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At least 991 records · Page 55

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NASA NTRS1986

The coalescence of two merged interaction regions between 6.2 and 9.5 AU - September 1979 event

A simulation study, based on an unsteady, one-dimensional, one-fluid MHD model, and using the plasma and magnetic field data from the Voyager 1 at 6.2 AU, was conducted on the evolution and interaction of solar wind structures to explain the two interaction regions observed by Voyager 1 within a large-scale interplanetary compound stream that was recorded at a heliocentric distance of 6.2 AU. A strong forward shock F(D) with a speed of 960 km/s was present at the front of the second interaction region, and two reverse shocks, R1 and R2, were at the end of the first interaction region. The model shows that the forward shock passed through the two reverse shocks and into the first interaction region, becoming weaker in each of these interactions. The reverse shocks coalesced to form a stronger reverse shock R; thus, the shock signature changed from R1-R2-F(D) to F(D)-R between 6.2 and 9.5 AU. The major stream structures at 9.5 AU predicted by the simulation model agree well with those directly observed from Pioneer 11.

Whang, Y. C.↗

NASA NTRS1987

The plasma environment during particle beam injection into space plasmas. I - Electron beams. II - Charge-neutral beams

A realistic electrostatic simulation model is used to investigate the plasma environment in the near vicinity of a spacecraft during the injection of electon beams from the spacecraft. The model is described, and the cases of injection into vacuum and into a low-density plasma are addressed. The relationship of the two-dimensional results to the purely field-aligned one-dimensional simulations is discussed. The dependence of the results on the ambient plasma density is investigated. Then, the properties of a charge-neutral beam and the plasma response are examined for the case where the beam has nonzero velocity components parallel and perpendicular to the magnetic field. The parameters used in the simulation are described, and the properties of the beam injection into the vacuum and in the case where a plasma is present are given.

Pritchett, P. L.↗

NASA NTRS1987

A scanning modulation collimator observation of the high-energy X-ray source in the Crab Nebula

Two-dimensional maps of the 22-64 keV emission from the Crab Nebula with an angular resolution of 15 arcsec has been synthesized. The maps are generated by application of a maximum entropy method operating on a series of one-dimensional scans obtained with a balloon-borne modulation collimator telescope. The two-dimensional size, shape and orientation of the hard X-ray nebula relative to the pulsar have been measured for the first time. The implications of these results for models of electron transport in the Crab are discussed, and the geometry of the observed X-ray nebula is related to other features of the Crab Nebula system.

Pelling, R. M.↗

NASA NTRS1987

Faceting and roughening in quasicrystals

The question whether quasi-crystal shapes should be faceted is studied in a simple model of quasi-crystalline order. At T = 0, the model is proved to yield a completely faceted equilibrium shape in both two and three dimensions. At T greater than 0, an interface model is derived for a two-dimensional Penrose tiling. By mapping it onto a one-dimensional quasi-periodic Schroedinger equation, it is shown that the roughness exponent varies continuously with T at low T.

Garg, Anupam↗

NASA NTRS1988

Overview of numerical codes developed for predicted electrothermal deicing of aircraft blades

An overview of the deicing computer codes that have been developed at the University of Toledo under sponsorship of the NASA-Lewis Research Center is presented. These codes simulate the transient heat conduction and phase change occurring in an electrothermal deicier pad that has an arbitrary accreted ice shape on its surface. The codes are one-dimensional rectangular, two-dimensional rectangular, and two-dimensional with a coordinate transformation to model the true blade geometry. All modifications relating to the thermal physics of the deicing problem that have been incorporated into the codes will be discussed. Recent results of reformulating the codes using different numerical methods to increase program efficiency are described. In particular, this reformulation has enabled a more comprehensive two-dimensional code to run in much less CPU time than the original version. The code predictions are compared with experimental data obtained in the NASA-Lewis Icing Research Tunnel with a UH1H blade fitted with a B. F. Goodrich electrothermal deicer pad. Both continuous and cyclic heater firing cases are considered. The major objective in this comparison is to illustrate which codes give acceptable results in different regions of the airfoil for different heater firing sequences.

