Mid Term Review

Self-propelling Particles with Designed Geometry in Turbulence

Mr Jia Yang
Centre for Plasma and Fluids, ANU

Two-dimensional turbulence driven by Faraday Waves (FWT) has been extensively studied in recent years. These studies resulted in the discovery of the coherent bundle structure underlying turbulent flows. It has been demonstrated that the energy contained in the bundles of particle trajectories can power specially shaped rotors. This discovery suggested a new method of designing floating objects capable of converting energy of turbulence into self-propulsion.

I will report new results on the design and experimental tests of a non-circular pacman-like surface swimmers. We conducted extensive investigations of the translational and rotational diffusion of such swimmers and compared the result with circular disc drifters. By changing the energy of the flow (driving forces, a), the characteristic length of the flow (wavelength of the surface wave,f) and the size of surfers (rs), the propulsion forces generated around the wedge can be finely controlled. Long-time behaviour of the Mean-Square-Displacement of the surfer can be modified from pure directional motion to a random diffusive walk. Figures below illustrate sample trajectories of the swimmer (40mmdiameter) exposed to the wave fields at 30, 60 and 120Hz. While translational motion of the swimmers changes substantially, the rotational behaviour remain diffusive regardless of the conditions.

 We also investigate on the relation between the orientation of the surfer qs, and direction of the velocity qv. The coupling effect between translation and rotation diffusivity is found. The optical fibre cantilever was used as force probes to better understand of the interaction of the flow with the surfers of different geometries.The results imply the fact that turbulent energy is both the fuel for directed motion and the force for randomisation of translation and rotation. Further work on this project will be on collective behaviour of multiple swimmers and the effects of the particle geometry (e.g. elongated particles).

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