Mixing efficiency in stratified turbulence : Joule's experiment revisited
| When |
Jan 28, 2019
from 11:00 to 12:00 |
|---|---|
| Where | Amphi. Schrödinger |
| Attendees |
Louis Gostiaux |
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Deep ocean mixing is today one of the top three most uncertain parametrizations in climate science (along with clouds and ice parametrization). I recently approached this challenging question, after studying linear internal waves in the laboratory, and observing nonlinear internal waves breaking in the ocean. The amount of energy transferred by waves and turbulence to irreversible mixing in the open ocean is still an open question, that can be addressed, fundamentally, by an analogy with a well known historical physics experiment. In 1850, the experiment of Joule on the "mechanical equivalent of heat" quantified the transfer of turbulent kinetic energy into heat in a closed water tank. So far, the stratified version of this experiment does not exist ! In an homogeneous fluid, viscous dissipation converts 100% of the mechanical energy into heat; in a stratified fluid, part of the energy is converted into an irreversible increase of the background potential energy. This ratio is directly related to the so-called mixing efficiency, which is of particular importance in the oceanic context and for mixing parametrization. We have considered by means of Direct Numerical Simulations a closed domain, linearly stratified, where turbulent kinetic energy is initially injected. A closed energy budget is proposed, for which all transfer terms are resolved in time and space and integrated over the flow decay. This approach fundamentally differs from forced-stationary numerical simulations, often used as a reference in the literature; as oceanic turbulence is known to be unstationary and intermittent, the distinction between instantaneous and cumulative mixing efficiency in stratified turbulence is addressed here.