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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

The influence of the computational mesh on accuracy for initial value problems with discontinuous or nonunique solutions

Discontinuous, or weak, solutions of the wave equation, the inviscid form of Burgers equation, and the time-dependent, two-dimensional Euler equations are studied. A numerical method of second-order accuracy in two forms, differential and integral, is used to calculate the weak solutions of these equations for several initial value problems, including supersonic flow past a wedge, a double symmetric wedge, and a sphere. The effect of the computational mesh on the accuracy of computed weak solutions including shock waves and expansion phenomena is studied. Modifications to the finite-difference method are presented which aid in obtaining desired solutions for initial value problems in which the solutions are nonunique.

Maccormack, R. W.↗
NASA NTRS1974

Propagation of solar disturbances in interplanetary space

Time-dependent solutions of a one-fluid model of the interplanetary medium are investigated. This set of unsteady hydrodynamic equations has been written in conservation form in order to apply the Lax-Wendroff method for the solution of this problem. The initial condition is specified by a pulse at 1 solar radius. The equilibrium condition is chosen to be the steady solution of a quiet solar wind. The specified solar disturbances in this calculation are allowed to be both sub- and supersonic by the present theoretical formulation. The results are presented in terms of density, velocity, and temperature profiles of the interplanetary gas flow at heliocentric distances up to about 10 AU at any particular time. The trajectories of disturbances for various initial pulses are shown. Some 1972 solar-flare observational data are compared with these theoretical calculations. From these calculations, the effects on the interplanetary environment, due to the propagation of solar disturbances, can be determined.

Wu, S. T.↗
NASA NTRS1975

Interaction of strong shock wave with turbulent boundary layer

The reported investigation represents an extension of the time-dependent solution of separated laminar flows based on the complete Navier-Stokes equations reported by MacCormack (1971) and Carter (1973). The current study includes turbulence models in conjunction with the compressible flow equations. The calculations start with a uniform flow except for values imposed along the upstream and outer boundaries. The basic numerical method is discussed along with questions concerning the exponential accuracy and the resolution of the viscous sublayer in a compressed region.

Baldwin, B. S.↗
NASA NTRS1975

High Reynolds number transonic flow simulation

A code has been developed for simulating high Reynolds number transonic flow fields of arbitrary configuration. An explicit finite-difference method with time splitting is used to solve the time-dependent equations for compressible turbulent flow. A nonorthogonal computational mesh of arbitrary configuration facilitates the description of the flow field. The code is applied to simulate the flow over a two-dimensional 18 percent thick circular-arc biconvex airfoil at zero angle of attack for several different Reynolds numbers and a free-stream Mach number of 0.775.

Deiwert, G. S.↗
NASA NTRS1975

Creep behavior of uranium carbide-based alloys

The present work gives the results of experiments on the influence of zirconium carbide and tungsten on the creep properties of uranium carbide. The creep behavior of high-density UC samples follows the classical time-dependence pattern of (1) an instantaneous deformation, (2) a primary creep region, and (3) a period of steady-state creep. Creep rates for unalloyed UC-1.01 and UC-1.05 are several orders of magnitude greater than those measured for carbide alloys containing a Zr-C and/or W dispersoid. The difference in creep strength between alloyed and unalloyed materials varies with temperature and applied stress.

Seltzer, M. S.↗
NASA NTRS1975

Estimates of saltation threshold and erosion rates on Mars

The Mariner 9 photographs of the Martian surface include many which show crater-associated streaks caused by atmospheric winds. Due to the low density atmosphere of Mars, the wind speeds necessary to move surface particles and thus cause the streaks are apparently very high. The atmospheric-boundary-layer wind tunnel has been used to determine threshold speeds for particles of varying density and diameter and the results for small particles have been extrapolated to estimate somewhat lower threshold speeds on Mars than previous estimates. A series of streak modeling tests is used to derive an erosion-rate correlation function, which is in turn used to estimate erosion rates to the lee of some of the craters on Mars which exhibited time-dependent streak behavior during the mission.

Iversen, J. D.↗
NASA NTRS1975

A lunar soil evolution model

Comminution, agglutination, and replenishment processes in a lunar soil are modeled by a system of time-dependent linear differential equations. In the model, a soil is subdivided into coarse-particle, fine-particle, and agglutinate fractions. The relative mass abundance of each component in a mature soil is found to be proportional to rates for the reworking processes. Evolution of the grain-size distribution from a fresh ejecta blanket to a mature soil is described quantitatively in terms of the changing proportions of the three soil constituents. If size data are available for an immature soil and a mature soil of the same system, rates for the various processes can be calculated under certain simplifying assumptions.

Mendell, W. W.↗
NASA NTRS1975

An optimal control of a flexible spinning satellite

A modification of optimal spinning satellite control theory is proposed, based on introducing an additional vector into the equation for control torque components. This modification essentially means that the amplitudes of the time-dependent satellite perturbation functions can be made small when the amplitudes of the control torque components are allowed to be different. An algorithm for computer simulation of the satellite control theory is provided. A singular perturbation scheme is used which greatly reduces the number of degrees of freedom in the flexible satellite motion equations.

