Autumn 2019 Seminars and Abstracts Autumn 2019 Seminars and Abstracts

Seminars take place on Monday afternoons 14:00–15:00, unlesss specified otherwise. Everyone is welcome.

Talks

Date Location Speaker Title
7 Oct  ARTS 01.01 Bartosz Protas A Calculus of Shapes for Free Boundary Problems:a Case Study in Vortex Dynamics
14 Oct ARTS 01.01 Duncan Hewitt Swimming in mud: viscoplastic locomotion and slender-body theory
21 Oct LT3 Andrey Cherdantsev Experimental Study of Air Entrapment at Oblique Impact of a Body at a Free Surface
28 Oct ARTS 01.01 Wei Guo Visualization Study of Quantum Fluid Dynamics in Superfluid He-4
4 Nov - - No Seminar
11 Nov LT3 Linda Cummings TBA

Wed 20 Nov

TBA Michikazu Kobayashi TBA
25 Nov LT3 Scott McCue Using Time-Frequency Analysis to Identify Features of Steady ands Unsteady Ship Wakes
2 Dec  LT3 Stefan Llewelyn Smith TBA
9 Dec TBA TBA TBA
16 Dec - - No Seminar

 

Abstracts

7 Oct - Bartosz Protas, McMaster University, Ontario, Canada

 

A Calculus of Shapes for Free-Boundary Problems: a Case Study in Vortex Dynamics

 

Many problems in science and engineering are described in terms of equilibrium shapes on which certain conditions are imposed and which separate regions where the solution may have different properties. A prototypical problem of this type involves inviscid vortex equilibria in 2D and axisymmetric 3D geometries characterized by compact vortex regions embedded in a potential flow. Computation of such equilibrium configurations is made difficult by the fact that it requires finding the shape of the boundary separating the two regions. Similarly, studying the linear stability of such free-boundary problems is also challenging as it requires characterization of the sensitivity of the equilibrium solutions with respect to suitable perturbations of the boundary. We will demonstrate that such questions can be in fact systematically addressed using techniques of "shape calculus" applied to the boundary-integral formulations of such problems, leading to elegant and accurate computational approaches. In the context of vortex dynamics we use these techniques to efficiently compute the family of inviscid vortex rings initially discovered by Norbury (1973).  We also obtain an equation characterizing the stability of general vortex equilibria from which certain classical results of vortex stability can be derived as special cases. Finally, this approach is employed to solve open problems concerning the linear stability of Hill's and Norbury's vortices to 3D axisymmetric perturbations, which leads to some unexpected findings.

 

14 Oct - Duncan Hewitt, DAMTP, University of Cambridge 

 

Swimming in mud: viscoplastic locomotion and slender-body theory

 

Many natural fluids, suspensions, emulsions and foams are characterised by a plastic yield stress, above which they flow like a viscous fluid and below which they do not significantly deform. In this talk, a variety of mechanisms for translation and locomotion through such ‘visco-plastic’ materials are discussed, in the limit of slow motion or small spatial scales. Various classical models for micro-swimming are revisited in the case of a visco-plastic material, and a general slender-body analytical theory for such materials is developed. Numerical solutions are presented and discussed, with particular attention paid to the ‘plastic’ limit of very slow motion or large yield stress. Canonical flow structures and swimming gaits are explored. Implications for real swimmers, as well as comparison with some experimental results, are discussed. 

 

21 Oct - Andrey Cherdantsev, Kutateladze Institute of Thermophysics, Novosibirsk, Russia

 

Experimental study of air entrapment at oblique impact of a body at a free surface

 

In annular gas-liquid flow, droplets are torn from liquid film surface and entrained by turbulent gas stream. Impacts of the entrained droplets back onto the film occur at shallow angles and high impact velocities. In such a case, a droplet creates a long and narrow "furrow" on liquid surface, which is accompanied by massive entrapment of gas bubbles into the liquid film (see Fig. 1). This phenomenon is different from the known mechanisms of air entrapment such as air cushioning; its nature is not entirely clear and deserves intensive experimental and theoretical investigation.

 

First part of the presentation reports on experimental study of oblique high-speed droplet impact in "natural" conditions of annular two-phase flow in a horizontal rectangular duct using three-dimensional and stereoscopic Laser-Induced Fluorescence approaches. The results are focused on the effect of parameters of an individual droplet on the type and shape of liquid surface perturbation and intensity of bubbles entrapment.

The contribution of this phenomenon into total amount of bubbles inside the liquid film and its role in evolution of the whole ensemble of bubbles are analysed.

 

The second part present the results of model experiments on impact of a large (21.3 cm) solid sphere onto a stagnant layer of liquid. This study is focused on air-cushioning mechanism of entrapment and elucidates the very initial stage of impact, prior to the contact between the solid body and the liquid. Despite that the sphere may embed into liquid by 5-6 mm, it is still separated from the liquid by a thin air layer. Synthetic Schlieren method is employed for spatiotemporal measurements of the shape of the liquid surface and the air layer, including the dynamics of the crater and different kinds of waves produced by the impact.

 

Further experimental and theoretical studies will be aimed at clarification of the physical mechanism of air entrapment for oblique high-speed impact and at the influence of the impact angle on air-cushioning entrapment.

 

28 Oct - Wei Guo, National High Magnetic Field Laboratory, Florida, USA

Department of Mechanical Engineering, Florida State University, Florida, USA

 

Visualization study of quantum fluid dynamics in superfluid He-4

 

Helium-4 in the superfluid phase (He II) is a two-fluid system that exhibits fascinating quantum fluid dynamics with important scientific and engineering applications. It supports the most efficient heat-transfer mechanism (i.e. thermal counterflow), and it also allows the generation of flows with extremely high Reynolds numbers for turbulence modelling. However, the lack of high-precision flow measurement tools in He II has impeded the progress in understanding and utilizing its hydrodynamics. In recent years, there have been extensive efforts in developing quantitative flow visualization techniques applicable to He II. Two types of techniques based on the use of either particle tracers (i.e. micron-sized frozen particles) or molecular tracers (i.e. He2* excimer molecules) have been developed. We will discuss the advantages and issues associated with these visualization techniques and will highlight some recent progresses in our visualization study of counterflow and quasiclasscial turbulence in He II. We will also briefly introduce our on-going work on developing the next generation flow visualization techniques and our effort on imaging quantized vortices in a levitated drop of He II.

 

25 Nov - Scott McCue, Queensland University of Technology, Queensland, Australia

 

Title: Using time-frequency analysis to identify features of steady and unsteady ship wakes

 

The motivation here is to study how properties of a ship wake can be extracted from surface height data collected at a single point as the ship travels past.  The tool we use is a spectrogram, which is a heat map that visualises the time-dependent frequency spectrum of the surface height signal.  In this talk, the focus will be on presenting the theoretical framework which involves an idealised mathematical model with a pressure distribution applied to the surface.  A geometric argument based on linear water wave theory provides encouraging results for a range of ship speeds.  The effects of nonlinearity are also studied.  We compare our theoretical predictions with experimental results from the field and from data collected at the Australian Maritime College.  This type of work has the potential to inform ship design, the detection of irregular vessels, and how coastal damage is attributed to specific vessels in shipping channels.

 

For further details about the seminars, or to join our mailing list, please contact Hayder Salman. For details of previous talks, please use the menu links on the left.