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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Analysis of unsteady wake of a circular cylinder using Navier-Stokes equations

An unsteady Navier-Stokes analysis of the separated flow in the wake of a circular cylinder is presented in order to study the wake structure and the various interactions between its scales at low Re. The quasi-periodic and chaotic flows noted by Sreenivasan (1985) are searched for. The analysis is used to characterize the secondary vortex formation in the early stages of the flow development and its interaction with the primary vortex.

Ghia, K. N.↗
NASA NTRS1987

Numerical solution of the Navier-Stokes equations with emphasis on rotorcraft applications

Recent advances in CFD are surveyed, with a focus on the applicability of codes based on the Navier-Stokes (NS) equations to helicopter aerodynamics. The developmental history of two- and three-dimensional NS codes is recalled; the formulations employed by codes of increasing complexity are outlined; and previous reviews are noted. Particular attention is given to Reynolds-averaged NS methods and their use in analyzing subsonic, transonic, and vortex-dominated flows. Typical numerical results are compared with experimental data in diagrams and graphs, and a number of outstanding problems are considered.

Holst, Terry L.↗
NASA NTRS1987

Recent progress in the development and understanding of SUPG methods with special reference to the compressible Euler and Navier-Stokes equations

The current status of streamline-upwind/Petrov-Galerkin (SUPG) methods for the analysis of flow problems is surveyed in an analytical review. Problem areas addressed include classical Galerkin, upwind, artificial-diffusion, SUPG, discontinuous Galerkin, space-time FEM, and discontinuity-capturing approaches to the scalar advection-diffusion equation; incompressible flows; advective-diffusive systems; and the compressible Euler and Navier-Stokes equations. Graphs and diagrams are provided, and the good stability properties of state-of-the-art SUPG methods are pointed out.

Hughes, Thomas J. R.↗
NASA NTRS1988

Navier-Stokes solutions for transonic flow over a wing mounted in a tunnel

Three-dimensional viscous flow calculations are performed for a swept, NACA 0012 wing mounted inside a wind tunnel for which detailed experimental data is available. A Runge-Kutta time-stepping scheme is used for obtaining steady-state solutions to the thin-layer Navier-Stokes equations. Free-air comptuations are also performed to assess the wall-interference effects. The effects of grid density and artificial dissipation on the accuracy of numerical results are included. The effect of the wind-tunnel sidewall boundary layer on the flow pattern over the wing surface, particularly in the vicinity of the wing/wall juncture is found to be significant.

Vatsa, V. N.↗
NASA NTRS1988

Navier-Stokes simulations of axisymmetric vortex rings

The solution of the incompressible Navier-Stokes equations by means of a spectral method is used in the present numerical investigation of viscous axisymmetric vortex rings, in the cases of a single ring over the 0.001-1000 Re number range, and a pair of interacting rings. The propagation speeds of vortex rings of varying Re are computed and compared with the results of Saffman's (1970) theory; the present error estimate is found to be smaller than Saffman's, and to decrease with increasing Re at fixed core/ring radius ratio. The 'leapfrogging' of two vortex rings of equal sign and Re=1000 each is observed, indicating severe strain of the vortex core through the first pass and merging during the second pass.

Stanaway, S. K.↗
NASA NTRS1988

Some numerical and physical aspects of unsteady Navier-Stokes computations over airfoils using dynamic meshes

An upwind-biased implicit approximate factorization algorithm is applied to several unsteady flows on dynamic meshes. The thin-layer form of the compressible Navier-Stokes equations is used to solve both laminar and turbulent flows over airfoils pitching about the quarter chord. Numerical aspects of the solutions are investigated, including grid and time step effects. Two methods for determining fluxes - flux-vector splitting and flux-difference splitting - are compared. Flux-difference splitting predicts results more accurately than flux-vector splitting on a coarse mesh, but both methods agree on a fine mesh. Physical aspects of the computations are also examined. An equilibrium turbulent boundary layer model computes generally better unsteady results in comparison with experiment than a nonequilibrium model for the transonic case analyzed. Also, the size and location of the primary shed vortex for an airfoil pitching up at a constant rate is calculated in good agreement with experiment for two pitch rates.