Das, A.↗
NASA NTRS1975

Winter bulge and diurnal variations in hydrogen inferred from AE-C composition measurements

In situ mass spectrometric measurements of ion and neutral particle thermospheric compositions have been used to infer the latitudinal and diurnal variations of thermospheric atomic hydrogen for solstice conditions. Local time-dependent and local time-independent components of the observed hydrogen distribution were separated on the basis of a model generated by expanding the log of neutral hydrogen concentration in terms of spherical harmonics. Results are compared with analogous data on N2 concentration; the comparison reveals an anticorrelation between gas temperature and hydrogen concentration. The slope of this anticorrelation line represents the 'zero flux condition' from the exosphere theory of Hodges (1973), thus further bearing out the conclusion that exospheric flow is the dominant process governing the global distribution of hydrogen.

Brinton, H. C.↗
NASA NTRS1975

Experimental and Theoretical Studies of Pulsating Turbulent Flow

The objective of this investigation was to study the effects of small amplitude sinusoidal pulsations on fully developed turbulent flow in a tube from both experimental and theoretical viewpoints. Theoretical models for the macroscopic behavior of pulsating turbulent tube flow were developed for the two cases of very low and very high pulsation frequencies. The models are based on assumptions of quasi-steady and frozen eddy viscosity flow behavior, respectively. The models successfully predict unsteady velocity profiles, thereby supporting the currently proposed definitions of frequency regimes in pulsating turbulent flow. Experimental measurements were made of the time-dependent pressure drop and velocity profiles over the range of frequency-to-Reynolds number ratios from 0.0095 to 0.24. The two macroscopic models developed in this study predict unsteady velocity profiles which are in moderately good agreement with the experiments in their respective frequency regimes, and a previously developed quasi-steady model is found to predict experimental velocity profiles well in both the quasisteady and the frozen eddy viscosity frequency regimes. The effect of flow pulsations on the dissipation of turbulence energy in the vicinity of the wall was measured in the lower transition frequency regime. The long-time averaged dissipation was observed to be unchanged from the steady flow dissipation, within the accuracy of the experiment. A theoretical model of the periodic viscous sublayer was also developed and applied to pulsating flow in a tube, in order to investigate the effects of flow pulsations on the rate of production of turbulence in the region of the wall. The periodic viscous sublayer model predicts sublayer growth periods in steady flow which agree with the published experimental data. When the model is applied to pulsating flow, the response of the sublayer growth period falls into three frequency regimes, the parameters of which are in approximate agreement with the frequency regimes which are defined on the basis of macroscopic flow behavior. The sublayer renewal cycle exhibits quasi-steady flow behavior when the sublayer growth period is much less than the pulsation period, transition behavior when these two periods are approximately equal, and frozen eddy viscosity behavior when the sublayer period is much longer than the pulsation period. The effect of the sublayer growth and renewal cycle on the level of turbulence was investigated by two methods. The velocity fluctuations seen by a point velocity probe located close to the wall were predicted from the model in one method and the rate of turbulence production was estimated from the frequency of sublayer renewal events in the other.

Kingston, G. C.↗
NASA NTRS1975

Three-dimensional finite element analysis for high velocity impact

A finite element algorithm for solving unsteady, three-dimensional high velocity impact problems is presented. A computer program was developed based on the Eulerian hydroelasto-viscoplastic formulation and the utilization of the theorem of weak solutions. The equations solved consist of conservation of mass, momentum, and energy, equation of state, and appropriate constitutive equations. The solution technique is a time-dependent finite element analysis utilizing three-dimensional isoparametric elements, in conjunction with a generalized two-step time integration scheme. The developed code was demonstrated by solving one-dimensional as well as three-dimensional impact problems for both the inviscid hydrodynamic model and the hydroelasto-viscoplastic model.

Chan, S. T. K.↗
NASA NTRS1975

Numerical solutions of the unsteady Navier-Stokes equations for arbitrary bodies using boundary-fitted curvilinear coordinates

A method of automatic body-fitted curvilinear coordinate generation is described and used to construct a finite-difference solution of the full incompressible time-dependent Navier-Stokes equations for the unsteady laminar viscous flow arbitrary two-dimensional airfoils or any other two-dimensional body. A method of controlling the spacing of the coordinate lines encircling the body is developed in order to treat high Reynolds number flows, since the coordinate lines must concentrate near the surface to a greater degree as the Reynolds number increases. Multiple airfoils and submerged hydrofoils are treated as illustrative examples. The solution shows good agreement with the Blasius boundary layer solution for the flow past a semi-infinite flat plate.

