observed by other inves tigators. Complex spanwise-non-uniform pairing interactions between th e spanwise vortices are forced farther downstream by spanwise-amplitud e or phase variations of subharmonic excitation wavetrains.