Transition in swimming direction in a model self-propelled inertial swimmer

We propose a reciprocal, self-propelled model swimmer at intermediate Reynolds numbers (Re). Our swimmer consists of two unequal spheres that oscillate in antiphase, generating nonlinear steady streaming (SS) flows. We show computationally that the SS flows enable the swimmer to propel itself, and also switch direction as Re increases. We quantify the transition in the swimming direction by collapsing our data on a critical Re and show that the transition in swimming directions corresponds to the reversal of the SS flows. Based on our findings, we propose that SS can be an important physical mechanism for motility at intermediate Re

Animating jellyfish through numerical simulation and symmetry exploitation

This thesis presents an automatic animation system for jellyfish that is based on a physical simulation of the organism and its surrounding fluid. Our goal is to explore the unusual style of locomotion, namely jet propulsion, which is utilized by jellyfish. The organism achieves this propulsion by contracting its body, expelling water, and propelling itself forward. The organism then expands again to refill itself with water for a subsequent stroke. We endeavor to model the thrust achieved by the jellyfish, and also the evolution of the organism's geometric configuration. We restrict our discussion of locomotion to fully grown adult jellyfish, and we restrict our study of locomotion to the resonant gait, which is the organism's most active mode of locomotion, and is characterized by a regular contraction rate that is near one of the creature's resonant frequencies. We also consider only species that are axially symmetric, and thus are able to reduce the dimensionality of our model. We can approximate the full 3D geometry of a jellyfish by simulating a 2D slice of the organism. This model reduction yields plausible results at a lower computational cost. From the 2D simulation, we extrapolate to a full 3D model. To prevent our extrapolated model from being artificially smooth, we give the final shape more variation by adding noise to the 3D geometry. This noise is inspired by empirical data of real jellyfish, and also by work with continuous noise functions from the graphics community. Our 2D simulations are done numerically with ideas from the field of computational fluid dynamics. Specifically, we simulate the elastic volume of the jellyfish with a spring-mass system, and we simulate the surrounding fluid using the semi-Lagrangian method. To couple the particle-based elastic representation with the grid-based fluid representation, we use the immersed boundary method. We find this combination of methods to be a very efficient means of simulating the 2D slice with a minimal compromise in physical accuracy

Structured light enables biomimetic swimming and versatile locomotion of photoresponsive soft microrobots.

Microorganisms move in challenging environments by periodic changes in body shape. In contrast, current artificial microrobots cannot actively deform, exhibiting at best passive bending under external fields. Here, by taking advantage of the wireless, scalable and spatiotemporally selective capabilities that light allows, we show that soft microrobots consisting of photoactive liquid-crystal elastomers can be driven by structured monochromatic light to perform sophisticated biomimetic motions. We realize continuum yet selectively addressable artificial microswimmers that generate travelling-wave motions to self-propel without external forces or torques, as well as microrobots capable of versatile locomotion behaviours on demand. Both theoretical predictions and experimental results confirm that multiple gaits, mimicking either symplectic or antiplectic metachrony of ciliate protozoa, can be achieved with single microswimmers. The principle of using structured light can be extended to other applications that require microscale actuation with sophisticated spatiotemporal coordination for advanced microrobotic technologies.This work was in part supported by the European Research Council under the ERC Grant agreements 278213 and 291349, and the DFG as part of the project SPP 1726 (microswimmers, FI 1966/1-1). SP acknowledges support by the Max Planck ETH Center for Learning Systems.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nmat456

Experimental investigation of the water entry of rigid and deformable bodies using time-resolved particle image velocimetry

This PhD thesis describes the use of optical measurement techniques, namely Particle Image Velocimetry (PIV), in the study of the water impact of rigid and deformable structures

Interdependence of Flow and Shape Morphological Dynamics For Flow-Induced Erosion of Bluff Bodies

