Imperial College London
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Dr Ali Mashayek

Faculty of Engineering, Department of Civil and Environmental Engineering

Honorary Senior Lecturer
 
 
 
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Contact

 

mashayek Website

 
 
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Location

 

336Skempton BuildingSouth Kensington Campus

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Summary

 

Publications

Citation

BibTex format

@article{Mashayekhi:2021:10.1017/jfm.2021.740,
author = {Mashayekhi, A and Caulfield, CP and Alford, MH},
doi = {10.1017/jfm.2021.740},
journal = {Journal of Fluid Mechanics},
pages = {1--32},
title = {Goldilocks mixing in oceanic shear-induced turbulent overturns},
url = {http://dx.doi.org/10.1017/jfm.2021.740},
volume = {928},
year = {2021}
}
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RIS format (EndNote, RefMan)

TY  - JOUR
AB  - We present a new, simple and physically motivated parameterization, based on the ratio of Thorpe and Ozmidov scales, for the irreversible turbulent flux coefficient  ΓM=M/ , i.e. the ratio of the irreversible rate  M  at which the background potential energy increases in a stratified flow due to macroscopic motions to the dissipation rate of turbulent kinetic energy   . Our parameterization covers all three key phases (crucially, in time) of a shear-induced stratified turbulence life cycle: the initial, ‘hot’ growing phase, the intermediate energetically forced phase and the final ‘cold’ fossilization decaying phase. Covering all three phases allows us to highlight the importance of the intermediate one, to which we refer as the ‘Goldilocks’ phase due to its apparently optimal (and so neither too hot nor too cold, but just right) balance, in which energy transfer from background shear to the turbulent mixing is most efficient. The value of  ΓM  is close to 1/3 during this phase, which we demonstrate appears to be related to an adjustment towards a critical or marginal Richardson number for sustained turbulence  ∼0.2--0.25 . Importantly, although buoyancy effects are still significant at leading order for the turbulent dynamics during this intermediate phase, the marginal balance in the flow ensures that the turbulent mixing of the (density) scalar is nevertheless effectively ‘locked’ to the turbulent mixing of momentum. We present supporting evidence for our parameterization through comparison with six oceanographic datasets that span various turbulence generation regimes and a wide range of geographical location and depth. Using these observations, we highlight the significance of parameterizing an inherently variable flux coefficient for capturing the turbulent flux associated with rare energetic, yet fundamentally shear-driven (and so not strongly stratified) overturns that make a disproportionate
AU  - Mashayekhi,A
AU  - Caulfield,CP
AU  - Alford,MH
DO  - 10.1017/jfm.2021.740
EP  - 32
PY  - 2021///
SN  - 0022-1120
SP  - 1
TI  - Goldilocks mixing in oceanic shear-induced turbulent overturns
T2  - Journal of Fluid Mechanics
UR  - http://dx.doi.org/10.1017/jfm.2021.740
UR  - https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/goldilocks-mixing-in-oceanic-shearinduced-turbulent-overturns/56115ECF93E0E3071E5C1FB119A2C479
UR  - http://hdl.handle.net/10044/1/99155
VL  - 928
ER  -
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