Thompson, J. F.↗
NASA NTRS1975

Chlorofluoromethanes in the stratosphere and some possible consequences for ozone

Inert chlorofluoromethanes are used by man as refrigerants and aerosol propellants. These substances eventually escape and diffuse upward into the stratosphere. At great enough heights, UV sunlight can photodissociate these chlorofluorocarbons into chlorine atoms which will catalytically destroy ozone molecules. Chlorofluoromethane production has been increasing steadily in recent years to its present level of about a megaton per year, and chlorofluorocarbon usage will probably continue to grow in the future. Calculations with a one-dimensional time-dependent atmospheric model suggests that, if projected increases in chlorofluoromethane use materialize and there is no tropospheric destruction mechanism for these gases, the total global abundance of ozone may be reduced by more than 20 per cent over the next 50 years. If the residence times for these fluorocarbons in the troposphere are in the range of 10-30 years, however, predicted ozone depletions would be significantly smaller.

Turco, R. P.↗
NASA NTRS1950

Use of source distributions for evaluating theoretical aerodynamics of thin finite wings at supersonic speeds

A series of publications on the source-distribution methods for evaluating the aerodynamics of thin wings at supersonic speeds is summarized, extended, and unified. Included in the first part are the deviations of: (a) the linearized partial-differential equation for unsteady flow at a substantially constant Mach number. b) The source-distribution solution for the perturbation-velocity potential that satisfies the boundary conditions of tangential flow at the surface and in the plane of the wing; and (c) the integral equation for determining the strength and the location of sources to describe the interaction effects (as represented by upwash) of the bottom and top wing surfaces through the region between the finite wing boundary and the foremost Mach wave. The second part deals with steady-state thin-wing problems. The third part of the report approximates the integral equation for unsteady upwash and includes a solution of approximate equation. Expressions are then derived to evaluate the load distributions for time-dependent finite-wing motions.

Evvard, John C↗
NASA NTRS1955

Generalized indical forces on deforming rectangular wings in supersonic flight

A method is presented for determining the time-dependent flow over a rectangular wing moving with a supersonic forward speed and undergoing small vertical distortions expressible as polynomials involving spanwise and chordwise distances. The solution for the velocity potential is presented in a form analogous to that for steady supersonic flow having the familiar "reflected area" concept discovered by Evvard. Particular attention is paid to indicial-type motions and results are expressed in terms of generalized indicial forces. Numerical results for Mach numbers equal to 1.1 and 1.2 are given for polynomials of the first and fifth degree in the chordwise and spanwise directions, respectively, on a wing having an aspect ratio of 4.

Lomax, Harvard↗
NASA NTRS1995

Single-drop reactive extraction/extractive reaction with forced convective diffusion and interphase mass transfer

An algorithm has been developed for time-dependent forced convective diffusion-reaction having convection by a recirculating flow field within the drop that is hydrodynamically coupled at the interface with a convective external flow field that at infinity becomes a uniform free-streaming flow. The concentration field inside the droplet is likewise coupled with that outside by boundary conditions at the interface. A chemical reaction can take place either inside or outside the droplet, or reactions can take place in both phases. The algorithm has been implemented, and for comparison results are shown here for the case of no reaction in either phase and for the case of an external first order reaction, both for unsteady behavior. For pure interphase mass transfer, concentration isocontours, local and average Sherwood numbers, and average droplet concentrations have been obtained as a function of the physical properties and external flow field. For mass transfer enhanced by an external reaction, in addition to the above forms of results, we present the enhancement factor, with the results now also depending upon the (dimensionless) rate of reaction.

Kleinman, Leonid S.↗
NASA NTRS1961

A Comparison of Theory and Observation of the Echo I Satellite

Observations of the Echo I balloon satellite have been compared with a theory including the following perturbing effects: (1) solar radiation pressure; (2) lunar and solar gravitation; (3) second, third, and fourth harmonics of the earth's gravitational potential; and (4) atmospheric drag. With a set of orbital elements at the 26th day of the lifetime of the satellite, it was possible to match the observational data to 180 days with root mean square residuals as follows: Delta-a = 17.9 km, Delta-e = 0.0021, Delta-i = 0.0177 deg., Delta-omega = 1.1231 deg., Delta-Omega = 0.4821 deg., Delta-perigee height = 7.50 km. No differential correction has been applied as yet. Values of atmospheric density between 1500 and 930 km, assuming neutral drag effects only, have been inferred from the orbital data. The connection between solar activity and drag is also examined. As the Echo I perigee height continues to oscillate between 900 and 1500 km, more valuable orbital data will be obtained and atmospheric properties will be deduced. Further refinements in the mathematical model, especially in a time-dependent model atmosphere, should bring a substantial reduction in the residuals of the observations.

Bryant, R. W.↗
NASA NTRS1959

Theoretical Determination of Lifetime of Compressed Plates at Elevated Temperatures

A method for the theoretical determination of the lifetime of com- pressed plates at elevated temperatures is presented. In this approach, linearized equations are used throughout with the assumption that the plate material is a standard linear solid. The critical time (lifetime) is determined by reducing the time-dependent behavior to the time- independent response of purely elastic buckling. Theoretically predicted lifetimes of 2024-T3 (formerly 24S-T3) aluminum-alloy plates at 450 F are compared with experimental values obtained in previous work.

Herrmann, George↗