The spatial error due to non-conservative interpolation becomes first-order when the second-order conservative schemes are used and the overlapped length of one region is not nearly same as that of the other, even if discontinuities are located away from the overlapped regions. Therefore, the global solution accuracy can be ensured if two domains overlap each other with a fixed number of grid points and the interpolation is occurred in smooth flow regions.
To satisfy the above conditions, a new systemized procedure for chimera domain decomposition is presented. This procedure consists of a new cut-paste algorithm for optimal mesh interface and a two-step search method for donor cell identification. It is fully automated and requires minimal user input. The cut-paste algorithm is based on the advancing front technique in which the front are iteratively determined from initial fronts, which are a collection of fringe points obtained from conventional chimera hole-cutting. The final fronts are determined iteratively in such a way that the overlapping region is minimized. With this method, interpolation points are located away from solid walls where the flow gradient is high. This method also can reduce the error which may arise from interpolation in stiff gradient regions.
The method couples a highly robust CFD method and the cut-paste algorithm for the chimera domain decomposition in order to enhance the efficiency of store trajectory simulation. All procedure of chimera techniques are parallelized on the Cray T3E using MPI library. The time-step size limitation due to the grid movement is relaxed with the interpolation of solutions in the previous time step. A treatment for orphan cells is also devised to improve the robustness of the method. The computational results show that the current method is capable of simulating the store separation without user interruption and that a time-step size of 0.005 sec, which is more than three times larger than ever reported, can be used for the Eglin wing/pylon/store seapration problem.
To compute aerodynamic coefficients on structured overlapped surface grids, present cut-paste algorithm is extended. The structured non-overlapped surface grids with minimal gaps are constructed using a cut algorithm which is devised in this paper. The construction of minimal gaps are critical to accurately compute the aerodynamic coefficients. The resultant gaps are filled with unstructured grids which are strings of triangular cells that conform the original surface. A hybrid composite surface grid which consists of non-overlapped quadrilaterals and triangles is formed, where the quadrilaterals are un-blanked structured cells in the original surface grids and the triangules are newly generated triangular cells using boundary points of the un-blanked structured grids in the gaps. The flow variables which are known at the hybrid surface cells are used in computing of the areodynamic coefficients. To validate the present procedure, a steady ONERA M6 wing and the unsteady Eglin wing/pylon/store test cases are solved.