Flow-induced erosion encompasses all processes in which fluid-solid interactions result in the removal and transport of material from the solid. The removed material may change its physical state and/or chemical composition and may be redeposited onto the solid body or advected away by the fluid and deposited elsewhere. Common to all flow induced erosion processes is that they involve an eroding surface, and eroding agent, and a fluid flow which delivers the eroding agent to the eroding surface. Consequently, the study of erosion is difficult as it requires detailed knowledge of the material, mechanical, and/or thermophysical properties of the eroding surface; the transport mechanisms that deliver the eroding agent to the eroding surface; and the transport mechanisms that entrain and advect the eroded material into and within the fluid flow. This difficulty is compounded by the fact that that there is a feedback coupling between the eroding surface and the fluid dynamics that control the transport mechanisms important to erosion. Specifically, during erosion, surface morphological changes to the eroding surface will alter the flow field thereby increasing or decreasing the rate at which the eroding agent is delivered to the eroding surface. This in turn alters the surface morphology. Thus a complex feedback cycle exists between the fluid and surface dynamics. The study of this feedback cycle has received little attention in the fluid mechanics community. This relative neglect is understandable due to its non-equilibrium nature, yet surprising when one considers how much erosion by the action of a flow is an integral part of major scientific and engineering fields, for example geophysics, environmental, manufacturing, and aerospace. The underlying research objective of this dissertation is to better understand the two-way coupling between an eroding body and the surface flux of the eroding agent by evaluating the shape dynamics of eroding bluff bodies through the erosion process. The problem is challenging since, as described above, the surface flux of the eroding agent will vary as the surface morphology of the eroding body evolves. In order to investigate the complex interdependence between the flow and surface morphology of an eroding body during flow-induced erosion, physical ablation and dissolution experiments will be performed and existing numerical datasets will be analyzed to: (i) re-evaluate existing scaling laws regarding geometric properties (cross-sectional area, wetted perimeter, and curvature) of bluff bodies undergoing erosion in (a) uniform, unidirectional flow, (b) in spatially and temporally varying flow, and (c) in convectively driven flow; (ii) identify a shape parameter of the eroding surface that is well-correlated with local evolutional changes to the eroding agent surface flux; and (iii) develop a simple feedback erosion model that bypasses the fluid dynamics and adjusts the local eroding agent surface flux based on the evaluation of the identified shape parameter. The focus on the erosion of bluff bodies was chosen because, in principle, it is more amenable to the study of the erosion feedback cycle as the evolution of the shape dynamics and morphological changes to the surface of the eroding bluff body are a direct result of the, unknown, instantaneous magnitude of the local eroding agent surface flux. Since the evolution of the local eroding agent surface flux is a direct consequence of the feedback from the eroding surface on the flow dynamics, an improved understanding of the erosion feedback cycle is possible by evaluating only the morphological changes to the surface of the eroding bluff body

Human sperm accumulation near surfaces: a simulation study

A hybrid boundary integral/slender body algorithm for modelling flagellar cell motility is presented. The algorithm uses the boundary element method to represent the ‘wedge-shaped’ head of the human sperm cell and a slender body theory representation of the flagellum. The head morphology is specified carefully due to its significant effect on the force and torque balance and hence movement of the free-swimming cell. The technique is used to investigate the mechanisms for the accumulation of human spermatozoa near surfaces. Sperm swimming in an infinite fluid, and near a plane boundary, with prescribed planar and three-dimensional flagellar waveforms are simulated. Both planar and ‘elliptical helicoid’ beating cells are predicted to accumulate at distances of approximately 8.5–22 μm from surfaces, for flagellar beating with angular wavenumber of 3π to 4π. Planar beating cells with wavenumber of approximately 2.4π or greater are predicted to accumulate at a finite distance, while cells with wavenumber of approximately 2π or less are predicted to escape from the surface, likely due to the breakdown of the stable swimming configuration. In the stable swimming trajectory the cell has a small angle of inclination away from the surface, no greater than approximately 0.5°. The trapping effect need not depend on specialized non-planar components of the flagellar beat but rather is a consequence of force and torque balance and the physical effect of the image systems in a no-slip plane boundary. The effect is relatively weak, so that a cell initially one body length from the surface and inclined at an angle of 4°–6° towards the surface will not be trapped but will rather be deflected from the surface. Cells performing rolling motility, where the flagellum sweeps out a ‘conical envelope’, are predicted to align with the surface provided that they approach with sufficiently steep angle. However simulation of cells swimming against a surface in such a configuration is not possible in the present framework. Simulated human sperm cells performing a planar beat with inclination between the beat plane and the plane-of-flattening of the head were not predicted to glide along surfaces, as has been observed in mouse sperm. Instead, cells initially with the head approximately 1.5–3 μm from the surface were predicted to turn away and escape. The simulation model was also used to examine rolling motility due to elliptical helicoid flagellar beating. The head was found to rotate by approximately 240° over one beat cycle and due to the time-varying torques associated with the flagellar beat was found to exhibit ‘looping’ as has been observed in cells swimming against coverslips