The hydrodynamics of water-walking insects and spiders

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 142-152).We present a combined experimental and theoretical investigation of the numerous hydrodynamic propulsion mechanisms employed by water-walking arthropods (insects and spiders). In our experimental study, high speed cinematography and flow visualization techniques are used to determine the form of the flows generated by water-walkers. In our supporting theoretical study we provide a formal fluid mechanical description of their locomotion. We focus on the most common means of walking on water such as the alternating tripod gait, rowing, galloping and leaping. We also examine quasi-static modes of propulsion in which the insect's legs are kept stationary: specifically, Marangoni propulsion and meniscus-climbing. Special attention is given to rationalizing the propulsion mechanisms of water-walking insects through consideration of the transfer of forces, momentum and energy between the creature and its environment.by David Lite Hu.Ph.D

Biomimetic and Live Medusae Reveal the Mechanistic Advantages of a Flexible Bell Margin

Flexible bell margins are characteristic components of rowing medusan morphologies and are expected to contribute towards their high propulsive efficiency. However, the mechanistic basis of thrust augmentation by flexible propulsors remained unresolved, so the impact of bell margin flexibility on medusan swimming has also remained unresolved. We used biomimetic robotic jellyfish vehicles to elucidate that propulsive thrust enhancement by flexible medusan bell margins relies upon fluid dynamic interactions between entrained flows at the inflexion point of the exumbrella and flows expelled from under the bell. Coalescence of flows from these two regions resulted in enhanced fluid circulation and, therefore, thrust augmentation for flexible margins of both medusan vehicles and living medusae. Using particle image velocimetry (PIV) data we estimated pressure fields to demonstrate a mechanistic basis of enhanced flows associated with the flexible bell margin. Performance of vehicles with flexible margins was further enhanced by vortex interactions that occur during bell expansion. Hydrodynamic and performance similarities between robotic vehicles and live animals demonstrated that the propulsive advantages of flexible margins found in nature can be emulated by human-engineered propulsors. Although medusae are simple animal models for description of this process, these results may contribute towards understanding the performance of flexible margins among other animal lineages

Hydrodynamics of swimming microorganisms in complex fluids

Swimming motion of microorganisms, such as spermatozoa, plankton, algae and bacteria, etc., ubiquitously occurs in nature. It affects many biological processes, including reproduction, infection and the marine life ecosystem. The hydrodynamic effects are important in microorganism swimming, their nutrient uptake, fertilization, collective motions and formation of colonies. In nature, microorganisms have evolved to use various fascinating ways for locomotion and transport. Different designs are also developed for the locomotion of artificial nano- and microswimmers. In this study, we use several different computational models to investigate the behavior of microswimmers. Microorganisms typically swim in the low Reynolds number regime, where inertia is negligible. They interact with each other, surfaces and external flow field. Microorganisms often swim in complex fluids, exhibiting non-Newtonian behavior, including viscoelasticity and shear-thinning viscosity. These biological materials contain network of glycoprotein fibers and gel-like polymers. Therefore on the scale of microorganisms, their fluid environments are heterogeneous rather than homogenous. In this study, we develop a computational platform to investigate swimming motion of a single and multiple microorganism(s) in the bulk fluid and near surfaces in complex fluids. We also investigate the role of fluid rheological properties and flow field on the migration of inert particles in a channel flow of viscoelastic fluids

The fluid mechanics of floating and sinking

This thesis is concerned with the fluid mechanics of floating and sinking. More specifically, the majority of this thesis considers the role played by surface tension in allowing dense objects to float. We first derive the conditions under which objects can float at an interface between two fluids. We obtain the conditions on density and size for various objects to float and show that being ‘super-hydrophobic’ does not generally help small, dense objects to float. Super-hydrophobicity does, however, dramatically reduce the energy required to remove an object from the interface. We then show that two floating objects can sink if they come into close proximity with one another. We extend this to show that a raft consisting of many interfacial objects can become arbitrarily large without sinking, providing that its density is below a critical value. Above this critical value, there is a threshold size at which sinking occurs. We then consider the surface tension dominated impact of an object onto a liquid–gas interface. We determine a similarity solution, valid shortly after impact, for the shape of the interface and study the asymptotic properties of the capillary waves generated by impact. We also show how the interfacial deformation slows down the impacting body. We use a boundary integral simulation to study the motion at later times and determine the conditions under which the object either sinks or is trapped by the surface. We find that for an object of a given weight there is a threshold impact speed above which it sinks. We study the waterlogging of a floating porous body as a model for the waterlogging of the pumice ‘rafts’ that often form on bodies of open water after a volcanic eruption. We study the inflow of water that is driven by capillary suction and hydrostatic pressure imbalances, and determine the time taken for this inflow to cause the object to sink. Finally, we study the effects of a natural slope on the spreading of carbon dioxide sequestered into aquifers. We use laboratory models and numerical techniques to study the spreading of the resulting gravity current. Initially the current spreads axisymmetrically, while at later times it spreads predominantly along any slope in the overlying cap rock. We show that in industrial settings the time scale over which this asymmetry develops is typically a few years. This effect may have important practical implications since the current propagates faster in the asymmetric state