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At least 739 records · Page 41

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NASA NTRS1978

The dynamical halo and the variation of cosmic-ray path length with energy

It is shown that the dynamical halo model offers a natural explanation for the form of the variation of the cosmic-ray path length with energy. The variation above approximately 1 GeV/nucleon can be understood as due to the variation of the diffusion coefficient, and hence the resident time, with energy. The flattening of the curve below 1 GeV/nucleon is seen to mark a transition to a convection dominated regime where coefficient is no longer the determining parameter. A fit to the observations yields a halo outflow velocity of 8 km sec/1. An attempt is made to determine the overall scale of the halo and the diffusion coefficient using recent Be-10 flux measurements but the data do not agree well enough to pin down these variables to within less than four or five orders of magnitude.

Jones, F. C.↗

NASA NTRS1977

Numerical determination of the fundamental eigenvalue for the Laplace operator on a spherical domain

Methods for obtaining approximate solutions for the fundamental eigenvalue of the Laplace-Beltrami operator (i.e., the membrane eignevalue problem for the vibration equation) on the unit spherical surface are developed. Two types of spherical surface domains are considered: the interior of a spherical triangle, and the exterior of a great circle arc extending for less than pi radians (a spherical surface with a slit). In both cases, zero boundary conditions are imposed. In order to solve the resulting second-order elliptic partial differential equations in two independent variables, a finite difference approximation is employed. The fundamental eigenvalue is approximated by iteration utilizing the power method and point successive overrelaxation. Some numerical results are given and compared, in certain special cases, with analytical solutions to the eigenvalue problem. The significance of the numerical eigenvalue results is discussed in terms of the singularities in the solution of three-dimensional boundary-value problems near a polyhedral corner of the domain.

Walden, H.↗

NASA NTRS1978

A comparative study of Conroy and Monte Carlo methods applied to multiple quadratures and multiple scattering

An efficient numerical method of multiple quadratures, the Conroy method, is applied to the problem of computing multiple scattering contributions in the radiative transfer through realistic planetary atmospheres. A brief error analysis of the method is given and comparisons are drawn with the more familiar Monte Carlo method. Both methods are stochastic problem-solving models of a physical or mathematical process and utilize the sampling scheme for points distributed over a definite region. In the Monte Carlo scheme the sample points are distributed randomly over the integration region. In the Conroy method, the sample points are distributed systematically, such that the point distribution forms a unique, closed, symmetrical pattern which effectively fills the region of the multidimensional integration. The methods are illustrated by two simple examples: one, of multidimensional integration involving two independent variables, and the other, of computing the second order scattering contribution to the sky radiance.

Deepak, A.↗

NASA NTRS1979

The dynamical halo and the variation of cosmic-ray path length with energy

It is shown that the dynamical halo model offers a natural explanation for the form of the variation of the cosmic-ray path length with energy. The variation above about 1 GeV per nucleon can be understood as due to the variation of the diffusion coefficient, and hence the resident time, with energy. The flattening of the curve below 1 GeV per nucleon is seen to mark a transition to a convection-dominated regime where the diffusion coefficient is no longer the determining parameter. A fit to the observations yields a halo outflow velocity of 8 km/s. An attempt is made to determine the overall scale of the halo and the diffusion coefficient using recent Be-10 flux measurements, but the data do not agree well enough to pin down these variables to within less than four or five orders of magnitude.

Jones, F. C.↗

NASA NTRS1979

Minimum-variance fixed-form compensation of linear systems

The problem of determining the linear time-invariant compensator of a specified dimension which minimizes the asymptotic expected value of a quadratic form in the state variables of a linear stochastic system of arbitrary order, is considered. It is shown that under appropriate assumptions, the solution of this problem can be interpreted as a minimum-order observer-based or dual observer-based compensator for an optimally aggregated model of the plant.

Johnson, T. L.↗

NASA NTRS1981

Top-mounted inlet system feasibility for transonic-subsonic fighter aircraft applications

To inlet flow field and engine inlet performance data for an advanced fighter aircraft configuration were obtained over the Mach 0.6 to 2.0 range. The studies not only provided extensive data for the baseline arrangement, but also evaluated the effects of key aircraft configuration variables (inlet location, canopy-dorsal integration, wing leading-edge extension planform area, and variable incidence canards) on top inlet performance. In order to set these data in the context of practical aircraft systems top inlet performance is compared with that of more conventional inlet/airframe integrations. The results of these evaluations show that, for the top inlet configuration tested, relatively good inlet performance and compatibility characteristics are maintained during subsonic and transonic maneuver. However, at supersonic speeds, flow expansion over the forebody and wings causes an increase in local inlet Mach number subsequently reduces inlet performance levels. These characteristics infer that although top inlets many not pose a viable design option for aircraft requiring a high degree of supersonic maneuverability, they have distinct promise for vehicles with subsonic and transonic maneuver capabilities.

