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Evolution of stream wise vortices and generation of small-scale motion in a plane mixing layer

Published online by Cambridge University Press:  26 April 2006

K. J. Nygaard  and
A. Glezer
K. J. Nygaard
Affiliation:
Department of Aerospace and Mechanical Engineering, The University of Arizona. Tucson, AZ 85721, USA
A. Glezer
Affiliation:
Department of Aerospace and Mechanical Engineering, The University of Arizona. Tucson, AZ 85721, USA
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Abstract

The evolution of streamwise vortices in a plane mixing layer and their role in the generation of small-scale three-dimensional motion are studied in a closed-return water facility. Spanwise-periodic streamwise vortices are excited by a time-harmonic wavetrain with span wise-periodic amplitude variations synthesized by a mosaic of 32 surface film heaters flush-mounted on the flow partition. For a given excitation frequency, virtually any span wise wavelength synthesizable by the heating mosaic can be excited and can lead to the formation of streamwise vortices before the rollup of the primary vortices is completed. The onset of streamwise vortices is accompanied by significant distortion in the transverse distribution of the streamwise velocity component. The presence of inflexion points, absent in corresponding velocity distributions of the unforced flow, suggests the formation of locally unstable regions of large shear in which broadband perturbations already present in the base flow undergo rapid amplification, followed by breakdown to small-scale motion. Furthermore, as a result of spanwise-non-uniform excitation the cores of the primary vortices are significantly altered. The three-dimensional features of the streamwise vortices and their interaction with the base flow are inferred from surfaces of r.m.s. velocity fluctuations and an approximation to cross-stream vorticity using three-dimensional single component velocity data. The striking enhancement of small-scale motion and the spatial modification of its distribution, both induced by the streamwise vortices, can be related to the onset of the mixing transition.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 231 , October 1991 , pp. 257 - 301
Copyright
© 1991 Cambridge University Press

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