Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session E26: Focus Session: From Single Swimmers to Swarms: Active Matter in Fluids at Intermediate Reynolds Numbers |
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Chair: Daphne Klotsa, University of North Carolina Room: Georgia World Congress Center B314 |
Sunday, November 18, 2018 5:10PM - 5:23PM |
E26.00001: Hydrodynamics of swarms at intermediate Reynolds number Isabel A. Houghton, Jeffrey R Koseff, Stephen Monismith, John O. Dabiri The significance of biologically generated turbulence is dependent upon the ability of swimming organisms to generate mixing eddies at scales comparable to the length scales of stratification in the ocean. Despite their small size, marine zooplankton undergo diurnal vertical migration over hundreds of meters and aggregate in dense swarms ranging from 10-50 m in vertical extent, which introduces additional length scales of relevance to their interaction with the surrounding water column. In recent work, we show that representative centimeter-scale swimmers (Artemia salina) migrating collectively perturb a stable density stratification at scales corresponding to the vertical extent of the laboratory controlled swarm, approaching 50 cm. This observed formation of aggregation-scale mixing eddies is the result of coalescence of the flows in the wakes of the individual organisms and leads to mixing of a stratified water column at a rate three orders of magnitude larger than molecular action alone. These results illustrate the potential for zooplankton to significantly alter the physical and biogeochemical structure of the water column with consequences for local or regional dynamics and ecology. |
Sunday, November 18, 2018 5:23PM - 5:36PM |
E26.00002: Random jellyfish: energetics and diffusion in a sea of inertial swimmers Thomas Morrell, Saverio Eric Spagnolie, Jean-Luc Thiffeault We address the energetics and diffusion of particles amidst a random distribution of swimmers in a viscous, inertial fluid. At intermediate Reynolds number, the main mechanism of mixing is the induced net particle displacement (drift). Several experiments have examined this drift for small jellyfish, which produce vortex rings that trap and transport a fair amount of fluid. Inviscid theory implies infinite particle displacements for the trapped fluid, so the effect of viscosity must be included to understand the damping of real vortex motion. We use a model viscous vortex ring to compute particle displacement and other moments. Fluid entrainment at the tail end of a growing vortex 'envelope' is found to play an important role in the total fluid transport and drift. Newer vortices produced by other jellyfish in the bloom overwhelm older vortices, limiting drift and dictating the effective particle diffusion. Our results are robust in the sense that any self-propulsion by isolated impulses produces the same long-time and far-field velocity fields, which determine how the density of swimmers scales with the effective diffusivity. |
Sunday, November 18, 2018 5:36PM - 5:49PM |
E26.00003: Directed percolation theory and experiments of ultra-fast hydrodynamic quorum sensing Arnold JTM Mathijssen, Josh Culver, Saad Bhamla, Manu Prakash Responding to external stimuli promptly is key to survival, so the biophysical relationships between physiological sensors and actuators were fundamental to the development of complex life forms. We study the protist Spirostomum ambiguum, which is unicellular but can grow up to 4mm in size. As a defence against predators, this ciliate releases toxins by contracting its long body within milliseconds. These rapid contractions also generate long-ranged vortex flows that trigger neighbouring cells, in turn, which collectively leads to an ultra-fast hydrodynamic signal transduction across a colony that moves hundreds of times faster than the swimming speed. By combining high-speed PIV and rheosensing experiments we determine the critical colony density required to sustain these signal waves. Synchronised toxin discharges could facilitate the repulsion of large-scale predators cooperatively, but false triggers are costly. We investigate this decision-making process in a framework of quorum sensing and percolation theory. |
Sunday, November 18, 2018 5:49PM - 6:02PM |
E26.00004: The structure and dynamics of simple models of swimming collectives Michael Shelley, Anand Oza, Eva Kanso, Leif Ristroph Fish schools are complex structures organized by the immersing fluid interacting with organismal sensing and behavior. How these aspects are integrated together by the school to yield its dynamics is a wide open question. I will discuss two types of models of swimming collectives, asking the simple question of what types of schools are stable under hydrodynamic interactions. Both types of models were inspired by experiments in the Courant Institute Applied Math Lab that studied the interactions of freely moving ensembles of wings. In one model we extend the seminal work of Weihs to study the dynamics and stability of one- and two-dimensional schools, with crystalline order, whose swimmers produce and interact with the collective vortical background flows. In the second model, we study a simplified version of the body/fluid dynamics, and move to a continuum setting. This model lays bare the role of time-delay in body-vortex interactions, which makes high Reynolds schooling much different from active suspensions at the micro-scale. We find and analyze traveling waves of swimmers, among other things. |
Sunday, November 18, 2018 6:02PM - 6:15PM |
E26.00005: Transition in motility and collective behavior of a simple, self-propelled swimmer at intermediate Reynolds numbers Thomas Dombrowski, Shannon K Jones, Amneet Pal Singh Bhalla, Georgios Katsikis, Boyce Griffith, Daphne Klotsa We propose a simple, self-propelled model swimmer that uses steady streaming flows in a novel way, i.e. for propulsion, and computationally study its motility for a single swimmer and collective behavior for multiple swimmers. Our model swimmer is composed of two unequal spheres that oscillate with respect to each other. For all Re>0, our reciprocal swimmer swims, and interestingly, switches its swimming direction from a small-sphere-leading to a large-sphere-leading regime. Varying a broad range of parameters (viscosity, amplitude, distance between the spheres, sphere radii and sphere-radii ratio), we can collapse the transition point data to a critical value when the appropriate Reynolds number is used. Analyzing the flow fields, we show that propulsion occurs as a result of the interfering steady streaming flows generated by the two spheres forced to oscillate close to one another; the small sphere acts like a flagellum and generates flows that are blocked on one side by the large sphere which causes an asymmetry and net momentum flux. We continue by investigating the interactions between multiple swimmers in both swimming regimes. |
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