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An Experimental Investigation of the Formation of Secondary Vortices and the Generation of Small-Scale Motion in a Spanwise Forced Plane Mixing Layer

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Final rept. 1 Aug 1988-30 Sep 1991,

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The evolution of spanwise instability modes and 3-D pulsed disturbances leading to the formation of streamwise vortices in a plane mixing layer and subsequently to the onset of small-scale 3-D motion were studied in a closed-return water facility. Streamwise vortices may result from a spanwise core instability of the primary vortices, or a spanwise instability of the nominally 2-D strain field between them. These two instability modes were excited by time-harmonic wavetrains with spanwise phase or amplitude variations, respectively, synthesized by a mosaic of surface film heaters flush-mounted on the flow partition. The appearance of the streamwise vortices is accompanied by significant distortion in the transverse distribution of the streamwise velocity component. Inflection points, which are not present in corresponding velocity distributions of the unforced flow, indicate the formation of locally unstable regions of large shear in which broadband perturbations already present in the base flow undergo rapid amplification which is followed by breakdown to turbulence and mixing transition. The core instability of the primary vortices suggests itself as viable mechanism for the continuation of the mixing process far downstream of mixing transition. Pulsed excitation was produced by pulsed spanwise amplitude modulation of a spanwise-uniform time-harmonic carrier wave train synthesized by the surface film heaters. Schlieren visualization showed that the disturbance spreads rapidly in the streamwise and lateral directions as it is advected downstream, and causes a substantial distortion of the adjacent spanwise vortices.

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  • Fluid Mechanics

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