Numerical Investigation Of Fluid And Flexible-thin-plate Interaction

Flow-structure interaction problems widely exist in nature and engineering. A com-prehensive study of such problems remains a challenge due to their strong nonlinearity and multidisciplinary. The numerical methods and the relevant numerical investigation of fluid-structure interaction are important for understanding fundamental mechanisms. In this thesis, a novel numerical method based on hybrid algorithms is developed, and some topics on interaction between deformable thin-plate structure and viscous fluid are studied. The results and conclusions are briefly given as follows:(1) An immersed boundary-lattice Boltzmann-finite element method based on hybrid algorithms is developed to solve the fluid-structure interaction problems involving flexible thin-plate structures. In this method, the fluid flow and plate motion are described by the incompressible Naiver-Stokes equation and structural equation with large displacement, large rotation but small strain, respectively, which are numerical solved by lattice Boltzmann method and finite element method, re-spectively. The coupling is implemented by an immersed boundary method using penalty parameters. This method has high efficiency and robustness, and can handle a variety of fluid-structure interaction problems. The method and code are validated by some test problems.(2) The dynamic responses of the filament in the wake of the cylinder are numeri-cally studied. It is revealed that there exist two flapping states of the filament depending on the cylinder-filament separation distance and the relevant critical distance distinguishing the two states is associated with the Reynolds number and the filament length. It is also found that the drag coefficient of the cylinder is reduced but that of the filament may be increased or decreased depending on its length. Compared with a single filament in a uniform flow, the filament of the same mechanical properties flapping in the wake of the cylinder has a lower frequency and a larger amplitude.(3) The locomotion of a flapping flexible plate in a viscous incompressible stationary fluid is numerically studied. When the leading-edge of the flexible plate is forced to heave sinusoidally, the entire plate starts to move freely as a result of the fluid-structure interaction. Mechanisms underlying the dynamics of the plate are elucidated. Three distinct states of the plate motion are identified and can be described as forward, backward, and irregular. Which state to occur depends mainly on the heaving amplitude and the bending rigidity of the plate. In the forward motion regime, analysis of the dynamic behaviors of the flapping flexible plate indicates that a suitable degree of flexibility can improve the propulsive performance. Moreover, there exist two kinds of vortex streets in the downstream of the plate which are normal and deflected wake. Further the forward motion is compared with the flapping-based locomotion of swimming and flying animals.(4) The dynamics of fluid flow over a circular flexible plate are numerically studied. As the plate is clamped at its center and placed in a uniform flow, the deformation of plate is induced by flow-induced loads on it. Mechanisms underlying the dynamics are elucidated. A series of distinct deformation modes of the plate are identified in terms of the azimuthal fold number from axial symmetry to multi-fold deformation patterns. The developing process of deformation modes is analyzed and both steady and unsteady states of the fluid-structure system are noticed. The drag reduction due to reconfiguration and the elastic potential energy due to flexibility are investigated. Moreover, theoretical analysis is carried out and is helpful in understanding the physical mechanisms on the plate deformation modes.