Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow
Stratified shear flows occur in many geophysical contexts, from oceanic overflows and river estuaries to winddriven thermocline layers. In this study, we explore a turbulent wallbounded shear flow of lighter miscible fluid into a quiescent fluid of higher density with a range of Richardson numbers $$0.05\lesssim Ri\lesssim 1$$. In order to find a stability parameter that allows close comparison with linear theory and with idealized experiments and numerics, we investigate different definitions of$Ri$$. We find that a gradient Richardson number defined on fluid interface sections where there is no overturning at or adjacent to the maximum density gradient position provides an excellent stability parameter, which captures the Miles–Howard linear stability criterion. For small $$Ri$$ the flow exhibits robust Kelvin–Helmholtz instability, whereas for larger $$Ri$$ interfacial overturning is more intermittent with less frequent Kelvin–Helmholtz events and emerging Holmboe wave instability consistent with a thicker velocity layer compared with the density layer. We compute the perturbed fraction of interface as a quantitative measure of the flow intermittency, which is approximately 1 for the smallest $$Ri$$ but decreases rapidly as $$Ri$ increases, consistent with linear theory. For the perturbed regions, we use the Thorpe scale to characterize the overturning properties of these flows. The probability distribution of the nonzero Thorpe length yields a universal exponential form, suggesting that much of the overturning results from increasingly intermittent Kelvin–Helmholtz instability events. Finally, the distribution of turbulent kinetic energy, conditioned on the intermittency fraction, has a similar form, suggesting an explanation for the universal scaling collapse of the Thorpe length distribution.
 Authors:

^{[1]};
^{[2]}
 Universite de Lyon, ENS de Lyon, Universite Claude Bernard Lyon 1, CNRS (France)
 Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
 Publication Date:
 Report Number(s):
 LAUR1720090
Journal ID: ISSN 00221120
 Grant/Contract Number:
 AC5206NA25396
 Type:
 Accepted Manuscript
 Journal Name:
 Journal of Fluid Mechanics
 Additional Journal Information:
 Journal Volume: 815; Journal ID: ISSN 00221120
 Publisher:
 Cambridge University Press
 Research Org:
 Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
 Sponsoring Org:
 USDOE National Nuclear Security Administration (NNSA); USDOE Laboratory Directed Research and Development (LDRD) Program
 Country of Publication:
 United States
 Language:
 English
 Subject:
 58 GEOSCIENCES; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Earth Sciences; Planetary Sciences; turbulence, stratified, instability
 OSTI Identifier:
 1356145
Odier, Philippe, and Ecke, Robert E. Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow. United States: N. p.,
Web. doi:10.1017/jfm.2017.48.
Odier, Philippe, & Ecke, Robert E. Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow. United States. doi:10.1017/jfm.2017.48.
Odier, Philippe, and Ecke, Robert E. 2017.
"Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow". United States.
doi:10.1017/jfm.2017.48. https://www.osti.gov/servlets/purl/1356145.
@article{osti_1356145,
title = {Stability, intermittency and universal Thorpe length distribution in a laboratory turbulent stratified shear flow},
author = {Odier, Philippe and Ecke, Robert E.},
abstractNote = {Stratified shear flows occur in many geophysical contexts, from oceanic overflows and river estuaries to winddriven thermocline layers. In this study, we explore a turbulent wallbounded shear flow of lighter miscible fluid into a quiescent fluid of higher density with a range of Richardson numbers $0.05\lesssim Ri\lesssim 1$. In order to find a stability parameter that allows close comparison with linear theory and with idealized experiments and numerics, we investigate different definitions of$Ri$. We find that a gradient Richardson number defined on fluid interface sections where there is no overturning at or adjacent to the maximum density gradient position provides an excellent stability parameter, which captures the Miles–Howard linear stability criterion. For small $Ri$ the flow exhibits robust Kelvin–Helmholtz instability, whereas for larger $Ri$ interfacial overturning is more intermittent with less frequent Kelvin–Helmholtz events and emerging Holmboe wave instability consistent with a thicker velocity layer compared with the density layer. We compute the perturbed fraction of interface as a quantitative measure of the flow intermittency, which is approximately 1 for the smallest $Ri$ but decreases rapidly as $Ri$ increases, consistent with linear theory. For the perturbed regions, we use the Thorpe scale to characterize the overturning properties of these flows. The probability distribution of the nonzero Thorpe length yields a universal exponential form, suggesting that much of the overturning results from increasingly intermittent Kelvin–Helmholtz instability events. Finally, the distribution of turbulent kinetic energy, conditioned on the intermittency fraction, has a similar form, suggesting an explanation for the universal scaling collapse of the Thorpe length distribution.},
doi = {10.1017/jfm.2017.48},
journal = {Journal of Fluid Mechanics},
number = ,
volume = 815,
place = {United States},
year = {2017},
month = {2}
}