Keith, Theo G.↗

NASA NTRS1988

Velocity-coupled flow oscillations in a simulated solid-propellant rocket environment

A comprehensive numerical analysis has been carried out to study the unsteady flowfields in a simulated rocket-motor environment. The model is based on the time-dependent compressible Navier-Stokes equations with a two-equation turbulence closure scheme. Various important aspects of the coupling between acoustic oscillations and mean flowfields, including flow reversal, modification of transport properties, etc., are addressed. Results indicate that multi-dimensional effects play important roles in determining local flow structures and wave characteristics. In much of the domain, acoustic velocity nodal points are observed in the near-wall region. The classical one-dimensional theory fails to describe several important mechanisms associated with velocity-induced flow instabilities.

Yang, Vigor↗

NASA NTRS1987

Time-dependent approximation schemes for some problems of parameter estimation in distributed systems

A parameter estimation method that can be used to estimate functional parameters in delay differential equations and moving boundary problems is discussed. In either problem, the original model equation (which is infinite-dimensional) is approximated by a system of ordinary differential equations that can be solved numerically in an efficient way. The approximation scheme is based on time-dependent spline elements. For the delay equation with time-varying delay, convergence results are presented that indicate the estimates obtained using the approximating system. Numerical test examples converge in some sense to a best-fit parameter for the original system are included by means of which time-varying and state-dependent delays and a time-varying diffusion coefficient in a one-phase, one-dimensional Stefan problem are estimated.

Murphy, K. A.↗

NASA NTRS1988

Development of a thermal and structural analysis procedure for cooled radial turbines

A procedure for computing the rotor temperature and stress distributions in a cooled radial turbine is considered. Existing codes for modeling the external mainstream flow and the internal cooling flow are used to compute boundary conditions for the heat transfer and stress analyses. An inviscid, quasi three-dimensional code computes the external free stream velocity. The external velocity is then used in a boundary layer analysis to compute the external heat transfer coefficients. Coolant temperatures are computed by a viscous one-dimensional internal flow code for the momentum and energy equation. These boundary conditions are input to a three-dimensional heat conduction code for calculation of rotor temperatures. The rotor stress distribution may be determined for the given thermal, pressure and centrifugal loading. The procedure is applied to a cooled radial turbine which will be tested at the NASA Lewis Research Center. Representative results from this case are included.

Kumar, Ganesh N.↗

NASA NTRS1989

Analysis of internal ablation for the thermal control of aerospace vehicles

A new method of thermal protection for transatmospheric vehicles is introduced. The method involves the combination of radiation, ablation and transpiration cooling. By placing an ablating material behind a fixed-shape, porous outer shield, the effectiveness of transpiration cooling is made possible while retaining the simplicity of a passive mechanism. A simplified one-dimensional approach is used to derive the governing equations. Reduction of these equations to non-dimensional form yields two parameters which characterize the thermal protection effectiveness of the shield and ablator combination for a given trajectory. The non-dimensional equations are solved numerically for a sample trajectory corresponding to glide re-entry. Four typical ablators are tested and compared with results obtained by using the thermal properties of water. For the present level of analysis, the numerical computations adequately support the analytical model.

Camberos, Jose A.↗

NASA NTRS1989

Dimensionality of high temperature superconductivity in oxides

Many models have been proposed to account for the high temperature superconductivity observed in oxide systems. Almost all of these models proposed are based on the uncoupled low dimensional carrier Cu-O layers of the oxides. Results of several experiments are presented and discussed. They suggest that the high temperature superconductivity observed cannot be strictly two- or one-dimensional, and that the environment between the Cu-O layers and the interlayer coupling play an important role in the occurrence of such high temperature superconductivity. A comment on the very short coherence length reported is also made.

