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Streamwise vortex structure in plane mixing layers

Bernal, L. P. and Roshko, A. (1986) Streamwise vortex structure in plane mixing layers. Journal of Fluid Mechanics, 170 . pp. 499-525. ISSN 0022-1120. doi:10.1017/S002211208600099X.

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The development of three-dimensional motions in a plane mixing layer was investigated experimentally. It is shown that superimposed on the primary, spanwise vortex structure there is a secondary, steamwise vortex structure. Three aspects of this secondary structure were studied. First, the spanwise vortex instability that generates the secondary structure was characterized by measurements of the critical Reynolds number and the spanwise wavelength at several flow conditions. While the critical Reynolds number was found to depend on the velocity ratio, density ratio and initial shear-layer-profile shape, the mean normalized wavelength is independent of these parameters. Secondly, flow visualization in water was used to obtain cross-sectional views of the secondary structure associated with the streamwise counter-rotating vortices. A model is proposed in which those vortices are part of a single vortex line winding back and forth between the high-speed side of a primary vortex and the low-speed side of the following one. Finally, the effect of the secondary structure on the spanwise concentration field was measured in a helium-nitrogen mixing layer. The spatial organization of the secondary structure produces a well-defined spanwise entrainment pattern in which fluid from each stream is preferentially entrained at different spanwise locations. These measurements show that the spanwise scale of the secondary structure increases with downstream distance.

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Additional Information:© Cambridge University Press 1986. Reprinted with permission. (Received 9 September 1985 and in revised form 13 March 1986) We wish to thank our colleagues at GALCIT for numerous helpful discussions and other assistance during the course of this investigation, in particular R.E. Breidenthal, J.E. Broadwell, D.E. Coles and P.E. Dimotakis. The research was sponsored by the Office of Naval Research under contract nos. 8960-1 and N00014-79-C-0365 of Project SQUID and contract no. N00014-C-0260 of the Fluid Dynamics Program.
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Deposited On:13 Apr 2008
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