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The effects of a vertical contraction on turbulencedynamics in a stably stratified fluid

Published online by Cambridge University Press:  26 April 2006

S. T. Thoroddsen  and
C.W. Van Atta
S. T. Thoroddsen
Affiliation:
Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, CA 92093-0411, USA Present address: Department of Theoretical and Applied Mechanics, University of Illinois at Urbana-Champaign, Urbana IL 61801-2935, USA.
C.W. Van Atta
Affiliation:
Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, CA 92093-0411, USA
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Abstract

We have experimentally studied the effects of mean strain on theevolution of stably stratified turbulence. Grid-generated turbulence($Re_{\lambda \leqslant25}$) in a stable linear mean backgrounddensity gradient was passed through a two-dimensional contraction,contracting the stream only in the vertical direction. This inducesan increase in stratification strength, which reduces the largestvertical overturning scales allowed by buoyancy forces. The meanstrain through the contraction causes, on the other hand, stretchingof streamwise vortices tending to increase the fluctuation levels ofthe transverse velocity components. This competition betweenbuoyancy and vortex stretching dominates the turbulence dynamicsinside and downstream of the contraction. Comparison betweennon-stratified and stratified experiments shows that thestratification significantly reduces the vertical velocityfluctuations. The vertical heat flux is initially enhanced throughthe contraction. Then, farther downstream the flux quickly reverses,leading to very strong restratification coinciding with an increasein the vertical velocity fluctuations. The vertical heat fluxcollapses much more rapidly than in the stratified case without anupstream contraction and the restratification intensity is also muchstronger, showing values of normalized flux as strong as −0.55.Velocity spectra show that the revival of vertical velocityfluctuations, due to the strong restratification, starts at the verylargest scales but is then subsequently transferred to smallerscales. The distance from the turbulence-generating grid to theentrance of the contraction is an important parameter which wasvaried in the experiments. The larger this distance, the larger theintegral length scale can grow, approaching the limit set bybuoyancy, before entering the contraction. The evolution of thevarious turbulence length scales is described. Two-pointmeasurements of velocity and temperature transverse integral scaleswere also performed inside the contraction. The emergence of‘zombie’ turbulence, for large buoyancy times, is in goodquantitative agreement with the numerical simulations of Gerz &Yamazaki (1993) for stratification number larger than 1.

Information

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 285 , 25 February 1995 , pp. 371 - 406
Copyright
© 1995 Cambridge University Press

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