Generation of unstructured grids for viscous flow applications

Abstract

A procedure is presented for efficient generation of high-quality unstructured grids suitable for viscous flow applications. The present procedure uses an iterative point creation and insertion scheme wherein points are created using either advancing-front type or advancing-normal type point placement. Advancing-normal point placement is used to generate high aspect ratio elements. Initially, the connectivity for these generated points is obtained by directly subdividing the element which contains them, without regard to quality. This connectivity is then improved by iteratively using local reconnection subject to a quality criterion. For two-dimensions, a min-max criterion is used and for three-dimensions, a Delaunay in-sphere criterion followed by a min-ma% type criterion is used. The overall procedure is applied repetitively until a complete field grid is generated with a desired point distribution. In two-dimensions, triangular elements in the viscous regions are optionally combined to form quadrilateral elements. Results are presented for a variety of two-dimen-& sional configurations. Preliminary results are also presented for extension of the present viscous grid generation methodology to three-dimensions. The results demonstrate that highquality unstructured grids suitable for inviscid or viscous flow applications can be generated about geometrically complex configurations.

Development and Application of New Algorithms for the Simulation of Viscous Compressible Flows with Moving Bodies in Three Dimensions

“…In addition, a blending procedure to transition smoothly between these zones was provided. Typically, the stretched mesh region was generated first (Kallinderis, 1992;Löhner, 1993; Pirzadeh, 1994;Marcum, 1995;Pirzadeh, 1996). Although we have used such a scheme (Löhner, 1993) for a number of years, we have found several situations in which the requirement of a semi-structured element or point placement close to wetted surfaces is impossible, prompting us to search for a more general technique.…”

“…The same applies to turbulent flows simulated using the Reynolds-Averaged Navier-Stokes (RANS) equations with suitable turbulence models. The reliable generation of high quality grids for RANS simulations has been attempted with varying degrees of success by several authors during the last decade (Nakahashi, 1987;Kallinderis, 1992;Löhner, 1993; Pirzadeh, 1994;Marcum, 1995;Pirzadeh, 1996; Morgan, 1996-1998). Given that the generation of highly stretched grids is not trivial, and that computing power is increasing steadily, the immediate question that comes to mind is when RANS simulations will be replaced by Large-Eddy Simulations (LES) or even Direct Navier-Stokes (DNS) simulations.…”

“…However, they fail when attempting to generate highly stretched elements, a key requirement for Reynolds-Averaged Navier Stokes (RANS) calculations with turbulence models that reach into the sublayer. A number of specialized schemes have been proposed to remedy this situation (Nakahashi, 1987;Kallinderis, 1992;Löhner, 1993; Pirzadeh, 1994;Marcum, 1995;Pirzadeh, 1996;Peraire and Morgan, 1996). The domain to be gridded was divided into isotropic and stretched element regions.…”

Assessment of Hybrid RANS/LES Turbulence Models for Aeroacoustics Applications

“…As mentioned earlier, this structured grid could not be used directly in the FUN3D unstructured grid code because of the hanging nodes in the patched (non point-matched) regions of the grid. Instead the AFLR2 grid generation code 17 was used to create the unstructured grids used here. A sectional plot of the unstructured grid is shown in Fig.…”

“…However, we encountered numerical difficulties in obtaining FUN3D solutions on this grid with two-equation based turbulence models. As an alternative, a two-dimensional hybrid grid consisting of a structured grid in the boundary layer regions surrounding the cylinders and then transitioning into unstructured grid was created using the AFLR2 17 grid generator. The grid spacing near the cylinder surfaces is approximately 0.0002 D, as was the case for the structured grid.…”