Unsteady Numerical Method Based On Dynamic Hybrid Mesh And Simulation Of Bio-fluid Flows

There are a great number of fishes and insects with various species in the world. Their ability for swimming or flying is much greater than any human machine all over the world. Lots of fluid-dynamics scientists had been absorbed into this field because of the undiscovered mystery of mechanism of their swimming or flying.Numerical simulations have become one of the main and important methods for studying fluid dynamics along with the development of computer technology. For bio-fluid flows, experiments and observations are difficult because of the self-dependent movement of the body. However, numerical simulations, which don't meet the problem above, have got quite great progress. Here in this paper, we expect to set up an effective numerical method to study the hydrodynamics and/or aerodynamics of swimming and/or flying animals.For fish and insects, their bodies deform flexibly with swimming or flying, and the flows around them are strongly unsteady. To simulate the unsteady flows over swimming fishes and flying insects, dynamic grid technique and corresponding unsteady flow solver should be set up firstly. Based on the previous work of the author's advisor on 2D dynamic hybrid grid generation for rigid bodies, a dynamic hybrid grid generator is presented in this paper for morphing bodies. Firstly, the initial quadrilateral/Cartesian/triangular hybrid grids are generated to cover the computational domain. With the motion of morphing bodies, the body-fitted quad grids deform with the bodies with same grid topology, while the adaptive Cartesian grids in the outer field remains stationary. Meanwhile, the triangular grids between them are relaxed according to the motion of morphing bodies with a"spring"analogy approach or an interpolation strategy based on Delaunay graph proposed by Liu and Qin. If the triangular grids become too skewed, or even the adaptive Cartesian grid crosses into the quad grids, the triangular grids are regenerated. On the other hand, an implicit solver based on the dynamic hybrid meshes is set up for unsteady incompressible flows. The solver adopts the artificial compressibility method with dual-time stepping approach. The dynamic hybrid grid generator and the incompressible unsteady flow solver are validated by some typical steady and unsteady cases, and good agreement with experimental data and other numerical results has been shown in the paper.In order to find out the mechanism for the great thrust for fishes, the undulated swimming for a fish is simulated numerically with the dynamic hybrid grid generator and the unsteady flow solver above. The effects of undulating frequency are considered, and the mechanism for the thrust is analyzed also. Furthermore, the undulated swimming for two in-line fishes with different undulating frequency, phases and distances in space is simulated and the interactions between the fore and back fishes are analyzed.Finally, the stroke for a insect wing and the"Weis-Fogh"mechanism for very tiny insects are simulated numerically. The mechanism for the great unsteady lift in the wing stroke is analyzed through the flow structure. The effect for enhancing the lift in"clap-fling"movement is analyzed too.