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At least 1,261 records · Page 70

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NASA NTRS1988

Navier-Stokes calculations using Cartesian grids. I - Laminar flows

A finite-volume formulation for the Navier-Stokes equations using Cartesian grids is used to study flows past airfoils. In addition to the solution of the complete equations, solutions for two simplified versions of the governing equations were obtained and compared with those using body-fitted grids. Results are presented for two airfoil sections, NACA 0012 and RAE 2822, for a range of Mach numbers, angles of attack, and Reynolds numbers. It is shown that the results are highly dependent on the smoothness of the surface grid. Without such smoothness, the skin friction and pressure converge to nonuniform distributions. On the other hand, when surface cells with smoothly varying areas are used, the results compared favorably with calculations employing body-fitted grids.

Frymier, Paul D., Jr.↗

NASA NTRS1989

Grid refinement studies of turbine rotor-stator interaction

Results from a three-dimensional, time-accurate Navier-Stokes simulation of rotor-stator interaction in an axial turbine stage are presented. The present study uses a fine grid in the spanwise direction to better resolve endwall and tip cllearance effects and complements coarse-grid calculations that were reported earlier. A realistic turbine stage with 22 stator vanes and 28 rotor blades is simulated as a single-stator, single-rotor airfoil combination with the stator geometry modified to properly account for blockage effects. This is in contrast to the earlier coarse-grid calculations where the rotor geometry was modified. The improved grid resolution and the unmodified rotor geometry result in a more accurate simulation of the flow field, particularly in the rotor channel where the interaction effects are more severe. The numerical results are compared to experimental data wherever possible and to earlier calculations.

Madavan, N. K.↗

NASA NTRS1989

A multigrid and upwind viscous flow solver on 3-D embedded and overlapped grids

A numerically efficient method is presented for solving the three-dimensional governing equations of the viscous compressible flow about complex configurations with topologically different components. The physical domain is decomposed into regions for which the grid generation is relatively simple and virtually with no significant restrictions. The Navier-Stokes equations are solved by an implicit, approximately factored, upwind, finite-volume scheme. The block inversions and the diagonalized scalar inversions of the coefficient matrices are modified to allow the holes created in the computational domain by the embedded and overlapped grids. The convergence is accelerated by a multigrid algorithm despite the existence of such holes. The solution for s supersonic flow past a blunt-nose-cylinder at high angle-of-attack is obtained using a C-O grid embedded in a global Cartesian grid.

Baysal, Oktay↗

NASA NTRS1989

The effect of adaptive grid on hypersonic nozzle flow calculations

Adaptive grid has been applied to the numerical study of generic hypersonic nozzles for the NASP vehicle to evaluate the effect of adaptive grid on solution accuracy. Several cases are calculated with and without adaptive grids; the numerical results are compared with experimental data wherever available, and with numerical results from other researchers. The present work shows that in most situations, especially when free shear layers and shocks exist in the flowfield, adaptive grid is essential in improving solution accuracy.

Hsu, Andrew T.↗

NASA NTRS1990

Adaptive grid embedding for the two-dimensional flux-split Euler equations

A numerical algorithm is presented for solving the 2-D flux-split Euler equations using a multigrid method with adaptive grid embedding. The method uses an unstructured data set along with a system of pointers for communication on the irregularly shaped grid topologies. An explicit two-stage time advancement scheme is implemented. A multigrid algorithm is used to provide grid level communication and to accelerate the convergence of the solution to steady state. Results are presented for a subcritical airfoil and a transonic airfoil with 3 levels of adaptation. Comparisons are made with a structured upwind Euler code which uses the same flux integration techniques of the present algorithm. Good agreement is obtained with converged surface pressure coefficients. The lift coefficients of the adaptive code are within 2 1/2 percent of the structured code for the sub-critical case and within 4 1/2 percent of the structured code for the transonic case using approximately one-third the number of grid points.

