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The effect of double diffusion on entrainment in turbulent plumes

Published online by Cambridge University Press:  03 December 2019

Maksim Dadonau Open the ORCID record for Maksim Dadonau [Opens in a new window] ,
J. L. Partridge Open the ORCID record for J. L. Partridge [Opens in a new window]  and
P. F. Linden Open the ORCID record for P. F. Linden [Opens in a new window]
Maksim Dadonau*
Affiliation:
Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, CambridgeCB3 0WA, UK
J. L. Partridge
Affiliation:
Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, CambridgeCB3 0WA, UK
P. F. Linden
Affiliation:
Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, CambridgeCB3 0WA, UK
*
†Email address for correspondence: [email protected]
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Abstract

We investigate experimentally the effect of double diffusion in the salt-fingering configuration on entrainment in turbulent plumes. Plumes over a range of source buoyancy fluxes $B_{0}$ and source density ratios $R_{\unicode[STIX]{x1D70C}}$ are examined. When the plumes are double diffusive ($R_{\unicode[STIX]{x1D70C}}>0$) the entrainment coefficient $\unicode[STIX]{x1D6FC}$ is not constant, with an up to 20 % reduction from the value found for single-diffusive plumes, that is, plumes with $R_{\unicode[STIX]{x1D70C}}=0$. The scale of reduction is found to be in direct relation to the source density ratio and is inversely related to the distance travelled by the plume, indicating that double-diffusive effects decrease as the plume evolves. We propose an explanation for the observed reduction in the entrainment coefficient on the basis of differential diffusion hindering large-scale engulfment at the edge of the plume.

JFM classification

Convection: Plumes/thermals
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 884 , 10 February 2020 , A6
Copyright
© 2019 Cambridge University Press

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