Rumsey, Christopher L.↗
NASA NTRS1988

Multi-airfoil Navier-Stokes simulations of turbine rotor-stator interaction

An accurate numerical analysis of the flows associated with rotor-stator configurations in turbomachinery can be extremely helpful in optimizing the performance of turbomachinery. In this study the unsteady, thin-layer, Navier-Stokes equations in two spatial dimensions are solved on a system of patched and overlaid grids for a rotor-stator configuration from an axial turbine. The governing equations are solved using a finite-difference, upwind algorithm that is set in an iterative, implicit framework. Results in the form of pressure contours, time-averaged pressures, unsteady pressure amplitudes and phase are presented. The numerical results are compared with experimental data and the agreement is found to be good. The results are also compared with those of an earlier study which used only one rotor and one stator. The current study uses multiple rotors and stators and a pitch ratio that is much closer to the experimental ratio. Consequently the results of this study are found to be closer to the experimental data.

Rai, Man Mohan↗
NASA NTRS1988

An upwind parabolized Navier-Stokes code for real gas flows

A real gas, upwind, parabolized Navier-Stokes (PNS) code has been developed to compute the two-dimensional/axisymmetric hypersonic flow of equilibrium air around various body shapes. The new code is an extension of the upwind (perfect gas) PNS code of Lawrence, Tannehill and Chaussee. The upwind algorithm is based on Roe's flux-difference splitting scheme which has been modified to account for real gas effects. Simplified curve fits are used to obtain the thermodynamic and transport properties of equilibrium air. The new code has been validated by computing the hypersonic laminar flow of air over a flat plate, a wedge, a ramp, and a cone. The results of these computations are compared with the results from a conventional centrally-differenced, real gas, PNS code and the agreement is excellent, except in the vicinity of shock waves where the present code exhibits far superior shock capturing capabilities.

Tannehill, John C.↗
NASA NTRS1988

Explicit upwind algorithm for the parabolized Navier-Stokes equations

A new explicit upwind algorithm based on Roe's flux-difference splitting (FDS) method has been developed for the three-dimensional Parabolized Navier-Stokes (PNS) equations. For three-dimensional flows, FDS's are determined separately for the two nonmarching directions and modified to account for the calculated shock angle in the crossflow plane. Second-order FDS is applied to the pressure and convection terms with the streamwise pressure gradient limited in the subsonic region to maintain a hyperbolic inviscid equation set. Second-order central differencing is obtained in the two-step algorithm for the shear and heat flux terms. The new algorithm is demonstrated for three laminar flow test cases: supersonic flow over a flat plate, hypersonic flow over a 15 deg ramp, and hypersonic flow past a 10 deg cone at a 24 deg angle of attack. The computed results agree well with experimental measurements.

Korte, John J.↗
NASA NTRS1988

A solution method for the unsteady incompressible Navier-Stokes equations in generalized coordinate systems

A solution method based on a fractional step approach is developed for obtaining time-dependent solutions of the three-dimensional, incompressible Navier-Stokes equations in generalized coordinate systems. The governing equations are discretized conservatively by finite volumes using a staggered mesh system. The primitive variable formulation uses the volume fluxes across the faces of each computational cell as dependent variables. This procedure, combined with accurate and consistent approximations of geometric parameters, is done to satisfy the discretized mass conservation equation to machine accuracy as well as to gain favorable convergence properties of the Poisson solver. The discretized equations are second-order-accurate in time and space and no smoothing terms are added. An approximate-factorization scheme is implemented in solving the momentum equations. A novel ZEBRA scheme with four-color ordering is devised for the efficient solution of the Poisson equation. Several two and three-dimensional solutions are compared with other numerical and experimental results to validate the present method.

Rosenfeld, Moshe↗
NASA NTRS1988

Consistent boundary conditions for reduced Navier-Stokes (RNS) scheme applied to three-dimensional internal viscous flows

Three-dimensional internal viscous flow problems are presently addressed by a consistent and efficient set of boundary conditions for the multisweep space-marching, pressure-elliptic Reduced Navier-Stokes scheme. Since continuity is by this means directly satisfied at all points in the flow domain, the first-order momentum equations are directly solvable for pressure without the requirement for a Poisson pressure-correction equation. Incompressible flow solutions are obtained for straight and curved ducts of square cross section, in order to validate the procedure. Usefulness is demonstrated for internal flows with strong interactions, as would be found in turbomachine geometries.

