Numerical Simulation Technique Research For Unsteady Multi-Body Flowfield Involving Moving Boundaries

Numerical methods using dynamic unstructured meshes for time-accurately solving unsteady multi-body flow problems involving moving boundaries are implemented.A MUSCL type cell-centered finite-volume scheme is presented for solving the 3-D time-dependent Euler equations cast in an Arbitrary Lagrangian-Eulerian (ALE) framework. The flux across each cell face is calculated using a flux-vector splitting technique. Higher-order scheme is obtained by expanding the cell-centered solution to each cell face using a linear reconstruction formula based on geometrical invariant features of triangles and tetrahedra, which reduces computer memory requirements. A simple limiter similar to the one of Earth and Jespersen is presented to eliminate unphysical oscillations near discontinuities. Solutions are advanced in time by explicit four-stage Runge-Kutta integration with up to second-order temporal accuracy. To avoid grid motion induced error, the geometric conservation law (GCL) is satisfied numerically. For the purpose of simulating aerodynamically determined body motion, the governing equations of rigid body dynamics are coupled in the overall solution algorithm.Mesh movement strategy is implemented by the combination of mesh deforming and local remeshing, but just using mesh deforming is sufficient for cases with relative small boundary displacement. The model used to control mesh deforming is improved spring analogy with boundary improvement and torsional effect improvement. For cases with relative large boundary displacement, a window is created enclosing the moving body in which the mesh elements are specified to be deforming adapted to the body motion. The window boundary may be extracted efficiently by the so-called nearest-neighbour-search algorithm. When severely distorted elements appear, the meshes in the window are regenerated by the advancing-front method and flow properties are interpolated from the old mesh to the new one linearly. Attaching mesh generation sources to the moving boundary allows a reasonable mesh distribution after remeshing. Quadtree data structure is used to accelerate the searching operation during interpolation.Results of mesh deforming about a rotating airfoil show that the new spring analogy greatly improves the deforming ability and mesh quality without lowering the efficiency. The same capability of the mesh deforming method in 3-D is validated by simulating the interaction of propagating shock and two cubic objects. This deforming method coupled with ALE Euler solver is applied to simulate unsteady transonic flow about oscillating airfoil (2-D) and oscillating rigid rectangular wing (3-D). Computational results are in good agreement with experimental data. The whole dynamic mesh strategy including deforming and remeshing and the overall flow solver coupled with rigid body dynamics equations are validated by simulating the free fall of a 2-D store from a wing section. It is the high performance of the mesh deforming method that reduces the remeshing requirements and interpolation induced diffusion loss, hi addition, as a new application of dynamic unstructured mesh technique, a method for direct numerical determination of trim angles of attack for space vehicles is presented and verified.