Chu, C. W.↗

NASA NTRS1989

Preconditioning matrices for the pseudospectral approximation of first-order operators

The behavior of the eigenvalues of preconditioning matrices for the pseudospectral approximation to the derivative operator has been analyzed in one and two dimensions. The one-dimensional analysis resulted in real and positive eigenvalues for the selected tridiagonal matrices. In the two-dimensional analysis, the eigenvalues of the selected block-diagonal matrices behaved well, but the preconditioner is full and therefore not suitable for applications. The Richardson scheme has been applied in the unpreconditioned as well as the preconditioned version to find the solution of the model problem.

Funaro, D.↗

NASA NTRS1990

Numerical simulations of the decay of Martian global dust storms

The decay of Martian global (great) dust storms is investigated. One-dimensional (vertical, static atmosphere) and two-dimensional (latitude-height, steady state circulation) simulations carried out with an aerosol transport-microphysical model indicate that atmospheric motions play a significant role in the observed decay of global dust storms. Spacecraft observations (Mariner 9, Viking) of the 1971 and the two 1977 planet-encircling dust storms have provided suggestions about some characteristics of storm decay. Specifically, the dust particle size distribution is inferred to have remained essentially unchanged for particles with radii between 1 and 10 microns during decay of the 1971 storm, and surface visible opacity declined quasi-exponentially with time in northern midlatitudes during the decay of the two 1977 storms. The results from this investigation indicate that two- and three-dimensional dynamical processes play a significant role the observed decay features of Martian global dust storms. The most important processes are the lofting of dust by vertical motions in the dust source region of the Southern Hemisphere subtropics and a continuing advective resupply of atmospheric dust into the dust sink regions of the Northern Hemisphere. This work has implications for Viking data analyses and future Mars observer observations and requires that the particle size distribution be treated as a time and latitude dependent quantity.

Murphy, James R.↗

NASA NTRS1990

Hypersonic vehicle simulation model: Winged-cone configuration

Aerodynamic, propulsion, and mass models for a generic, horizontal-takeoff, single-stage-to-orbit (SSTO) configuration are presented which are suitable for use in point mass as well as batch and real-time six degree-of-freedom simulations. The simulations can be used to investigate ascent performance issues and to allow research, refinement, and evaluation of integrated guidance/flight/propulsion/thermal control systems, design concepts, and methodologies for SSTO missions. Aerodynamic force and moment coefficients are given as functions of angle of attack, Mach number, and control surface deflections. The model data were estimated by using a subsonic/supersonic panel code and a hypersonic local surface inclination code. Thrust coefficient and engine specific impulse were estimated using a two-dimensional forebody, inlet, nozzle code and a one-dimensional combustor code and are given as functions of Mach number, dynamic pressure, and fuel equivalence ratio. Rigid-body mass moments of inertia and center of gravity location are functions of vehicle weight which is in turn a function of fuel flow.

Shaughnessy, John D.↗

NASA NTRS1991

Latitudinal dynamics of steady solar wind flows

In many previous studies, it has been assumed that the streamlines of the solar wind are in a purely radial direction beyond a few solar radii. Therefore, the nonradial modulations by the magnetic force and gas pressure is neglected at large heliocentric distances. The work reported in this paper includes a two-dimensional magnetohydrodynamic (MHD) study of the latitudinal structure of solar wind flows. The dynamical effects of the interplanetary magnetic field (IMF) are investigated by MHD solutions and by comparing them with both one-dimensional and two-dimensional hydrodynamic (HD) solutions. The corotational effect of the sun and the influence of transient events in the solar atmosphere are neglected. With such a simplification, the latitudinal structure of the steady solar wind is controlled by the dynamic balance of the magnetic force and the gas pressure. Several different patterns of the evolution of the solar wind latitudinal structure are exhibited in the calculation. The results indicate the existence of a proton number density maximum at the magnetic neutral line, whether or not there is a density maximum or minimum at the inner boundary. The drift motion of magnetic field lines toward the magnetic neutral line enhances the magnetic field strength around the neutral sheet, which may provide a possible explanation of the discrepancy between the measured IMF at 1 AU and that extrapolated from the photospheric magnetic field by the current source-surface modeling.

