Dynamic Hybrid Grid Generation Method For Complex Geomtries And Numerical Method For Unsteady Compressible Flows

The next generation of fighter aircraft is being designed to fly at very high angles of attack as an inherent part of both offensive and defensive maneuvering. To achieve this increasing demand for super maneuverability and controllability, the designer should firstly understand the unsteady aerodynamics and fluid mechanism of the flight vehicles.Generally, the unsteady movements can be classified into following three categories, due to the manner of movement. The first type is that the body remains stationary, but the flow field over it is unsteady itself, such as flow separation in the leeward of slender bodies at high angles of attack. The second type is that the body considered moves rigidly (relative to the case of morphing/deforming), such as aircraft pitching, wing-rock and their coupling. The third one is the cases with geometric morphing/deforming or multi-body separation, such as dynamic aero-elastics of aircrafts, exterior/interior weapon launching, fairing separation, fish swimming and insect/bird wing flapping, and so on. The second and the third types of unsteady flow problems widely exist in fields of aeronautics, astronautics and bio-fluid mechanics.For numerical simulations of the later two types of unsteady problems with moving and/or morphing boundaries, the computational grids should move and/or deform with the moving/morphing boundaries. Moreover, efficient unsteady flow solver should be set up to match the dynamic grids. So in this thesis, an efficient dynamic hybrid grid generation technique is proposed for two-dimensional and three-dimensional complex configurations with morphing/deformation or relative body movement. Meanwhile, an unsteady finite volume solver is developed to simulate the unsteady flows, based on the dynamic hybrid grids.In previous literatures, the pure unstructured grid technique was mostly adopted to generate moving grids, because of the flexibility for complex geometries. However, some disadvantages come with it as well. For example, it is hard to generate high aspect ratio mesh near to the solid wall for viscous flow simulations, and hard to reach fine mesh quality to decrease the simulation accuracy of viscous effects. For unsteady flow simulations with great demand for computer resources, the moving hybrid grids should be a better choice for complex configurations, because they combine the advantages of structured and unstructured grids, which had been proven in numerical simulations for steady cases over complicate geometries.The most straightforward approach is to deform the computational grids locally using a'spring-analogy'type algorithm to follow the motion of the moving boundaries. However, the'spring'relaxation of grid points is limited, so this kind of methods can only deal with the problems with small motion. Recently, Liu and Qin developed a novel method based on'Delaunay graph'mapping. This method can generate high-quality dynamic grids efficiently for those problems with moderate displacement or distortion, because it can keep the node-connectivity topology within acceptable movement range. Despite of this improvement, this method still suffer from some difficulties to generate dynamic grids for very large motion problems, especially for multi-body separation with very large displacement. To deal with these kinds of problems, integrating the'Delaunay graph'mapping with local-remeshing approach may be a better choice. That is the main point of present thesis.There are six chapters in this thesis as follow.Chapter I is the introduction, in which the research background is discussed briefly, and the state-of-arts of static/dynamic grid generation methods are reviewed, and then the work of this paper is introduced.In Chapter II, the dynamic hybrid grid generation technique is presented in details, including the main idea and the strategy of present hybrid mesh generation method, the stationary hybrid mesh generation approach, the'Delaunay graph'mapping method and the mixed method of'Delaunay graph'mapping and local remeshing.A series of 2D and 3D test cases are listed in Chapter III to validate present moving hybrid grid generator, such as 2D fish swimming and multiple fish schooling, simple airfoil pitching, flap deforming of multi-element airfoils, the'clap-fling'motion of tiny insect wings. Three-dimensional test cases include Hummel delta-wing pitching, dual delta-wing pitching, Clipper-like capsule pitching, and store separation from a wing, etc.The unsteady flow solver is presented in Chapter IV based on the moving hybrid grids in Chapter II and III. The well-known dual-time stepping method is introduced into our solver to improve the efficiency of unsteady simulations, while a block LU-SGS implicit method is adopted in sub-iteration to accelerate the convergence history. Moreover, the geometric conservation law (GCL) is considered also.In Chapter V, some typical steady and unsteady cases are simulated to validate the unsteady flow solver, including steady flow over airfoils and delta-wing, and unsteady flows over pitching airfoils in inviscid and viscous cases, and a moving sphere with supersonic speed.The last chapter is the concluding remarks.