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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

A critical evaluation of two-equation models for near wall turbulence

A basic theoretical and computational study of two-equation models for near-wall turbulent flows was conducted. Two major problems established for the K-epsilon model are discussed, the lack of natural boundary conditions for the dissipation rate and the appearance of higher-order correlations in the balance of terms for the dissipation rate at the wall. The K-omega equation is shown to have two problems also: an exact viscous term is missing, and the destruction of the dissipation term is not properly damped near the wall. A new K-tau model (where tau = 1/omega is the turbulent time scale) was developed by inclusion of the exact viscous term, and by introduction of new wall damping functions with improved asymptotic behavior. A preliminary test of the new model yields improved predictions for the flat-plate turbulent boundary layer.

Speziale, Charles G.↗
NASA NTRS1990

Assessment of shock capturing schemes for resonant flows in nonlinear instability analysis

The paper presents computational assessment of advanced numerical schemes for nonlinear acoustic problems related to combustion instabilities in liquid rocket engines. Several time-accurate, shock capturing schemes have been evaluated on a benchmark, closed-end resonant pipe flow problem. It involves the numerical solution of inviscid, compressible gas dynamics equations to predict acoustic wave propagation, wave steepening, formation of shocks, acoustic energy dissipation and wave-wall reflection for several hundred wave cycles. It was demonstrated that high accuracy TVD type schemes can be used for direct, exact nonlinear analysis of combustion instability problems, preserving high harmonic energy content for long periods of time. The selected scheme was then applied to analyze the acoustic responses of resonant pipe-resonator, radial acoustic modes and hub-baffle configurations. Interesting observations of wave shape and damping characteristics have been drawn from presented computational studies.

Przekwas, A. J.↗
NASA NTRS1990

Simulation of leading-edge vortex flows

An implicit upwind-relaxation finite-difference algorithm solving the incompressible Navier-Stokes equations is employed to simulate low-speed, three-dimensional, laminar, leading-edge vortex flows over three round-edged low-aspect-ratio wings. The effects of grid density, angle of attack, Reynolds number, and wing planform on the flowfield structures and integral values are studied. Computed results are presented and compared with experimental data.

Hsu, C.-H.↗
NASA NTRS1991

Transonic Flows About A Fighter Airplane

Report describes computational study of transonic flows about airplane having shape similar to F-16A fighter. Navier-Stokes calculations simulate complicated flow fields and interactions.

Reznick, Steve↗
NASA NTRS1991

Computed Hypersonic Flow About A Sharp Cone

Report describes mostly computational study of hypersonic flow about sharp cone. Part of continuing effort to understand and predict complicated aerodynamic and heat-transfer phenomena in flows over forebodies of such hypersonic vehicles as Space Shuttle and proposed National Aerospace Plane. Focuses on transition from laminar to turbulent flow, with particular reference to heat-transfer predictions.

Kaul, Upender K.↗
NASA NTRS1990

Data reduction of digitized images processed from calibrated photographic and spectroscopic films obtained from terrestial, rocket and space shuttle telescopic instruments

The Microvax 2 computer, the basic software in VMS, and the Mitsubishi High Speed Disk were received and installed. The digital scanning tunneling microscope is fully installed and operational. A new technique was developed for pseudocolor analysis of the line plot images of a scanning tunneling microscope. Computer studies and mathematical modeling of the empirical data associated with many of the film calibration studies were presented. A gas can follow-up experiment which will be launched in September, on the Space Shuttle STS-50, was prepared and loaded. Papers were presented on the structure of the human hair strand using scanning electron microscopy and x ray analysis and updated research on the annual rings produced by the surf clam of the ocean estuaries of Maryland. Scanning electron microscopic work was conducted by the research team for the study of the Mossbauer and Magnetic Susceptibility Studies on NmNi(4.25)Fe(.85) and its Hydride.

Hammond, Ernest C., Jr.↗
NASA NTRS1991

Navier-Stokes simulation of the supersonic combustion flowfield in a ram accelerator

A computational study of the ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, is presented. The analysis is performed using a fully implicit TVD scheme that efficiently solves the Reynolds-averaged Navier-Stokes equations and the species continuity equations associated with a finite rate combustion model. Previous analyses of this concept were based on inviscid assumptions. The present results indicate that viscous effects are of primary importance; in all the cases studied, shock-induced combustion always started in the boundary layer. The effects of Mach number, mixture composition, pressure, and turbulence are investigated for various configurations. Two types of combustion processes, one stable and the other unstable, were observed depending on the inflow conditions. In the unstable case, a detonation wave is formed, which propagates upstream and unstarts the ram accelerator. In the stable case, a solution that converges to steady-state is obtained, in which the combustion wave remains stationary with respect to the ram accelerator projectile. The possibility of stabilizing the detonation wave by means of a backward facing step is also investigated. In addition to these studies, two numerical techniques were tested. These two techniques are vector extrapolation to accelerate convergence, and a diagonal formulation that eliminates the expense of inverting large block matrices that arise in chemically reacting flows.

