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The Fluid Mechanics Webinar Series



Date & time Jun 4, 04:00PM
Ends on Jun 18
University of Cambridge, United Kingdom
Creator JanineAtteney


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The Fluid Mechanics Webinar Series

The Leeds Institute for Fluid Dynamics is delighted to partner with the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge, the UK Fluids Network, and the Journal of Fluid Mechanics to deliver a regular webinar series on fluids-related topics.

Registration for this week's seminar is now closed. Registration closes at 5:00pm UK time on Thursdays prior to the Friday webinar.

We will resume on the 30th April 2021 for our Easter term. The next series of the Fluid Mechanics Webinar will be dedicated to the George Batchelor Centenary celebration, and in the spirit of Professor G. K. Batchelor, 15 talks will be given by Early Career Scientists. These speakers all contributed papers to the JFM Special Volume in Celebration of the George Batchelor Centenary. All papers in this volume are free to read in perpetuity and we invite you to read and share.

Video recordings of past webinars can be found within the biographies and abstracts of previous speakers further down this page.

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.

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.

Speaker: Jane Bae, Harvard University, USA 

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

Title: Nonlinear interaction of the self-sustaining process in the near-wall region of wall-bounded turbulence

Abstract: We investigate the nonlinear interaction in the self-sustaining process of wall-bounded turbulence. Resolvent analysis is used to identify the principal forcing (nonlinear) mode which produces the maximum amplification in direct numerical simulations of the minimal channel for the buffer layer. The identified mode is then removed from the nonlinear term of the Navier-Stokes equations at each time step from a direct numerical simulation of a minimal channel. The results show that the removal of the principal forcing mode is able to inhibit turbulence in the buffer layer, while the removal of subsequent modes only marginally affects the flow. Analysis of the dyadic interactions in the nonlinear term shows that contributions toward the principal forcing mode come from a limited number of wavenumber interactions. Using conditional averaging, the flow structures that are responsible for generating the principal forcing mode, and thus the nonlinear interaction to self-sustain turbulence, are identified to be spanwise rolls interacting with meandering streaks.

Speaker: Axel Huerre, Laboratoire Matière et Systèmes Complexes (MSC), Université de Paris 

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

Title: Freezing a rivulet

Abstract: We investigate experimentally the formation of the particular ice structure obtained when a capillary trickle of water flows on a cold substrate. We show that after a few minutes the water ends up flow-ing on a tiny ice wall whose shape is permanent. We characterize and understand quantitatively the formation dynamics and the final thickness of this ice structure. In particular, we identify two growth regimes. First, a 1D solidification diffusive regime, where ice is building independently of the flowing water. And second, once the ice is thick enough, the heat flux in the water comes into play, breaking the 1D symmetry of the problem, and the ice ends up thickening linearly downward. This linear pattern is explained by considering the competition between the water cooling and its convection.

Speaker: Guohua Wang, Lanzhou University, China 

Date/Time: Friday 18th June 2021 4:00pm BST/11am EDT

Title: Very-large scale motions in the atmospheric surface layer

Abstract: Very-large scale motions (VLSMs) are typical structures in wall-bounded turbulence at high Reynolds number, which make important contributions to mass and energy transport. This talk will present the atmospheric surface layer observations carried out at the Qingtu Lake observation array (QLOA) site. Furthermore, some studies on the VLSMs in the atmospheric surface layer based on the observed data are introduced, including the morphological and dynamic characteristics of the VLSMs and its effect on the sand dust transportation. The influences of dust particles and heat flux on the VLSMs are discussed as well. It is found that the length scale of the VLSMs has Reynolds number invariance, and evidenced that the VLSMs in the atmospheric surface layer evolute with a top-down mechanism. In the sand-laden flow, the energy of the VLSMs increases, while their energy fraction decreases. The inclination angle of the VLSMs increases with the increase of dust concentration. It is revealed that the VLSMs dominates the streamwise transport of PM10 (tiny particles with size less than 10 μm), but suppress the vertical transport of PM10 near the surface. Finally, it is demonstrated that the temperature and PM10 in the atmospheric surface layer have large structural feature similar as the VLSMs, though the shape of temperature and PM10 structures are different with the VLSMs.

The Wall

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