Warren, Gary Patrick↗

NASA NTRS1990

Applications of algebraic grid generation

Techniques and applications of algebraic grid generation are described. The techniques are univariate interpolations and transfinite assemblies of univariate interpolations. Because algebraic grid generation is computationally efficient, the use of interactive graphics in conjunction with the techniques is advocated. A flexible approach, which works extremely well in an interactive environment, called the control point form of algebraic grid generation is described. The applications discussed are three-dimensional grids constructed about airplane and submarine configurations.

Eiseman, Peter R.↗

NASA NTRS1990

Generation and adaptation of 3-D unstructured grids for transient problems

Grid generation and adaptive refinement techniques suitable for the simulation of strongly unsteady flows past geometrically complex bodies in 3-D are described. The grids are generated using the advancing front technique. Emphasis is placed not to generate elements that are too small, as this would severely increase the cost of simulations with explicit flow solvers. The grids are adapted to an evolving flowfield using simple h-refinement. A grid change is performed every 5 to 10 timesteps, and only one level of refinement/coarsening is allowed per mesh change.

Loehner, Rainald↗

NASA NTRS1990

The fundamentals of adaptive grid movement

Basic grid point movement schemes are studied. The schemes are referred to as adaptive grids. Weight functions and equidistribution in one dimension are treated. The specification of coefficients in the linear weight, attraction to a given grid or a curve, and evolutionary forces are considered. Curve by curve and finite volume methods are described. The temporal coupling of partial differential equations solvers and grid generators was discussed.

Eiseman, Peter R.↗

NASA NTRS1988

Three-dimensional adaptive grid generation for body-fitted coordinate system

This report describes a numerical method for generating 3-D grids for general configurations. The basic method involves the solution of a set of quasi-linear elliptic partial differential equations via pointwise relaxation with a local relaxation factor. It allows specification of the grid spacing off the boundary surfaces and the grid orthogonality at the boundary surfaces. It includes adaptive mechanisms to improve smoothness, orthogonality, and flow resolution in the grid interior.

Chen, S. C.↗

NASA NTRS1988

GENIE - Generation of computational geometry-grids for internal-external flow configurations

Progress realized in the development of a master geometry-grid generation code GENIE is presented. The grid refinement process is enhanced by developing strategies to utilize bezier curves/surfaces and splines along with weighted transfinite interpolation technique and by formulating new forcing function for the elliptic solver based on the minimization of a non-orthogonality functional. A two step grid adaptation procedure is developed by optimally blending adaptive weightings with weighted transfinite interpolation technique. Examples of 2D-3D grids are provided to illustrate the success of these methods.

Soni, B. K.↗

NASA NTRS1991

Generating Three-Dimensional Grids About Anything

Three-Dimensional Grids About Anything by Poisson's Equation (3DGRAPE) computer program designed to make computational grids in or about almost any shape. Generated by solution of Poisson's differential equations in three dimensions. Program automatically finds its own values for inhomogeneous terms giving near-orthogonality and controlled grid-cell height at boundaries. Grids generated applied to both viscous and inviscid aerodynamic problems, and to problems in other areas of fluid dynamics. Written in 100 percent FORTRAN 77.

Sorenson, Reese L.↗

NASA NTRS1991

Adaptive Grids For Computations Of Three-Dimensional Flows

Self-adaptive-grid method devised and enhanced for efficient computation of complicated three-dimensional flows. Extension of method developed for two-dimensional flows. Fictitious springs connecting points of computational grid used to determine optimum grid spacing. Each point connected to neighboring points by tension and torsion springs. Stiffnesses of tension springs related to flow-field quantities. Stiffnesses of torsion springs resist departure of grid from orthogonality.