Yungster, Shaye↗
NASA NTRS1991

Supersonic quasi-axisymmetric vortex breakdown

An extensive computational study of supersonic quasi-axisymmetric vortex breakdown in a configured circular duct is presented. The unsteady, compressible, full Navier-Stokes (NS) equations are used. The NS equations are solved for the quasi-axisymmetric flows using an implicit, upwind, flux difference splitting, finite volume scheme. The quasi-axisymmetric solutions are time accurate and are obtained by forcing the components of the flowfield vector to be equal on two axial planes, which are in close proximity of each other. The effect of Reynolds number, for laminar flows, on the evolution and persistence of vortex breakdown, is studied. Finally, the effect of swirl ration at the duct inlet is investigated.

Kandil, Osama A.↗
NASA NTRS1991

Thin-layer and full Navier-Stokes, locally-conical and three-dimensional asymmetric solutions

The unsteady, compressible thin-layer and full Navier-Stokes equations are used to numerically simulate steady and unsteady asymmetric, supersonic, locally-conical flows around a 5-deg-semiapex angle circular cone. The main computational scheme used for the present computations is the implicit, upwind, flux-difference splitting, finite-volume scheme. Comparisons of the solutions using the two sets of equations are presented for the flow asymmetry and its control. Computational studies are also presented to investigate the effects of the freestream Reynolds number and the locally-scaled Reynolds number on the flow asymmetry. These studies are carried out using the full Navier-Stokes equations. Three-dimensional, asymmetric flow solutions are also presented for a 5-deg-semiapex angle cone of unit length and a cone-cylinder configuration. The three-dimensional solutions are obtained by using the thin-layer equations and short-duration transient side-slip disturbances along with a very fine grid.

Kandil, O. A.↗
NASA NTRS1990

Mapping considerations for optimal binary correlation filters

The optimality of correlation filters is an important issue in applications of pattern recognition. Both binary phase-only filters (BPOFs) and amplitude encoded binary phase-only filters (AE BPOFs) are considered and the results of optimizing the filters for a real world object (the Space Shuttle) are studied. It is found that while only small improvements result from optimizing a BPOF, optimization of the AE BPOF is quite important in obtaining a useful correlation function. In the case of an AE BPOF, both signal-to-noise and peak-to-sidelobe measures must be studied. Computer simulation and experimental correlation results are presented.

Downie, John D.↗
NASA NTRS1991

Ignition delay of a gas mixture above a liquid fuel pool

A computational study has been made of transient heat transfer and fluid flow of an axisymmetric two-layer gas-and-liquid system heated from above by a small hot spot (e.g., a hot wire or pilot flame) located close to the liquid surface. The gas phase is unconfined above the liquid pool. The effects of varying gravity level (0.0001 to 1 gn), liquid pool height, and heater height are investigated. Thermocapillary convection induced by the nonuniform heating of the liquid surface combines with buoyancy forces to affect the heat transfer and the transport of fuel vapor toward the heat source. At reduced gravity, gas-phase conduction is comparable to the incident radiation at the liquid surface, whereas at 1 gn, buoyant convection carries the heat upward from the heat source and, therefore, the liquid is heated primarily by radiation.

Schiller, D. N.↗
NASA NTRS1991

Navier-Stokes simulation of the supersonic combustion flowfield in a ram accelerator

A computational study of the ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, is presented. The analysis is performed using a fully implicit TVD scheme that efficiently solves the Reynolds-averaged Navier-Stokes equations and the species continuity equations associated with a finite rate combustion model. The present results indicate that viscous effects are of primary importance in all the cases studied, shock-induced combustion always started in the boundary layer. The effects of Mach number, mixture composition, pressure and tubulence are investigated for various configurations. Two types of combustion processes, one stable and the other unstable, were observed depending on the inflow conditions. The possibility of stabilizing the detonation wave by means of a backward facing step is also investigated. Two numerical techniques were tested: vector extrapolation, to accelerate convergence, and a diagonal formulation that eliminates the expense of inverting large block matrices which arise in chemically reacting flows.

