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Dr. Neal T. Frink, Principal Investigator
NASA Langley Research Center
Co-Investigators: Dr. Shahyar Pirzadeh, NASA Langley Research Center, and Dr. Paresh Parikh, Paragon Research, Inc.
To calibrate an emerging unstructured Navier-Stokes capability within TetrUSS, a Tetrahedral Unstructured Software System, on an unconventional aircraft configuration operating at transonic, separated flow conditions.
TetrUSS is an unstructured-grid computational fluid dynamic (CFD) software system which is being used by a broad range of users for performing rapid aerodynamic analysis and design of complex configurations. While its Euler capability is well established, the Navier-Stokes component is maturing rapidly under extensive validation testing.
For the present computation, a thin-layered tetrahedral grid of 1,062,928 cells (186,936 nodes) was generated for a joined-wing configuration with the VGRIDns unstructured tetrahedral grid generator. Grid size is kept small by utilizing anisotropic cell stretching which results in a third less cells than an isotropic grid with similar chordwise resolution. The grid was completed in three days.
Turbulent flow computations were performed at several transonic separated flow conditions for wind-tunnel Reynolds number using the tetrahedral cell-centered upwind Navier-Stokes flow solver USM3Dns. Turbulence is modeled in the intermediate logarithmic layer of the boundary layer by the Spalart-Allmaras one-equation model, and in the inner region by a wall function. By exploiting the wall function, a considerable savings in grid size, hence core memory, is achieved while retaining reasonable solution accuracy. The normal grid spacing was sized to yield a nominal midchord y-plus of 50 for the first node above the surface and 18 to 20 tetrahedra across the boundary layer.
The first figure depicts the surface triangulation and representative "oilflow" patterns on the joined-wing configuration. Significant shock-induced and trailing-edge flow separations are observed on the fore and aft wings. Severe flow reversal is evident on the tip panel. The surface flow patterns are in good agreement with experimental oilflow data (not shown) obtained in the NASA Langley Research Center 16-Foot transonic tunnel. The second figure portrays the longitudinal pressure coefficient (Cp) distribution along three representative spanwise stations of the fore and aft wings, and tip panel. The computed pressures are in good agreement in each flow region with the experimental data.
The computation required 188MW of memory and 8 hours on the CRAY C-90.
This study represents a significant contribution toward a broader goal of validating a next-generation CFD methodology for rapid and cost effective Navier-Stokes analysis and design of complex aerodynamic configurations. The primary motivation for this new technology is rapid grid generation on the order of days with solution accuracies comparable to established structured-grid methodology.
Unstructured Navier-Stokes computations are presently underway on a variety of complex geometries, including an F-16 fighter aircraft with fuel tank and finned store. The current focus is to validate this new capability for a broad range of complex geometries and flows in order to elevate user confidence.
1. Frink, N. T., "Assessment of an Unstructured-Grid Method for Predicting 3-D Turbulent Viscous Flows" , AIAA 96-0292, January 1996.
2. Pirzadeh, S.: "Progress Toward A User-Oriented Unstructured Viscous Grid Generator", AIAA Paper 96-0031, January 1996.
Figure 1: Surface triangulation and "oilflow" patterns for a joined-wing
configuration. magenta = low pressure and green = high pressure. - 68k color gif