Djomehri, M. Jahed↗

NASA NTRS1990

A three-dimensional application with the numerical grid generation code: EAGLE (utilizing an externally generated surface)

Program EAGLE (Eglin Arbitrary Geometry Implicit Euler) is a multiblock grid generation and steady-state flow solver system. This system combines a boundary conforming surface generation, a composite block structure grid generation scheme, and a multiblock implicit Euler flow solver algorithm. The three codes are intended to be used sequentially from the definition of the configuration under study to the flow solution about the configuration. EAGLE was specifically designed to aid in the analysis of both freestream and interference flow field configurations. These configurations can be comprised of single or multiple bodies ranging from simple axisymmetric airframes to complex aircraft shapes with external weapons. Each body can be arbitrarily shaped with or without multiple lifting surfaces. Program EAGLE is written to compile and execute efficiently on any CRAY machine with or without Solid State Disk (SSD) devices. Also, the code uses namelist inputs which are supported by all CRAY machines using the FORTRAN Compiler CF177. The use of namelist inputs makes it easier for the user to understand the inputs and to operate Program EAGLE. Recently, the Code was modified to operate on other computers, especially the Sun Spare4 Workstation. Several two-dimensional grid configurations were completely and successfully developed using EAGLE. Currently, EAGLE is being used for three-dimension grid applications.

Houston, Johnny L.↗

NASA NTRS1991

Implementation of control point form of algebraic grid-generation technique

The control point form (CPF) provides explicit control of physical grid shape and grid spacing through the movement of the control points. The control point array, called a control net, is a space grid type arrangement of locations in physical space with an index for each direction. As an algebraic method CPF is efficient and works well with interactive computer graphics. A family of menu-driven, interactive grid-generation computer codes (TURBO) is being developed by using CPF. Key features of TurboI (a TURBO member) are discussed and typical results are presented. TurboI runs on any IRIS 4D series workstation.

Choo, Yung K.↗

NASA NTRS1989

Local grid refinement for transonic flow problems

The present use of locally refined Cartesian grids to solve transonic flow problems about three-dimensional aircraft configurations obviates surface-conforming grid generation through an embedding of surface-geometry paneling in the grid. Accurate resolution of flow close to the boundary, and in regions with strong velocity gradients, is achieved via hierarchical local refinement which subdivides a given grid cell into eight cells. Fast and reliable convergence is obtained by combining several preconditioners and damping strategies. Methods are suggested for preclusion of global convergence problems.

Melvin, Robin G.↗

NASA NTRS1991

Effect of stiffness characteristics on the response of composite grid-stiffened structures

A study of the effect of stiffness discontinuities and structural parameters on the response of continuous-filament grid-stiffened flat panels is presented. The buckling load degradation due to manufacturing-introduced stiffener discontinuities associated with a filament cut-and-add approach at the stiffener intersections is investigated. The degradation of buckling resistance in isogrid flat panels subjected to uni-axial compression and combined axial compression and shear loading conditions and induced damage is quantified using FEM. The combined loading case is the most critical one. Nonsolid stiffener cross sections, such as a foam-filled blade or hat with a 0-deg dominant cap, result in grid-stiffened structures that are structurally very efficient for wing and fuselage applications. The results of a study of the ability of grid-stiffened structural concepts to enhance the effective Poisson's ratio of a panel are presented. Grid-stiffened concepts create a highly effective Poisson's ratio, which can produce large camber deformations for certain elastic tailoring applications.

Ambur, Damodar R.↗

NASA NTRS1991

Parallel unstructured grid generation

A parallel unstructured grid generation algorithm is presented and implemented on the Hypercube. Different processor hierarchies are discussed, and the appropraite hierarchies for mesh generation and mesh smoothing are selected. A domain-splitting algorithm for unstructured grids which tries to minimize the surface-to-volume ratio of each subdomain is described. This splitting algorithm is employed both for grid generation and grid smoothing. Results obtained on the Hypercube demonstrate the effectiveness of the algorithms developed.

Loehner, Rainald↗

NASA NTRS1992

Photofabricated Wire-Grid Polarizers

Freestanding metallic grids for use as polarizers for electromagnetic radiation at millimeter and submillimeter wavelengths made by simple modification of designs of freestanding square- and nearly-square cell metallic grids, according to proposal. Cross wires provide mechanical support, but distance between cross wires made greater than one wavelength so cross wires have little effect on polarizing characteristics of grid. Possible to fabricate grids commercially for frequencies up to several terahertz.

Siegel, Peter H.↗