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Growth and decay of turbulence in a stably stratified shear flow

Published online by Cambridge University Press:  21 April 2006

J. J. Rohr ,
E. C. Itsweire ,
K. N. Helland  and
C. W. Van Atta
J. J. Rohr
Affiliation:
Naval Ocean Systems Center, Code 634, San Diego, CA 92152, USA
E. C. Itsweire
Affiliation:
Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, MD 21211, USA
K. N. Helland
Affiliation:
Department of Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, CA 92093, USA Data Ready, Suite 150 4647T, Highway 280 East, Birmingham, AL 35243, USA
C. W. Van Atta
Affiliation:
Scripps Institution of Oceanography, University of California, San Diego, CA 92093, USA
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Abstract

The behaviour of an evolving, stably stratified turbulent shear flow was investigated in a ten-layer, closed-loop, salt-stratified water channel. Simultaneous single-point measurements of the mean and fluctuating density and longitudinal and vertical velocities were made over a wide range of downstream positions. For strong stability, i.e. a mean gradient Richardson number Ri greater than a critical value of Ricr ≈ 0.25, there is no observed growth of turbulence and the buoyancy effects are similar to those in the unsheared experiments of Stillinger, Helland & Van Atta (1983) and Itsweire, Helland & Van Atta (1986). For values of Richardson number less than Ricr the turbulence grows at a rate depending on Ri and for large evolution times the ratio between the Ozmidov and turbulent lengthscale approaches a constant value which is also a function of Richardson number.

Normalized velocity and density power spectra for the present experiments conform to normalized spectra from previous moderate- to high-Reynolds-number studies. With increasing $\tau = (x/\overline{U}) (\partial \overline{U}/\partial z)$ or decreasing stability, the stratified shear spectra exhibit greater portions of the universal non-stratified spectrum curve. The shapes of the shear-stress and buoyancy-flux cospectra confirm that they act as sources and sinks for the velocity and density fluctuations.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 195 , October 1988 , pp. 77 - 111
Copyright
© 1988 Cambridge University Press

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