Research And Application Of Topology-Changeable Dynamic Unstructured Grid Generation Technique

The simulation of compressible flows with large-scale moving and deforming boundaries is still one of the most important focuses of Computational Fluid Dynamics (CFD) researches currently. Traditional generation methods of unstructured dynamic grids are able to cope with most of the cases, however, one of the main challenges of dynamic grids is topological changes in geometry, such as the collapse or collision of objects, the fracture of object, the breakup of droplets or bubbles and the movement of a projectile traveling through the muzzle. With the support of National Science Foundations, a new topology-changeable dynamic adaptive grid generation technique is proposed in the thesis for problems of large-scale moving and deforming with topological changes in geometry and applied into the simulation of unsteady flows with gas-solid interfaces involving geometric deformation or relative movement, compressible multi-material flows with immiscible gas-liquid interface and complex reactive flows with moving boundaries; a general purpose CFD software code package is developed by using C++. The main contens are presented as following:A new topology-changeable dynamic adaptive grid generation technique is proposed for simulating the problems with large-scale moving and deforming boundaries with topological changes in geometry, in conjection with Spring Analogy Method and local grid regeneration method, which has been confirmed to be robust and efficient. In the thesis, an unstructured grid generator is developed by using a Delaunay triangulation algorithm or Advancing Front Technique (AFT) to create high-quality meshes quickly and easily, and an adaptive refinement/un-refinement strategy is implemented on this grid framework. Relatively, Delaunay triangulation algorithm is more faster and AFT performs better in local re-meshing process. To avoid the mesh distortion around moving boundaries, Spring Analogy Method is employed to propagate the boundary perturbation into the whole solution domain, local grid regeneration method is adopted to improve the grid quality in the region with serious grid distortion, boundary edge splitting and merging technique is developed to optimize the distribution of the boundary points and boundary re-constructing approach is proposed to deal with the topological change. The dynamic mesh generation technique performs well in the applications to merge or split solid moving boundaries or free boundaries.The topology-changeable dynamic grids generation technique proposed has been applied into the simulation of unsteady compressible flows with deforming and moving gas-solid boundaries and the blast risk assessment. The ALE form of governing equations is derived based on dynamic grids, the geometrical conservation law (GCL), the finite volume method and HLLC Riemann solver. A fourth-order Runge-Kutta scheme is employed for the time integration, and Godunov scheme or space vector approach is used to provide second order accuracy in space. Coupled multi-body dynamics, numerical method for unsteady flow is developed. The proposed numerical treatment is discussed thoroughly and validated against analytical solutions, which include the Riemann problem, the Emery test case, the impulse advancing and withdrawing of piston, the slow compress process of limited space, interaction of shock with cylinder and vibration of NACA0012airfoil, illustrating the validation of numerical methods. The numerical methods are also applied into the simulation with deformable moving boundaries successfully, including bursting of pressurized vessel, explosion in limited room with fragment (2D/3D).The surface tension on the material interface is considered in the deducing of Riemann solver of compressible multi-material flow, based on which, the material interface is tracked accurately and the muti-material flow problem is converted into independent single material flow problems which would be solved by using the dynamic grids and ALE solver. As a typical contact, immiscible material interface with surface tension satisfies Rankine-Hugoniot jump conditions and the points on which in the unstructured mesh move as an Lagrange surface by solving the Riemann problem, during which both of stiffened gas equation of state and arbitrary equation of states are considered.By imposing Modified Ghost Fluid Methos (MGFM), the original ALE interface tracking method is improved for suppressing spurious oscillations on material interface. The continuity of pressure and normal velocity are ensured by the employment of Rankine-Hugoniot conditions, however, material properties on both sides of the interface play an important role to determine the final interfacial dynamic parameters and entropy. With the employment of MGFM, isentropic fixing with predicted entropy is also considered in the ghost fluid point, together with the intermediate states obtained from multi-material Riemann problem. As a result of which, the numerical solution of single material flow problems could keep consistent with the former multi-material flow problem.Topology-changeable dynamic adaptive grids technique proposed has been extended into the application of ALE method in compressible multi-material flow, and deformation and breakup of bubble induced by shock is simulated. Although the motion of material interface could be tracked accurately, ALE method encounters serious difficulty in the situation of mesh distortion, split or merge of moving boundaries. With the employment of topology-changeable dynamic adaptive grids technique proposed and numerical method for compressible multi-material flow, the deformation of bubble is tracked accurately, breakup process is presented perfectly, and development of flow filed and broken bubbles is simulated continuously. The second jet and breakup are confirmed in in the computational result, which match the phonemes observed in experiment.The dynamic moving mesh is also applied into the simulation of complex reactive flow. The elementary reaction model and corresponding numerical treatments are used to describe processes of chemical reactions in multi-species system, and ALE governing equations is also extended into chemical reaction flows and solved by using a time splitting method. Good agreement between numerical predictions of test cases and theoretical results or experimental data indicates that the numerical treatment proposed in the thesis is feasible for simulating multi-species reacting flows. Detonation interaction with wedges, shock-induced combustion around projectile traveling at hypervelocity, projectile traveing through the muzzle and base bleed are discussed.