Numerical Investigation Of Bio-inspired Flow-Structure Interaction

Flow-structure interaction problems inspired from bio-locomotion are ubiquitous in nature. The relevant numerical methods and the basic mechanism of flow-structure interaction are important for understanding the biobehavior. In addition, this issue has the potential to revolutionize our sensing and information gathering capabilities in arcas such as environmental monitoring and homeland security. In this thesis, a new version of penalty immersed boundary method (IBM) based on multi-block lattice Boltzmann method (LBM) is developed, and several topics on interaction be-tween flexible filament(s)/plate and the ambient viscous fluid are studicd. The main conclusions are given as follows:(1) A modified penalty IBM based on a multi-block LBM is developed to solve the flow-structure interaction problems involving the massive filaments. The effect of the filament is handled by the IBM in which the stress exerted by the filament on the fluid is spread onto the collocated grid points near the boundary. The fluid motion is obtained by solving the discrete lattice Boltzmann equation. The inertial force of the filament is incorporated by connecting this filament through virtual springs to a ghost filament with the equivalent mass. This treatment ameliorates the numerical instability issue encountered in this type of problem.(2) A filament flapping in the bow wake of a rigid body is considered numerically in order to study the hydrodynamic interaction between flexible and rigid bodies in tandem arrangement. It is shown that the results largely depend on the gap between the two bodies and the Reynolds number. The flexible filament may have larger vibration amplitude but meanwhile experience a reduced drag force. On the other hand, the trailing rigid body enjoys a drag reduction. The qualitative behavior of the filament is independent of the filament's length and mass ratio, and the shape of the rigid body for the parameter regime considered. The result is in contrast with the interaction between two rigid or two flexible objects in tandem arrangement.(3) A viscous flow past three filaments in side-by-side arrangement at low Reynolds number is studied by numerical simulations and is accolnpanied by a linear sta-bility analysis for inviscid flow. The dynamic characteristics of the filaments are studied while varying the separation distance between the filaments, Reynolds number, and bending rigidity. Seven distinct coupling modes of the filaments including the single-filament mode, symmetrical mode, out-el-phase mode, half-frequency mode, irrational-frequency mode, in-phase synchronized mode, and an erratic flapping state are identified as the separation distance is varied. Four typical vortex structures are observed in the wake of the filaments and are de-scribed as the coalesced vortices, symmetrical vortices, erratic vortices and inde-pendent vortex streets. The vortex structures are highly related to the coupling modes and dynamic characteristics of the filaments. As the Reynolds number is increased or as the bending rigidity is reduced, the filaments gain more energy and may transition from one coupling mode to another.(4) A three dimensional flexible plate in a uniform flow is numerically and analyt-ically studied. The globally stable motion is observed for the massless plate. The sustained flapping of the plate only occurs when plate mass is involved. Within a certain range of parameters, the system is bistable, which means that the plate can settle into either rest state or sustained flapping depending on the initial conditions. The Squire's theorem is developed to explain the stability and the transformation is obtained using the incoming flow velocity or flutter speed and wave numbers as the transform variables. The analysis of the energy prop-erties in the plate-fluid system and the pressure distribution around the plate reveals that the onset of instability occurs when positive and negative energy waves coalesce and the energy exchange between the plate and the ambient fluid is necessary for the plate to perform sustained flapping.