Williams, T. L.↗

NASA NTRS1984

MAGSAT correlations with geoid anomalies

The MAGSAT data of the Gulf of Mexico were analyzed to define better the possible relation of the negative MAGSAT anomaly there to the negative residual geoid anomaly in the western Gulf of Mexico. The estimated magnetic crystal anomaly pattern has a magnetic low in the region of the residual geoid low, but the shape of the anomalies are different. Since the shape and location of the negative magnetic anomaly are variable depending upon the particular polynomial and curve orders used, the degree of correspondance between the residual geoid and MAGSAT lithosphere anomalies was not established definitively.

Bowin, C. O.↗

NASA NTRS1984

Multifrequency observations of the BL Lacertae object 0735+178

In each of the present four simultaneous spectra covering the radio-through-X-ray regimes, the IR-UV synchrotron continuum dominates the total observed power and presumably becomes opague between 10 to the 11th and 10 to the 13th Hz. Nonsimultaneous observations were also conducted, over a longer time period, in order to study long- and short-term variability at X-ray, optical, and radio frequencies. These data indicate that the rapid and dramatic variations evident at IR and optical wavelengths are absent at radio and X-ray frequencies, supporting a view of IR-UV flux emanation from a small region, while the X-rays are produced by the inverse Compton process in the radio-emitting region. Particles, photons and magnetic field may not be far from equipartition in this region. Theoretical suggestions are developed regarding the radial behavior of the electron density and magnetic field.

Bregman, J. N.↗

NASA NTRS1984

An iwatsubo-based solution for labyrinth seals - comparison with experimental results

The basic equations are derived for compressible flow in a labyrinth seal. The flow is assumed to be completely turbulent in the circumferential direction where the friction factor is determined by the Blasius relation. Linearized zeroth and first-order perturbation equations are developed for small motion about a centered position by an expansion in the eccentricity ratio. The zeroth-order pressure distribution is found by satisfying the leakage equation while the circumferential velocity distribution is determined by satisfying the momentum equation. The first-order equations are solved by a separation of variables solution. Integration of the resultant pressure distribution along and around the seal defines the reaction force developed by the seal and the corresponding dynamic coefficients. The results of this analysis are compared to published test results.

Childs, D. W.↗

NASA NTRS1985

Search for scattered X-ray halos around variable sources - The X-ray halo of Cygnus X-1

The results of a program to search for the presence of halos due to X-ray scattering by interstellar grains in the line-of-sight to variable X-ray sources are reported. As part of this program, four Einstein HRI images of Cyg X-1 were examined. The analysis technique exploits the intrinsic aperiodic variability of this source to map the point response function of the optics. Any scattered halo present will not reflect short-term central source time variability, since such variability is smoothed by differential time delays of order days. Thus, a residual, nonvariable component to the surface brightness distribution (comprising about 12 percent or more of the source flux) is interpreted as a scattered halo. The Cyg X-1 halo is consistent with those of other sources found in previous studies using different techniques. Comparison is also made with a scattering model and, despite uncertainties in source spectrum and distance, reasonable agreement with the observatons is found using a standard interstellar grain model. The potential of X-ray scattering as a probe of the properties of interstellar grains is demonstrated.

Bode, M. F.↗

NASA NTRS1986

Computer aided derivation of equations for composite mechanics problems and finite element analyses

Explicit equations are derived for analysis of multilayered fiber composites and for finite element analyses. The equations are obtained using a symbolic program and tested for various composite properties as well as for different fiber orientations. In order to analyze multilayered fiber composite structures, a variable thickness finite element is formulated. Examples of an airfoil geometry, simulated in a form of a cantilevered beam with various fiber orientations are studied.

Sarigul, N.↗

NASA NTRS1987

A Comparison of Experimental and Theoretical Results for Labyrinth Gas Seals

The basic equations are derived for a two control volume model for compressible flow in a labyrinth seal. The flow is assumed to be completely turbulent and isoenergetic. The wall friction factors are determined using the Blasius formula. Jet flow theory is used for the calculation of the recirculation velocity in the cavity. Linearized zeroth and first order perturbation equations are developed for small motion about a centered position by an expansion in the eccentricity ratio. The zeroth order pressure distribution is found by satisfying the leakage equation. The circumferential velocity distribution is determined by satisfying the momentum equations. The first order equations are solved by a separation of variable solution. Integration of the resultant pressure distribution along and around the seal defines the reaction force developed by the seal and the corresponding dynamic coefficients. The results of this analysis are compared to experimental test results.

Scharrer, Joseph Kirk↗

NASA NTRS1987

Theory versus experiment for the rotordynamic coefficients of labyrinth gas seals. I - A two control volume model

The basic equations are derived for a two-control-volume model for compressible flow in a labyrinth seal. The recirculation velocity in the cavity is incorporated into the model for the first time. The flow is assumed to be completely turbulent and isoenergetic. The wall friction factors are determined using the Blasius formula. Jet flow theory is used for the calculation of the recirculation velocity in the cavity. Linearized zeroth- and first-order perturbation equations are developed for small motion about a centered position by an expansion in the eccentricity ratio. The zeroth-order pressure distribution is found by satisfying the leakage equation while the circumferential velocity distribution is determined by satisfying the momentum equations. The first-order equations are solved by a separation of variable solution. Integration of the resultant pressure distribution along and around the seal defines the reaction force developed by the seal and the corresponding dynamic coefficients.

