Numerical Investigation Of Fluid And Flexible-Thin-Walled Structure Interaction

Flow-structure interaction is a common phenomenon in nature and engineering. Four typical flow-structure interaction problems are studied numerically by a lattice Boltzmann-finite element (finite difference)-immersed boundary (momentum exchange) method. The results and conclusions are briefly given as follows:(1) Motivated by collapse of blood vessels for both healthy and diseased situations under various circumstances in human body, we have performed computational studies on an incompressible viscous fluid past a collapsible channel with part of its upper wall being replaced by a deformable beam. The self-excited oscillation can be intrigued by application of a small bending stiffness or a large external pressure on the elastic portion of the channel and the part of the beam having the largest deformation tends to occur always towards the end portion of the deformable wall. The pressure and wall shear stress undergo significant variations near the portion of the greatest oscillation. The stretching motion has the most contribution to the total potential elastic energy of the oscillating beam. Based on the comparison between the flexible and rigid beams, the effects to the flow field, pressure and wall shear stress distribution by the flexibility are identified.(2) The dynamics of an inverted flexible plate with a free leading-edge and a fixed trailing-edge in a uniform flow is studied. Mechanisms underlying the dynamics of the fluid-plate system are elucidated systematically. A series of distinct states of the plate deformation and motion are identified and can be described as straight, flapping, deflected, deflected-flapping, and asymmetric-flapping states. Which state to occur depends mainly on the bending stiffness and aspect ratio of the plate. The forces exerted on the plate and the elastic strain energy of the plate are analyzed. It is found that the flapping state can efficiently produce elastic strain energy for harvesting fluid kinetic energy. In addition, the effects of the mass ratio of the plate and the fluid, the Reynolds number, the angle of attack of the uniform flow on the dynamics and the elastic strain energy of flexible plate are also investigated in detail. The vortical structures around the plate are presented to discuss the connection of the evolution of vortices with the plate deformation and motion.(3) The locomotion of a flapping flexible plate in the stationary fluid is studied. It is identified that the plate performs an improved propulsive performance for moder-ate bending stiffness. The effect of the aspect ratio on the propulsive performance depends on the bending stiffness. Moreover, a larger mass ratio and amplitude lead to a larger propulsive speed, but more input work is needed. The vortical structures in the wake of plate for different aspect ratios are investigated. Based on the comparison of our numerical results and previous experiments and obser-vations, the relationship between the flapping flexible plate and the locomotion of swimming and flying animals is analyzed.(4) The locomotion of a flapping flexible plate near the ground is studied. It is identified that the existence of the ground can enhance the propulsive speed and efficiency. For moderate bending stiffness, the ground may inhibit the deformation of the plate. Moreover, the effects of distance, aspect ratio and mass ratio on the propulsive performance are also investigated. Based on the pressure distributions and vortical structures for different bending stiffness, the flow field with the ground is analyzed.