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Licentiate seminar

Numerical investigation of swimming microorganisms in complex environments


Defendant Main Advisor Extra Advisor Date
Lailai Zhu Luca Brandt 2012-06-14

Opponent
Andrej Vilfan, J. Stefan Institute, Slovenia

Evaluation committee

Abstract

In this thesis, numerical simulations are performed to study the hydrodynamics of swimming microorganisms in complex media and geometries. Microorgan- isms are assumed to swim at zero Reynolds number and the squirmer model representing the propulsion of ciliated microorganisms is used. Finite element method is utilized to study the effect of fluid viscoelasticity on the swimming performance and a boundary element method code is developed to simulate the dynamics of swimming cells inside a tube. Several microorganisms have to swim through viscoelastic fluids and fluid elasticity changes swimming hydrodynamics significantly. In the first phase of our work, we study locomotion of the neutral squirmer with an emphasis on the change of swimming kinematics, energetics, and flow disturbance from Newto- nian to viscoelastic fluid. In the second phase, we focus on the dynamics of different swimming gaits resulting in different patterns of polymer deformation, as well as their influence on swimming performance. We correlate the change of swimming speed with extensional viscosity and that of power consumption with deformation delay in viscoelastic fluids. Microorganisms often swim in confined geometries. Proximity to solid or interfacial boundaries alter the locomotory features substantially. We utilize boundary element method to study swimming cells in a straight and torus-like bent tube, when the tube radius is a few times the cell radius. We investigate the effect of tube confinement to the swimming speed and power consumption. We analyze the dynamic motions of squirmers with different gaits which sig- nificantly affect the stability of the motion. Helical trajectories are produced for a neutral squirmer swimming in the tube, in qualitative agreement with experimental observations, this feature can be explained by hydrodynamic in- teractions.
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