| With the perfection of the unsteady flow theory, people begin to realize that the unsteady effect plays an important role in the flight of insect with wings. A Finite Volume Method (FVM) ALE model based on unstructured mesh is developed in this thesis, which is used to solve the problem of 2D unsteady viscous flow with moving boundary. Numerical simulation is made center on flapping flight of insect with wings, trying to clarify the principle of the generation of the lift and drag force for hovering insect flight.A novel unstructured mesh ALE mathematical formulation for two-dimensional unsteady incompressible viscous flow is developed in this thesis. The finite-volume ALE Navier-Stokes solver is based on the artificial compressibility method (ACM) with a high-resolution characteristics-based scheme on unstructured grids. A five-stage Runge-Kutta time integration algorithm is used between each physical time step to iterate the numerical solution in pseudo time until convergence is reached. ALE technique is adopted to keep the topological structure of the computational grid unchanged and exponential damping function is made for moving mesh, which makes the grid very rigid near the wall and soft far away from the wall.To validate and assess the ALE model presented in this thesis, flow around an oscillating circular cylinder is simulated and comparisons with experimental data and existed numerical results are made. It shows that the results got from the ALE model agree well with the experimental data and the problem of low-Reynolds-number flow around an oscillating circular cylinder can be simulated exactly. Meanwhile, the phenomenon of flow switching is captured for Re=210 and KC=6 using the ALE model, which agrees well with those discoveries in experiments. The case of Re=210 and KC=10 is simulated as well and the conclusion is reached that the vortices shed away from the cylinder at an angle of approximately 27° with respect to the axis along which the cylinder oscillated, which may be caused by the change of lift and drag force on the cylinder surface.Based on the research work above, the proposed ALE model is used to simulate the aerodynamic of the two-dimensional insect hovering in order to analyze the principle of the generation of the lift and drag force of flapping wing. With the ALE model, there are totally three elliptical foil simulation cases with different axial ratios 2:1,5:1 and 10:1, respectively flow patterns around oscillating foils and force situation are investigate. In the case of Re=100 and Ao=3.198, the amplitudes of Cl become smaller nonlinearly with the increasing of elliptical ratios of foils, and the amplitudes of Cd become larger nonlinearly with the increasing of elliptical ratios of foils. The tendency and amplitude of lift and drag change gently with 5:1 and 10:1. The lift of 2:1 is twice as big as the lift of 5:1 and 10:1. The lift of 2:1 is caused by the large area of 2:1 on the direction of the lift. Finally, for flapping wing flight, the lift is very high with the leading edge vortex adhered to the flapping wing in the half cycle. When the wake spread along the surface of flapping wing, the lift force increases. Flapping wing flight consists of two movements:translation and rotation. Translation of flapping provide power for hovering flight, rotation of flapping wing enhances the ratio of lift and drag coefficient, which leads to the formation of leading edge vortex and trailing vortex. After that, the influence of the lift and drag coefficient is analyzed by the movement model and the amplitude. The life increases with advanced model, while the drag decreases. For the delayed model, the change of life and drag is opposite with advanced model. On the other hand, with the increase of amplitude, lift increases and drag decreases. when the amplitude is greater than 2.229c,the effect of amplitude is reduced for the lift and drag. |
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