Journal of Fluid Mechanics Webinar Series: Guohua Wang, China and Keith Moffatt, UK

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.

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

Speaker: Keith Moffatt, Cambridge University, UK

Date/Time: Friday 18th June 2021 4:30pm BST/11.30am EDT

Title: Concluding remarks and some open problems

Abstract: This series of webinars was planned in commemoration of George Batchelor's promotion of the field of fluid mechanics in all its varied aspects and applications. In rounding off the series, I shall first comment on how much George Batchelor himself would have enjoyed the 15 brilliant presentations that we have heard. It is good to know that our subject is flourishing so vigorously with the young talent that has been so evident through these presentations. I shall use the short time available to me to discuss three questions that I touched on in my JFM Perspectives paper Some topological aspects of fluid dynamics:
(i) How are corner eddies affected by increasing the Reynolds number of the remote forcing?
(ii) How does a sheet of air enter through the free-surface cusp in a tightly controlled situation?
(iii) How can knotted or linked magnetic flux tubes most efficiently jump from one minimum-energy state to another?