Journal of Fluid Mechanics Webinar Series: Laura Cope, UK and Hugues Faller, France

Speaker: Laura Cope, Cambridge University, UK

Date/Time: Friday 4th June 2021 4:00pm BST/11am EDT

Title: The dynamics of stratified horizontal shear flows at low Péclet number

Abstract: Stratified flows are ubiquitous; examples include atmospheres and oceans in geophysics and stellar interiors in astrophysics. The interaction of a stable stratification with a background velocity distribution can develop into stratified turbulence, key to transport processes in many systems. Geophysical flows, in which the Prandtl number Pr ∼ O(1), are often strongly stratified, nevertheless, turbulence still occurs. Density layering is key to understanding the properties of this ‘layered anisotropic stratified turbulence’ (LAST) regime that is characterised by anisotropic length scales and velocity fields. Conversely, Pr ≪ 1 for astrophysical flows, inhibiting the formation of density layers. This suggests that LAST dynamics cannot occur, raising the question of whether analogous or fundamentally different regimes exist in the limit of strong thermal diffusion. This study addresses this question for the case of a vertically stratified, horizontally-forced Kolmogorov flow using a combination of linear stability theory and direct numerical simulations. Four distinct dynamical regimes emerge, depending upon the strength of the background stratification. By considering dominant balances in the governing equations, we derive scaling laws which explain the empirical observations.

Click here to read Cope's recent paper published in the Journal of Fluid Mechanics.

Speaker: Hugues Faller, Universite Paris-Saclay, France

Date/Time: Friday 4th June 2021 4:30pm BST/11:30am EDT

Title: On the nature of intermittency in a turbulent von Kármán flow

Abstract: We have conducted an extensive study of the scaling properties of small scale turbulence using both numerical and experimental data of a flow in the same von Kármán geometry. We have computed the wavelet structure functions, and the structure functions of the vortical part of the flow and of the local energy transfers. We find that the latter obeys a generalized extended scaling, similar to that already observed for the wavelet structure functions. We compute the multi-fractal spectra of all the structure functions and show that they all coincide with each other, providing a local refined hypothesis. We find that both areas of strong vorticity and strong local energy transfer are highly intermittent and are correlated. For most cases, the location of local maximum of energy transfer is shifted with respect to the location of local maximum of vorticity. We however observe a much stronger correlation between vorticity and local energy transfer in the shear layer, that may be an indication of a self-similar quasi-singular structure that may dominate the scaling properties at large order structure functions.

Enjoy free access to papers in support of Faller's webinar, courtesy of the Journal of Fluid Mechanics.