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Fractal scaling of the turbulence interface in gravity currents

Published online by Cambridge University Press:  09 May 2017

Dominik Krug Open the ORCID record for Dominik Krug [Opens in a new window] ,
Markus Holzner ,
Ivan Marusic  and
Maarten van Reeuwijk
Dominik Krug*
Affiliation:
Department of Mechanical Engineering, University of Melbourne, Victoria 3010, Australia
Markus Holzner
Affiliation:
Institute of Environmental Engineering, ETH Zürich, CH-8039 Zürich, Switzerland
Ivan Marusic
Affiliation:
Department of Mechanical Engineering, University of Melbourne, Victoria 3010, Australia
Maarten van Reeuwijk
Affiliation:
Department of Civil and Environmental Engineering, Imperial College London, SW7 2AZ London, UK
*
†Email address for correspondence: [email protected]
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Abstract

It was previously observed by Krug et al. (J. Fluid Mech., vol. 765, 2015, pp. 303–324) that the surface area $A_{\unicode[STIX]{x1D702}}$ of the turbulent/non-turbulent interface (TNTI) in gravity currents decreases with increasing stratification, significantly reducing the entrainment rate. Here, we consider the multiscale properties of this effect using direct numerical simulations of temporal gravity currents with different gradient Richardson numbers $Ri_{g}$. Our results indicate that the reduction of $A_{\unicode[STIX]{x1D702}}$ is caused by a decrease of the fractal scaling exponent $\unicode[STIX]{x1D6FD}$, while the scaling range remains largely unaffected. We further find that convolutions of the TNTI are characterized by different length scales in the streamwise and wall-normal directions, namely the integral scale $h$ and the shear scale $l_{Sk}=k^{1/2}/S$ (formed using the mean shear $S$ and the turbulent kinetic energy $k$) respectively. By recognizing that the anisotropy implied by the different scaling relations increases with increasing $Ri_{g}$, we are able to model the $Ri_{g}$ dependence of $\unicode[STIX]{x1D6FD}$ in good agreement with the data.

JFM classification

Geophysical and Geological Flows: Geophysical and Geological Flows Geophysical and Geological Flows: Gravity currents Turbulent Flows: Stratified turbulence
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 820 , 10 June 2017 , R3
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Copyright
© 2017 Cambridge University Press 

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