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A Possible Explanation of Counter-Gradient Fluxes in Homogeneous Turbulence

Gerz, T. and Schumann, U. (1996) A Possible Explanation of Counter-Gradient Fluxes in Homogeneous Turbulence. Theoretical and Computational Fluid Dynamics, 8, pp. 169-182. DOI: 10.1007/BF00418056

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Official URL: http://www.springer.com/physics/mechanics/journal/162

Abstract

This study addresses the phenomenon of persistent countergradient (PCG) fluxes of momentum and heat (density) as observed in homogeneous turbulence forced by shear and stratification. Countergradient fluxes may occur at large scales when stratification is strong. However, they always occur at small scales, independently of stratification. A conceptional model is introduced to explain PCG fluxes at small scales as the result of the collision of large-scale fluid parcels. The large parcels collide under the driving force of inclined vortex structures (in a shear-dominated flow) or of buoyancy (in a strongly stratified shear flow). This "collision model" also explains the PCG heat flux in an unsheared stratified flow with zero average momentum flux. It is found that the energy of the small-scale PCG motions is provided (i) by quick transport of kinetic energy from the scales of production to relatively slowly dissipating scales if the flow is shear-driven and (ii) by conversion of available potential energy to kinetic energy at small scales when the flow is stratified. The collision mechanism is an inherent property of the turbulence dynamics. Therefore, the PCG fluxes at small scales reflect a universal character of homogeneous turbulence, and are found over a large range of Reynolds numbers. The Prandtl (or Schmidt) number influences the rate of dissipation of temperature (or density) variance but not the dissipation rate of the velocity variance. In stratified flows, therefore, the number directly affects the strength of the PCG heat flux at small scales. It is found, however, that the PCG momentum flux is also altered slightly when the Prandtl number is large enough to sustain small buoyantly moving parcels after collision.

Item URL in elib:https://elib.dlr.de/32140/
Document Type:Article
Title:A Possible Explanation of Counter-Gradient Fluxes in Homogeneous Turbulence
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Gerz, T.UNSPECIFIEDUNSPECIFIED
Schumann, U.UNSPECIFIEDUNSPECIFIED
Date:1996
Journal or Publication Title:Theoretical and Computational Fluid Dynamics
Open Access:Yes
Gold Open Access:No
In SCOPUS:No
In ISI Web of Science:No
Volume:8
DOI :10.1007/BF00418056
Page Range:pp. 169-182
Status:Published
Keywords:homogeneous turbulence, counter-gradient fluxes
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Aeronautics
HGF - Program Themes:L VU - Air Traffic and Environment (old)
DLR - Research area:Aeronautics
DLR - Program:L VU - Air Traffic and Environment
DLR - Research theme (Project):L - Low-Emission Air Traffic (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics
Deposited By: DLR-Beauftragter, elib
Deposited On:29 Jan 2008
Last Modified:31 Jul 2019 19:17

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