Journal of Fluid Mechanics Webinar Series: George Fortune, UK and Xianyang Jiang, UK
- Date
- Friday 14 May 2021, 4:00pm BST/11am EST
- Location
- Zoom
- External URL
- https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/fluid-mechanics-webinar-series
- Category
- JFM Webinar Series
Speaker: George Fortune, University of Cambridge, UK
Date/Time: Friday 14th May 2021 4:00pm BST/11am EST
Title: Waltzing worms: the dynamics of plant-animal collective vortex structures
Abstract: Circular milling, a stunning manifestation of collective motion, is found across the natural world, from fish shoals to army ants. It has been observed recently that the plant-animal worm Symsagittifera roscoffensis exhibits circular milling behaviour, both in shallow pools at the beach and in Petri dishes in the laboratory. Here in this talk, we investigate this phenomenon, through experiment and theory, from a fluid dynamical viewpoint, focusing on the effect that an established circular mill has on the surrounding fluid. Unlike systems such as confined bacterial suspensions and collections of molecular motors and filaments that exhibit spontaneous circulatory behaviour, and which are modelled as force dipoles, the front-back symmetry of individual worms precludes a stresslet contribution. Instead, singularities such as source dipoles and Stokes quadrupoles are expected to dominate. A series of theoretical models is presented to understand the contributions of these singularities to the azimuthal flow fields generated by a mill, in light of the particular boundary conditions that hold for flow in a Petri dish. A model that treats a circular mill as a rigid rotating disc that generates a Stokes flow is shown to capture basic experimental results well, and gives insights into the emergence and stability of multiple mill systems.
Speaker: Xianyang Jiang, University of Cambridge, UK
Date/Time: Friday 14th May 2021 4:30pm BST/11:30am EST
Title: A metamorphosis of three-dimensional wave structure in transitional and turbulent boundary layers
Abstract: Laminar-turbulent transition in boundary layers is characterized by the generation and metamorphosis of flow structures. The early transition is usually associated with a process of the evolution from a three-dimensional (3-D) wave to a Λ-vortex. To develop a deeper understanding of the spatiotemporal wave-warping process and its roles in precipitating the development of other structures (e.g. hairpin-like structure and turbulent spot), we present numerical studies of both K-regime transition and bypass transition. In this talk, I will first illustrate a qualitative comparison of flow visualizations between a K-regime zero pressure gradient case and an adverse pressure gradient case, based on the method of Lagrangian tracking of marked particles. The underlying vortex dynamics will be presented using a proposed method of Lagrangian-averaged enstrophy. Next, I will draw attention to the 3-D wave structures in bypass transition and early turbulent boundary layer and will describe similar flow behaviours between transitional and turbulent boundary layers. Finally, I will discuss a path to transition, which hypothesizes that the amplification of a 3-D wave precipitates low-speed streaks and rotational structures in wall-bounded flows.