Scharrer, J. K.↗

NASA NTRS1989

Rotordynamic coefficients for stepped labyrinth gas seals

The basic equations are derived for compressible flow in a stepped labyrinth gas seal. The flow is assumed to be completely turbulent in the circumferential direction where the friction factor is determined by the Blasius relation. Linearized zeroth and first-order perturbation equations are developed for small motion about a centered position by an expansion in the eccentricity ratio. The zeroth-order pressure distribution is found by satisfying the leakage equation while the circumferential velocity distribution is determined by satisfying the momentum equations. The first order equations are solved by a separation of variables solution. Integration of the resultant pressure distribution along and around the seal defines the reaction force developed by the seal and the corresponding dynamic coefficients. The results of this analysis are presented in the form of a parametric study, since there are no known experimental data for the rotordynamic coefficients of stepped labyrinth gas seals. The parametric study investigates the relative rotordynamic stability of convergent, straight and divergent stepped labyrinth gas seals. The results show that, generally, the divergent seal is more stable, rotordynamically, than the straight or convergent seals. The results also show that the teeth-on-stator seals are not always more stable, rotordynamically, then the teeth-on-rotor seals as was shown by experiment by Childs and Scharrer (1986b) for a 15 tooth seal.

Scharrer, Joseph K.↗

NASA NTRS1991

Sensitivity of Space Station alpha joint robust controller to structural modal parameter variations

The photovoltaic array sun tracking control system of Space Station Freedom is described. A synthesis procedure for determining optimized values of the design variables of the control system is developed using a constrained optimization technique. The synthesis is performed to provide a given level of stability margin, to achieve the most responsive tracking performance, and to meet other design requirements. Performance of the baseline design, which is synthesized using predicted structural characteristics, is discussed and the sensitivity of the stability margin is examined for variations of the frequencies, mode shapes and damping ratios of dominant structural modes. The design provides enough robustness to tolerate a sizeable error in the predicted modal parameters. A study was made of the sensitivity of performance indicators as the modal parameters of the dominant modes vary. The design variables are resynthesized for varying modal parameters in order to achieve the most responsive tracking performance while satisfying the design requirements. This procedure of reoptimization design parameters would be useful in improving the control system performance if accurate model data are provided.

Kumar, Renjith R.↗

NASA NTRS1991

Biophysical information in asymmetric and symmetric diurnal bidirectional canopy reflectance

The authors present a theory for partitioning the information content in diurnal bidirectional reflectance measurements in order to detect differences potentially related to biophysical variables. The theory, which divides the canopy reflectance into asymmetric and symmetric functions of solar azimuth angle, attributes asymmetric variation to diurnal changes in the canopy biphysical properties. The symmetric function is attributed to the effects of sunlight interacting with a hypothetical average canopy which would display the average diurnal properties of the actual canopy. The authors analyzed radiometer data collected diurnally in the Thematic Mapper wavelength bands from two walnut canopies that received differing irrigation treatments. The reflectance of the canopies varied with sun and view angles and across seven bands in the visible, near-infrared, and middle infrared wavelength regions. Although one of the canopies was permanently water stressed and the other was stressed in mid-afternoon each day, no water stress signature was unambiguously evident in the reflectance data.

Vanderbilt, Vern C.↗

NASA NTRS1991

Nonlinear influence of mesoscale land use on weather and climate

It is shown that the influence of mesoscale landscape spatial variability on the atmosphere must be parameterized or explicitly modeled in larger-scale atmospheric model simulations including general circulation models. The mesoscale fluxes of heat that result from this variability are shown to be of the same order of magnitude but with a different vertical structure than found for the turbulent fluxes. These conclusions are based on experiments in which no phase changes of water were permitted. To parameterize surface thermal inhomogeneities, the influence of landscape must be evaluated using spectral analysis or an equivalent procedure. To include the nonlinear contribution of each scale, numerical model simulations for the range of observed surface and overlying atmospheric conditions must be performed.

Pielke, R. A.↗

NASA NTRS1992

Computational modeling of femtosecond optical solitons from Maxwell's equations

An algorithm is developed that permits the direct time integration of full-vector nonlinear Maxwell's equations. This capability permits the modeling of both linear and nonlinear instantaneous and dispersive effects in the electric polarization in material media. The modeling of the optical carrier is retained. The fundamental innovation is to notice that it is possible to treat the linear and nonlinear convolution integrals, which describe the dispersion, as new dependent variables. A coupled system of nonlinear second-order ordinary differential equations can then be derived for the linear and nonlinear convolution integrals, by differentiating them in the time domain. These equations, together with Maxwell's equations, are solved to determine the electromagnetic fields in nonlinear dispersive media. Results are presented of calculations in one dimension of the propagation and collision of femtosecond electromagnetic solitons that retain the optical carrier, taking into account as the Kerr and Raman interactions.

Goorjian, Peter M.↗