Numerical And Experimental Investigation On The Interaction Between Moving Fluid And Flexible Bodies

The interactions between moving fluid and one or multiply deformable bodies are commonly exhibited in animal kinematics, people's everyday life and various industry applications. This kind of fluid-flexible-structure coupling phenomena proves to be research challenge by involving many disciplines, including fluid dynamics, material mechanics, structural dynamics, computational mechanics and experimental mechanics. Studying these problems and collecting experimental data help researchers develop practical and efficient fluid-solid coupling numerical and theoretical methods. The research on the coupling kinematics of one or multiply flexible bodies in fluid flow can not only make us understand the nature better, but also provide technical supports for engineering application and national defense.In this dissertation, we developed a two-dimensional numerical method to study flexible-structure-fluid coupling problems. Using this method, we studied the flow-induced flapping of single and multiple deformable bodies and analyzed the dependence of the coupling mode and mechanical properties on each governing parameter. In addition, this paper verified the numerical results through the flapping experiments of flags and filaments. Finally, we studied the flow field structure around two tandem cylinders at low Reynolds number both experimentally and numerically.The main content and conclusions are as follow:1. Fluid-solid coupling flapping of a flexible body in a uniform flow.We simulated the kinematics of a cantilever beam in a moving flow using our in-house program, and investigated the flow-induced flapping frequency, amplitude and forces with different parameters. The critical velocities for different density ratios were calculated. Furthermore, we verified the accuracy of our numerical results through a flag flapping experiment in a low speed wind tunnel.2. The coupling flapping of two side by side identical flexible bodies in a uniform flow.We investigated the coupling flapping of two side by side flexible bodies in a moving flow numerically and experimentally, and summarized the affect of the arranging distance on the coupling mode, flapping frequency and amplitude, wake structure and forces of the two objects. The results revealed that, with the increasing distance, the critical velocities for two side by side flexible bodies would decrease first and then increase to the same value as in the case of a single flexible body in flow. There are four different coupling modes:in phase flapping, out of phase flapping, transition and decoupled modes. For a same distance, when the flow speed is relatively low the two flexible bodies flap in phase with a high frequency. Oppositely, they flap out of phase with a much lower frequency when the flow speed is high. Under fixed flow velocity condition, the two flexible bodies flap in phase when the distance is small and out of phase when the distance is large. One thing worth of mentioning is that, both our numerical and experimental results proved that a transition state exists between the in phase and out of phase modes, in which the flapping of the flexible bodies involves two frequencies.3. Numerical study on the coupling of two bodies in other arrangements.By simulating the interactions of two identical flexible bodies, two different flexible bodies, a flexible body and a rigid plate arranged in different manners, this paper studied the influencing mechanism of the distance on the coupling mode and drag coefficients. Our numerical results revealed that when two flexible bodies are set in line with a small distance, the flapping amplitude and the drag coefficient of the upstream body are decreased and those of the downstream body are increased obviously. The drags on both bodies decrease when two flexible bodies are closely stagger positioned. This might be a reason for fish schooling in diamond arrangement. We can also depress the body flapping by setting a short flexible body or a rigid plate either along the side or at the near downstream.4. Flapping of a filament in the bow wake and in the downstream of a cylinder.The experiments of a filament flapping in front or behind a cylinder were conducted in a flow film tunnel. Based on the experimental kinematic information we calculated the forces of the filament in uniform flow and in a Karman vortex street. The experimental results showed that, in the bow wake in front of a cylinder the filament will flap with larger amplitude and lower frequency than those in a free stream. In the Karman vortex street behind the cylinder, the filament flaps synchronously with the vortices. Numerical results revealed that the filament has the ability to derive energy and achieve thrust from the Karman vortex street. The drag coefficient depends strongly on the phase relation between the filament movement and the Karman vortex street. When the filament moves toward the vortices centers, it suffers a thrust. When it slaloms between vortices, a drag is presented.5. Experimental investigation on two tandem cylinders at low Reynolds number.We conducted experiments on two tandem cylinders in a horizontal flow film tunnel and investigated the single bluff-body (SBB), shear layer reattachment (SLR), synchronization of vortex shedding (SVS) and second vortex formation (SVF) modes with increasing distance. The SVF mode is first presented in this research and the formation of the secondary vortex in this mode was analyzed numerically. We also confirmed.the mode transitions hysteresis when the distance is increasing or decreasing continuously.