Yungster, Shaye↗
NASA NTRS1991

Investigation of vortex development on a pitching slender body of revolution

A computational study of the unsteady flow about a pitching 3.5 caliber tangent ogive forebody is presented. The flow is simulated using the full 3D unsteady Navier-Stokes equations and a time-accurate implicit algorithm. Comparison to available experimental data for a steady 20 deg case is presented as validation. Effects of grid resolution and a comparison of solutions using full Navier-Stokes and the thin-layer approximation are included. The forebody is simulated in a 'pitch-up to 20 deg and hold' maneuver, and two different pitch axis locations are used in the study. Examination of the unsteady vorticity field for the pitch-up cases reveals the formation of strong shear layers as the body decelerates, and their roll-up into vortical structures in a process similar to that observed in airfoil dynamic stall. Pronounced vortex/surface interactions are seen which produce multiple secondary separation regions, ejection of vorticity from the surface, and embedded regions of high suction.

Stanek, M. J.↗
NASA NTRS1991

Reynolds number effects on supersonic asymmetrical flows over a cone at high angle of attack

The supersonic viscous flow over a 5-degree half-angle cone at an angle of attack of four times the cone half-angle is studied computationally using both the conical and the three-dimensional Navier-Stokes equations. The numerical solutions were obtained with an implicit, upwind-biased algorithm. Asymmetrical flowfields of the absolute-instability type are found using the conical-flow equations which agree with published results. However, the absolute instabilities of the originally symmetric flow found with the conical equations do not occur in the three-dimensional simulations, although spurious asymmetric three-dimensional flows for symmetric bodies arise if the grid resolution is insufficient in the nose region. The asymmetric flows computed with the three-dimensional equations are convective instabilities and are possible if the local Reynolds number exceeds a critical value and a fixed geometric asymmetry is imposed. A continuous range of asymmetries can be developed, depending on the size of the disturbance and the Reynolds number. As the Reynolds number is increased, the asymmetries demonstrate a bistable behavior at levels of side force consistent with those predicted using the conical equations. Below a certain critical Reynolds number, any flow asymmetries arising from a geometrical asymmetry are damped with increasing distance downstream from the geometrical asymmetry.

Thomas, J. L.↗
NASA NTRS1992

Effects Of Spatial Disturbances On Asymmetries Of Vortexes

Report describes computational study of effects of small spatial disturbances on asymmetries in vortexes downstream of slender bodies of revolution in flows at large angle of attack. Flow phenomena affect stability and maneuvering characteristics of missiles and aircraft. Understanding phenomena helps answer questions as how precisely symmetrical aerodynamic body should be, and how roughness, bumps, or other imperfections affect maneuvering characteristics.

Degani, David↗
NASA NTRS1992

Shedding Of Vortices From A Missilelike Body

Report describing computational study of subsonic flows about slender bodies of revolution focuses on effect of angle of attack on development of unsteadiness in flow - particularly unsteady shedding of vortices on leeward side. At large angles of attack, flows exhibit self-sustained oscillations.

Schiff, Lewis B.↗
NASA NTRS1991

Engine dynamic analysis with general nonlinear finite element codes

A general engine dynamic analysis as a standard design study computational tool is described for the prediction and understanding of complex engine dynamic behavior. Improved definition of engine dynamic response provides valuable information and insights leading to reduced maintenance and overhaul costs on existing engine configurations. Application of advanced engine dynamic simulation methods provides a considerable cost reduction in the development of new engine designs by eliminating some of the trial and error process done with engine hardware development.

Adams, M. L.↗
NASA NTRS1992

Thermal stratification potential in rocket engine coolant channels

The potential for rocket engine coolant channel flow stratification was computationally studied. A conjugate, 3-D, conduction/advection analysis code (SINDA/FLUINT) was used. Core fluid temperatures were predicted to vary by over 360 K across the coolant channel, at the throat section, indicating that the conventional assumption of a fully mixed fluid may be extremely inaccurate. Because of the thermal stratification of the fluid, the walls exposed to the rocket engine exhaust gases will be hotter than an assumption of full mixing would imply. In this analysis, wall temperatures were 160 K hotter in the turbulent mixing case than in the full mixing case. The discrepancy between the full mixing and turbulent mixing analyses increased with increasing heat transfer. Both analysis methods predicted identical channel resistances at the coolant inlet, but in the stratified analysis the thermal resistance was negligible. The implications are significant. Neglect of thermal stratification could lead to underpredictions in nozzle wall temperatures. Even worse, testing at subscale conditions may be inadequate for modeling conditions that would exist in a full scale engine.

Kacynski, Kenneth J.↗