Fighting the flow: the stability of model flocks in a vortical flow

We investigate the stability of self-propelled particle flocks in the Taylor-Green vortex, a steady vortical flow. We consider a model where particles align themselves to a combination of the orientation and the acceleration of particles within a critical radius. We identify two distinct regimes, if alignment with orientation is dominant the particles tend to be expelled from regions of high vorticity. In contrast if anticipation is dominant the particles accumulate in areas of large vorticity. In both regimes the relative order of the flock is reduced. However we show that there can be a critical balance of the two effects which stabilises the flock in the presence of external fluid forcing. This strategy could provide a mechanism for animal flocks to remain globally ordered in the presence of fluid forcing, and may also have applications in the design of flocking autonomous drones and artificial microswimmers.Comment: 7 pages, 5 figure

Kelvin-Helmholtz instability in an atomic superfluid

We demonstrate an experimentally feasible method for generating the classical Kelvin-Helmholtz instability in a single component atomic Bose-Einstein condensate. By progressively reducing a potential barrier between two counter-flowing channels we seed a line of quantised vortices, which precede to form progressively larger clusters, mimicking the classical roll-up behaviour of the Kelvin-Helmholtz instability. This cluster formation leads to an effective superfluid shear layer, formed through the collective motion of many quantised vortices. From this we demonstrate a straightforward method to measure the effective viscosity of a turbulent quantum fluid in a system with a moderate number of vortices, within the range of current experimental capabilities.Comment: 7 pages, 8 figure

Gyrotactic suppression and emergence of chaotic trajectories of swimming particles in three-dimensional flows

We study the effects of imposed {three-dimensional flows} on the trajectories and mixing of gyrotactic swimming micro-organisms, and identify new phenomena not seen in flows restricted to two dimensions. Through numerical simulation of Taylor--Green and ABC flows, we explore the role that the flow and the cell shape play in determining the long-term configuration of the cells' trajectories, which often take the form of multiple sinuous and helical `plume-like' structures, even in the chaotic ABC flow. This gyrotactic suppression of Lagrangian chaos persists even in the presence of random noise. Analytical solutions for a number of cases reveal the how plumes form and the nature of the competition between torques acting on individual cells. \note{Furthermore, studies of Lyapunov exponents reveal that as the ratio of cell swimming speed relative to the flow speed increases from zero, the initial chaotic trajectories are first suppressed and then give way to a second unexpected window of chaotic trajectories at speeds greater than unity, before suppression of chaos at high relative swimming speeds

Acceleration statistics in thermally driven superfluid turbulence

New methods of flow visualization near absolute zero have opened the way to directly compare quantum turbulence (in superfluid helium) to classical turbulence (in ordinary fluids such as air or water) and explore analogies and differences. We present results of numerical simulations in which we examine the statistics of the superfluid acceleration in thermal counterflow. We find that, unlike the velocity, the acceleration obeys scaling laws similar to classical turbulence, in agreement with a recent quantum turbulence experiment of La Mantia et al.Comment: 5 pages, 5 figures, to appear in PR

Vortex density fluctuations in quantum turbulence

We compute the frequency spectrum of turbulent superfluid vortex density fluctuations and obtain the same Kolmogorov scaling which has been observed in a recent experiment in Helium-4. We show that the scaling can be interpreted in terms of the spectrum of reconnecting material lines. The calculation is performed using a vortex tree algorithm which considerably speeds up the evaluation of Biot-Savart integrals.Comment: 7 Pages, 7 figure

Video-Conferencing with Audio Software

An online conference is illustrated using the format of a TV talk show. The conference combined live audio discussion with visual images spontaneously selected by the moderator in the manner of a TV control-room director. A combination of inexpensive online collaborative tools was used for the event, based on the browser-based audio-conferencing software, iVocalize. The exercise illustrates how an impression of a fully featured online video-conference can be created without the need for complex video-conferencing software and high bandwidth