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Görtler vortices and induced mixing in gravity currents over curved boundaries

Numerical, theoretical and experimental approaches

Contact:

Christophe Brun
christophe.brun[A]univ-grenoble-alpes.fr

Eletta Negretti
eletta.negretti[A]legi.cnrs.fr

Guillaume Balarac
guillaume.balarac[A]legi.grenoble-inp.fr

Laboratoire des Ecoulements Géophysiques et Industriels (LEGI, UMR5519),
Domaine Universitaire, BP 53, 38041 Grenoble Cedex 9, France

Context and PhD work:

Gravity currents are relevant to many geophysical and environmental flows such as salt intrusions into lakes, reservoirs and estuaries, glacial runoff into the ocean, or cold downhill airflows in moun- tain areas. On a larger scale they play an important role for the transport of water masses in the oceans and the understanding of their dynamics is therefore important for ocean circulation models and climate models. The correct parametrization of sub-grid turbulent processes related to gravity flows over complex topography (sudden slope changes and curved slopes) are a problem of growing concern since predictions are often unreliable when the overflows themselves and the turbulent and mixing processes are not correctly included in atmospheric, coastal and oceanic circulation models.

While previous research on gravity currents have focused on dense currents descending a flat or uniform sloping bottom, where the dominant instability mechanism at the interface is given by the Kelvin-Helmholtz instability (KHI), this project aims at exploring the conditions of existence and amplification of centrifugal instabilities (Görtler instability GI) which may originate on curved boundaries (Saric, Ann Rev 1994). The novelty is brought by the presence of a background density stratification: its influence on their development and the and the interplay with the KHI are still open questions and deserve further study. At the present state, very few evidence is given of these instabilities within a stratified fluid (Brun, JGR 2017). The importance of the eventual existence of such instabilities is crucial for the dynamics of oceanic overflows, for katabatic flows, turbidity currents and snow avalanches as their induced entrainment/detrainment [Albayrak et al., 2008] modifies their mean properties significantly and the turbulent fluxes (Brun, JGR 2017).

The goal of this PhD thesis is thus to deeply understand these processes and to improve their modelization. For this, a fundamental understanding of the involved hydrodynamic instabilities is needed, which are finally responsible of most of the mixing mechanisms and properties. This will be realized using numerical, theoretical and experimental approaches.