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Lagrangian coherent structures and entrainment near the turbulent/non-turbulent interface of a gravity current

Published online by Cambridge University Press:  27 August 2019

Marius M. Neamtu-Halic Open the ORCID record for Marius M. Neamtu-Halic [Opens in a new window] ,
Dominik Krug Open the ORCID record for Dominik Krug [Opens in a new window] ,
George Haller Open the ORCID record for George Haller [Opens in a new window]  and
Markus Holzner Open the ORCID record for Markus Holzner [Opens in a new window]
Marius M. Neamtu-Halic*
Affiliation:
Institute of Environmental Engineering, ETH Zürich, CH-8039 Zürich, Switzerland
Dominik Krug
Affiliation:
Physics of Fluids Group and Twente Max Planck Center, Department of Science and Technology, Mesa+ Institute, and J. M. Burgers Center for Fluid Dynamics, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands
George Haller
Affiliation:
Institute of Mechanical Systems, ETH Zürich, CH-8092 Zürich, Switzerland
Markus Holzner
Affiliation:
Institute of Environmental Engineering, ETH Zürich, CH-8039 Zürich, Switzerland
*
†Email address for correspondence: neamtu@ifu.baug.ethz.ch
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Abstract

In this paper, we employ the theory of Lagrangian coherent structures for three-dimensional vortex eduction and investigate the effect of large-scale vortical structures on the turbulent/non-turbulent interface (TNTI) and entrainment of a gravity current. The gravity current is realized experimentally and different levels of stratification are examined. For flow measurements, we use a multivolume three-dimensional particle tracking velocimetry technique. To identify vortical Lagrangian coherent structures (VLCSs), a fully automated three-dimensional extraction algorithm for multiple flow structures based on the so-called Lagrangian-averaged vorticity deviation method is implemented. The size, the orientation and the shape of the VLCSs are analysed and the results show that these characteristics depend only weakly on the strength of the stratification. Through conditional analysis, we provide evidence that VLCSs modulate the average TNTI height, consequently affecting the entrainment process. Furthermore, VLCSs influence the local entrainment velocity and organize the flow field on both the turbulent and non-turbulent sides of the gravity current boundary.

JFM classification

Mixing: Turbulent mixing Turbulent Flows: Stratified turbulence Geophysical and Geological Flows: Gravity currents

Information

Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 877 , 25 October 2019 , pp. 824 - 843
Copyright
© 2019 Cambridge University Press 

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