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Lock-exchange release density currents over three-dimensional regular roughness elements

Published online by Cambridge University Press:  30 October 2017

Kiran Bhaganagar Open the ORCID record for Kiran Bhaganagar [Opens in a new window]  and
Narasimha Rao Pillalamarri
Kiran Bhaganagar*
Affiliation:
Laboratory of Turbulence, Sensing and Intelligence Systems, Department of Mechanical Engineering, University of Texas at San Antonio, San Antonio, TX 78249, USA
Narasimha Rao Pillalamarri
Affiliation:
Laboratory of Turbulence, Sensing and Intelligence Systems, Department of Mechanical Engineering, University of Texas at San Antonio, San Antonio, TX 78249, USA
*
†Email address for correspondence: kiran.bhaganagar@utsa.edu
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Abstract

A fundamental study has been conducted to understand the front characteristics and the mixing in the flow of density currents over rough surfaces. A large-eddy simulation (LES) has been performed for lock-exchange release density currents over rough walls to shed light on the unsteady mixing processes. A volume-penalization method, which is a special case of the immersed-boundary method, has been implemented to realize the bottom-mounted rough topology. In this study, the LES has been conducted in a channel with a lower wall covered with three-dimensional cube- and pyramid-shaped roughness elements, such that all cases have the same base area, but differences in the roughness solidity and volume fraction of roughness. Both cases of identical roughness elements and those with randomness in height have been considered. The maximum roughness height for all cases is kept at a constant fraction (10 %) of the total channel height. The study focuses on the instantaneous mixing processes in lock-exchange release currents over rough surfaces. An important contribution of the work is that qualitative and quantitative analysis has been conducted to demonstrate additional mixing mechanisms due to the presence of surface roughness that enhances dilution of the current. Enhanced mixing due to roughness is related to the strength of the shear layer resulting from the roughness, and hence depends on friction Reynolds number ( $Re_{\unicode[STIX]{x1D70F}}$ ). The combined role of current characteristics and $Re_{\unicode[STIX]{x1D70F}}$ together dictate the mixing processes and extent of dilution in density currents over surface roughness.

JFM classification

Geophysical and Geological Flows: Gravity currents Geophysical and Geological Flows: Topographic effects Mixing: Turbulent mixing

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Papers
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Journal of Fluid Mechanics , Volume 832 , 10 December 2017 , pp. 793 - 824
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© 2017 Cambridge University Press 

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