Swimmer Dynamics in Complex Flows

This PhD thesis presents a theoretical and numerical investigation of microswimmer dynamics in fluid flows. The original contributions are presented in two dedicated chapters. The first focuses on the effects of surface waves and shear on gyrotactic settling microorganisms. The second is centered on the numerical development of the immersed-boundary method for active swimmers. The effects of waves and shear on model microswimmers are analyzed using multiple-scale stability analysis and kinematic simulations. The analytical approach highlights the system’s phenomenology, characterized by identifying the possible fixed points and discussing their stability. Kinematic simulations confirm these results and extend them beyond the applicability limits of the stability analysis. The key finding from this study is the identification of swimmer trapping below the surface, which may be relevant to the formation of thin phytoplankton layers. The connection to real biological systems is further explored by analyzing broad ranges of the problem’s relevant parameters. The latter part of the thesis addresses the modeling of rigid swimmers with various shapes, incorporating the feedback effects of their motion on the fluid. These effects are known to influence the locomotion and distribution of microorganism populations, and may therefore play a crucial role in collective phenomena. The study is based on the immersed-boundary method, which enforces no-slip conditions on solid bodies moving through fluid while ensuring overall momentum conservation. Notably, this approach can, in principle, accommodate different swimming strategies (such as puller and pusher organisms) and flow regimes. A novel aspect of this study is the ability to model curved swimmer trajectories for so-called circle swimmers (bodies made of non-collinear elementary units or beads). The presentation of results follows a progressive structure: starting with a single swimmer, followed by pair interactions, and concluding with preliminary insights into collective behaviours, which reveal the emergence of transient ordered phases.