Zipper grids for force and moment computation on overset grids

Abstract

A scheme is developed to account for the overlapped zones on overset structured surface grids when computing forces and moments for the surface. Grid points in the overlapped zone between grids are blanked. The resulting gaps are then filled by 'zipper grids' which are strings of triangular cells that conform to the original surface. A hybrid composite surface grid is formed which consists of non-overlapping quadrilaterals and triangles, where the quadrilaterals are the unblanked cells in the original surface grids and the triangles are the newly generated triangular cells in the gaps. Pressure and viscous stresses which are known at the grid points can then be integrated accurately on the hybrid surface. Results obtained by applying the current scheme to numerical solutions are compared with data from wind tunnel tests for several complex configurations. These include the Space Shuttle Launch Vehicle, a subsonic transport, and a supersonic oblique all-wing configuration. 1. I n t r o d u c t i o n Chimera overset grid methods1 have been successfully applied in the numerical flow simulations of complex configurations such as the Space Shuttle Launch ~ e h i c l e , ~ , ~ , ~ the V-22 t i l t r ~ t o r , ~ the SOFIA Airborne O b ~ e r v a t o r ~ , ~ and many others. Very good agreement of many flow characteristics has been obtained between the numerical solution and wind tunnellflight tests. One of the most important set of quantities sought from numerical solutions is the set of force and moment coefficients for the solid surfaces in the flow field. Accurate computation of these quantities is crucial for study of aerodynamics, design optimization, and moving body problems. * Research Scientist, MCAT, Inc., Member AIAA. t Research Scientist, NASA Ames Research Center, Senior Member AIAA Forces and moments are usually computed by integrating the pressure and viscous stresses on the surface grids. Since the surface grids for overset grids are allowed to overlap arbitrarily, the regions in the overlapped zones are counted more than once if integration is performed over all the surface grids. In order for overset grid methods to be widely accepted for use in practical applications, a scheme must be available to accurately extract force and moment information from the numerical solutions. A procedure developed to solve this problem is described in this paper. The method for computing force and, moment cot efficients is broken into two main steps performed by two separate computer codes. The first step is to construct a hybrid composite surface grid consisting of non-overlapping quadrilaterals (quads) and triangles (the MIXSUR code), while the second step is to integrate the pressure and viscous stresses on the hybrid grid (the OVERINT code). Generation of the hybrid composite surface grid only needs to be performed once before the flow solver is called. Integration of pressure and viscous stresses can be performed just once a t the end of the run to compute the final force and moment coefficients. I t can also be performed every N number of iterations to supply the force and moment coefficients as indicators of convergence, or as input to a six-degrees-of-freedom rigid body dynamics package for moving body applications. In both cases, the connections between the quads and the triangles in the hybrid composite surface grid do not change with