On a turbulent flow, gravity wave generation and quasilinear approximations

by Cristóbal Arratia (Nordita )

Wednesday 28 Oct 2020, 13:30 → 14:30 Europe/Stockholm

This talk will be divided in three parts, with an overarching motivation of understanding the appearance of coherence at large scales in turbulent flows. First, I will motivate that general problem with experimental results showing the emergence of a free surface gravity wave in a highly turbulent swirling flow. The origin of this phenomenon is not understood. I will then present a methodology for characterizing the transient dynamics of linearized perturbations. Its application to a horizontal shear layer with vertical stratification unveils a robust mechanism of strong generation of internal gravity waves which is unrelated to instabilities in the classical sense. It is linked to the non-normality of the linearized Navier-Stokes equations, which is generic to shear flows. In the third and last part of the talk, I will introduce quasilinear approximations obtained by decomposing the flow fields into mean and fluctuating parts and neglecting the fluctuating self interactions. I will present results on saturation of laminar-flow instabilities with such models, and explain how these approximations provide a framework that could relate the type of linear amplification mechanisms of the second part to the emerging coherent structure of the first part of the talk.

https://stockholmuniversity.zoom.us/j/530682073

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Arratia_28-Oct-2020.mp4