Yang, W.-H.↗

NASA NTRS1991

Nighttime reactive nitrogen measurements from stratospheric infrared thermal emission observations

IR thermal emission spectra of the earth's atmosphere in the 700-2000/cm region were obtained with a cryogenically cooled high-resolution interferometer spectrometer on a balloon flight from Palestine, Texas, on September 15-16, 1986. The observations exhibit spectral features of a number of stratospheric constituents, including important species of the reactive nitrogen family. An analysis of the observed data for simultaneously measured vertical distributions of O3, H2O, N2O, NO2, N2O5, HNO3, and ClONO2 is presented. These measurements permit the first direct determination of the nighttime total reactive nitrogen concentrations, and the partitioning of the important elements of the NO(x) family. Comparisons of the total reactive nitrogen budget are made with the measurements by the ATMOS experiment and with the predictions of one-dimensional and two-dimensional photochemical models.

Abbas, Mian M.↗

NASA NTRS1992

Progress in multi-dimensional upwind differencing

Multi-dimensional upwind-differencing schemes for the Euler equations are reviewed. On the basis of the first-order upwind scheme for a one-dimensional convection equation, the two approaches to upwind differencing are discussed: the fluctuation approach and the finite-volume approach. The usual extension of the finite-volume method to the multi-dimensional Euler equations is not entirely satisfactory, because the direction of wave propagation is always assumed to be normal to the cell faces. This leads to smearing of shock and shear waves when these are not grid-aligned. Multi-directional methods, in which upwind-biased fluxes are computed in a frame aligned with a dominant wave, overcome this problem, but at the expense of robustness. The same is true for the schemes incorporating a multi-dimensional wave model not based on multi-dimensional data but on an 'educated guess' of what they could be. The fluctuation approach offers the best possibilities for the development of genuinely multi-dimensional upwind schemes. Three building blocks are needed for such schemes: a wave model, a way to achieve conservation, and a compact convection scheme. Recent advances in each of these components are discussed; putting them all together is the present focus of a worldwide research effort. Some numerical results are presented, illustrating the potential of the new multi-dimensional schemes.

Vanleer, Bram↗

NASA NTRS1992

Formation and stability of the self-consistent one-dimensional tail current sheet

The paper investigates the formation, the structure, and the stability of self-consistent one-dimensional current sheets in which the ions carry most of the current and momentum (the occurrence of which was suggested by observations of Mitchell et al., 1990; and Sergeev et al., 1990). Results of the analysis showed that, for the case of a cold current sheet, the characteristic thickness lamba equals to about (Bz/B0) exp 4/3 c/omega(p0), where Bz is the normal field component, B0 is the asymptotic magnitude of the reversing field, and c/omega(p0)is the collisionless ion skin depth based on lobe density. A two-dimensional self-consistent dynamical simulation model is developed, which demonstrates that these idealized current sheets are unstable to kink perturbations driven by the anisotropic pressure distribution produced by the chaotic nature of the particle orbits in a field-reversal region.

Pritchett, P. L.↗

NASA NTRS1991

An experimental and analytical investigation on the response of GR/EP composite I-frames

Six-foot diameter, semicircular graphite/epoxy specimens representative of generic aircraft frames were loaded quasi-statically to determine their load response and failure mechanisms for large deflections that occur in an airplane crash. These frame-skin specimens consisted of a cylindrical skin section cocured with a semicircular I-frame. Various frame laminate stacking sequences and geometries were evaluated by statically loading the specimen until multiple failures occurred. Two analytical methods were compared for modeling the frame-skin specimens: a two-dimensional branched-shell finite element analysis and a one-dimensional, closed-form, curved beam solution derived using an energy method. Excellent correlation was obtained between experimental results and the finite element predictions of the linear response of the frames prior to the initial failure. The beam solution was used for rapid parameter and design studies, and was found to be stiff in comparison with the finite element analysis. The specimens were found to be useful for evaluating composite frame designs.

Moas, E., Jr.↗