Reddy, D. R.↗
NASA NTRS1987

Streamline-upwind/Petrov-Galerkin procedures for the vorticity-stream function form of the Navier-Stokes equations

The paper presents procedures for the solution of the Navier-Stokes equations in the vorticity-stream function form. The difficulties involved are related to the convection term in the vorticity transport equation and to the lack of boundary conditions for voritcity at no-slip surfaces. The use of a streamline-upwind/Petrov-Galerkin finite element formulation for the solution of the vorticity transport equation. In the present scheme, the weighting functions are dependent on both spatial and temporal discretizations. A proper numerical treatment of the boundary conditions leading to an implicit treatment of the vorticity at no-slip surfaces is presented. These procedures have successfully been employed to simulate various flows of engineering interest.

Tezduyar, T. E.↗
NASA NTRS1987

A Navier-Stokes solver using the LU-SSOR TVD algorithm

A new Navier-Stokes solver is developed by combining the efficiency of the LU-SSOR scheme and the accuracy of the flux-limited dissipation scheme. Application to laminar and turbulent flows and hypersonic flows proves the reliability of the new algorithm.

Yoon, Seokkwan↗
NASA NTRS1988

On pressure boundary conditions for the incompressible Navier-Stokes equations using nonstaggered grids

Pressure boundary conditions satisfying the normal momentum equation at solid boundaries with second-order accuracy are developed. Implementation of these conditions in an explicit numerical procedure for the two-dimensional incompressible Navier-Stokes equations enables convergent and accurate solutions for the driven cavity problem provided that the integral constraint of the Neumann boundary condtions is satisfied.

Biringen, S.↗
NASA NTRS1988

Polarized D2 Stokes-Raman Q branch broadened by He - A numerical calculation

Numerical calculations are presented for the high-pressure dependence of the linewidths of the D2 polarized Q branch of the Stokes Raman nu = 0-1 transition. A comparison is made with the experimental linewidths. The relationship between the line shape and S matrices is reviewed as well as the calculation of the latter for the D2-He system.

Blackmore, Robert↗
NASA NTRS1987

Simulation of separated flow past a bluff body using Navier-Stokes equations

Two-dimensional flow past a bluff body is presently simulated on the basis of an analysis that employs the incompressible, unsteady Navier-Stokes equations in terms of vorticity and stream function. The fully implicit, time-marching, alternating-direction, implicit-block Gaussian elimination used is a direct method with second-order spatial accuracy; this allows it to avoid the introduction of any artificial viscosity. Attention is given to the simulation of flow past a circular cylinder with and without symmetry, requiring the use of either the half or the full cylinder, respectively.

Ghia, K. N.↗
NASA NTRS1988

Navier Stokes computation of the flow field over delta wings with spanwise leading edge blowing

The concept of spanwise leading edge blowing, a means of controlling the position and strength of leading edge vortices, is analyzed by numerical solutions of the three-dimensional Thin-Layer Navier Stokes equations. The leading edge jet is simulated by defining a permeable boundary, corresponding to the jet slot, where suitable boundary conditions are implemented. Numerical results agree favorably with experimental measurements. It is found that the use of spanwise leading edge blowing not only magnifies the size and strength of the leading edge vortices, but also moves the vortex cores outboard and upward. As a result, the increase in lift comes primarily from the greater nonlinear vortex lift. The presence of the leading edge jet stream displaces the flow outboard, thereby increasing the effective aspect ratio of the delta wing. However, blowing causes earlier vortex breakdown, thus decreasing the stall angle.

Yeh, David T.↗
NASA NTRS1988

Computational validation of a parabolized Navier-Stokes solver on a sharp-nose cone at hypersonic speeds

Perfect gas computational results from a newly-developed upwind, parabolized Navier-Stokes (PNS) solver are compared with an existing set of experimental laminar results for a 10-deg half-angle circular cone at freestream Mach number of 7.95. Comparisons were performed with surface pressure and heat transfer data, as well as with flowfield pitot measurements. The PNS code predicted the surface quantities accurately up through 20-deg angle-of-attack, including crossflow separation, and correctly defined the location of the bow shock and the edge of the boundary layer. The importance of cell Reynolds number, grid density, and thermal boundary conditions to the accurate prediction of the flowfield are examined through numerical emamples.

Huebner, Lawrence D.↗