MACGS - A zonal grid generation system for complex aero-propulsion configurations

Abstract

A comprehensive grid generation system has been developed which enables grid generation for complex aircraft configurations. This system is especially suited for generation of zonal grids without point continuity at zoneto-zone interfaces. This approach reduces the complexity of the grid generation process by allowing grids to be generated with fewer zones, and fewer unnecessary grid points, thereby reducing solution time and cost. The capabilities of the system include geometry acquisition and manipulation, grid generation on the boundaries of each zone, three-dimensional field grid generation, grid quality evaluation for the field grid, boundary condition specification, coupling parameter identification for zone-to-zone interfaces, and output directly compatible with a zonal Euler or Navier-Stokes flowfield analysis code. The system operates in an interactive environment through a userfriendly graphical interface running on several computer platforms with numerous graphics devices. In order to further improve the efficiency of the grid generation process, the system utilizes several macro operations to perform sequences of steps ranging from generating various farfield boundary shapes to generating an entire zone for an isolated forebody or diffuser type geometry. A prototype expert system has also been implemented to assist the user in identifying and correcting flaws in the field grid. Applications of the system to inlet-forebody analyses, a duct with turning vanes, and a thrust-vectoring nozzle configuration will be presented. I n t r o d u c t i o n Numerical solution of the Navier-Stokes equations is becoming increasingly important for the analysis of advanced propulsion systems and their integration with an airframe. The ability to generate a suitable grid is often the pacing i tem in obtaining solutions for these complex configurations. High quality grids are necessary to accurately predict flowfields. High quality grids a180 increase the convergence rate of the solution, thereby re* Lend Engineer Aemdynsmics ** Technical Specisiixt Aerodyrmnics t Senior Engineer Aerodynamics t Technical Specialist Propulsion 5 Senior Engineer Propulsion Copyright @ 1991 by McDonnell Douglas Cormration Published by the American Institute 01 Aeronautics and Astronautics, Inc. with permission. ducing computer resource requirements. The grid must accurately represent the geometry. It must also be tnilored to the flowfield, Le., i t must provide high density near solid boundaries, shock locations, and other regions where high Bowfield gradients may be experienced. Any flaws in the grid must be identified and eliminated before the grid is used in the flowfield solution algorithm or else costly regridding and repeated analysis will be required. The McDonnell Aircraft Computational Grid System (MACGS) was developed to address these issues. It uses an interactive zonal approach which does not require point continuity a t zone-to-zone interfaces. The zonal approach maximizes flexibility and simplifies the grid gcneration problem. In a zonal approach, the physical domain around the configuration is subdivided into simpler regions 01 zones. These zones are non-overlapping and ., boundary conforming. A zonal approach dlows various strategies for subdividing the field into zones or the use of a single zone, where appropriate. Current zonal grid generation systems, such as EAGLE (Reference 1) and GRIDGEN (Reference Z), are highly focused on generation of grids with point continuity at the zonal interfaces. Although these grid generators are very versatile and allow zones without point continuity to be generated independently, these systems lose some of their power for such cases. They do not address how adjacent zones will be treated. MACGS generates grids without point continuity at the zonal interface and computes the interpolation factors t o allow flowfield da t a to be transferred between zones. Lack of point continuity restrictions makes grid generation much more flexible because the user can distribute points where they are needed, based on flow properties within the zone rather than on where points were distributed i n an adjacent zone. This allows the user t o have as many grid points as required in a critical flowfield region, such as a boundary layer, with far fewer points in a less critical adjacent zone, such as the region upstream of an inlet. The zonal approach without point continuity was previously implemented for specific configurations in the McDonnell Aircraft Company (MCAIR) three-dimensional batch grid generator, ZGRID (Reference 3), and has been used successfully with the MCAIRzonal three-dimensional Euler or Navier-Stokes flow solver, NASTD (Reference 4). Interactive methods provide improved grids by giving the user more direct control of and feedback from the i /