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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 649 records · Page 36
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NASA NTRS1986

Missing matter in the vicinity of the sun

The Poisson and Vlasov equations are solved numerically for realistic Galaxy models which include multiple disk components, a Population II spheroid, and an unseen massive halo. The total amount of matter in the vicinity of the sun is determined by comparing the observed distributions of tracer stars, samples of F dwarfs, and K giants with the predictions of the Galaxy models. Results are obtained for a number of different assumed distributions of the unseen disk mass. For all the observed samples, typical models imply that about half of the mass in the solar vicinity must be in the form of unobserved matter. The volume density of unobserved material near the sun is about 0.1 solar mass/cu pc; the corresponding column density is about 30 solar mass/sq pc. This so far unseen material must be in a disk with an exponential scale height of less than 0.7 kpc.

Bahcall, John N.↗
NASA NTRS1987

Dark matter in the galactic disk

Observational data on the distributions of tracer stars, F dwarfs and K giants were used as input to obtain self-consistent solutions for the Poisson and Vlasov equations to set bounds on the amount of missing matter in the solar neighborhood. The numerical computations were carried out using Galaxy models which feature multiple disk components and an unseen massive halo. The star data included the mass components and velocity dispersions. Consideration of various possible distributions of the unseen matter leads to the conjecture that half of the disk material in the solar neighborhood has yet to be observed. Techniques for determining if brown dwarfs are a significant component of the missing mass are discussed, as are improved models which would use limited numbers of tracer stars to set further constraints on the amount and distribution of the missing mass.

Bahcall, John N.↗
NASA NTRS1986

A coupled marching procedure for the partially parabolized Navier-Stokes equations

A coupled finite-difference formulation is described for solving a reduced form of the compressible Navier-Stokes equations by a multiple space marching procedure. The properties of the equations are discussed and the solution algorithm presented. The scheme is used to compute incompressible flows by taking the incompressible limit (M about 0.1) of the compressible formulation. A separate Poisson equation is not required for the pressure. Results are compared with experimental data and other numerical predictions for low Reynolds number channel inlet flow, flow over a rearward-facing step in a channel, and flow near the trailing edge of a flat plate.

Liu, Xuezong↗
NASA NTRS1986

EUV astronomical spectroscopy with CCD detectors

The applicability of CCD detectors to astronomical extreme ultraviolet (EUV) spectroscopy (100-1250 A) is discussed. The advantages of CCDs in this spectral region include internal electron yield, the potential for very high quantum efficiency (about 50-90 percent), and broad wavelength response. Visible light suppression is achieved by a combination of low grating scattering, greater than unity electron yield in the EUV, and various filter techniques. For the current generation of CCDs, detection of only a few EUV photons will rapidly overwhelm the read noise; thus, for all practical S/N ratios used in astronomical spectroscopy, read noise will be negligible compared to the poisson statistics of the detected photons. A model based on experimental data for the quantum efficiency and electron yield of CCDs in the EUV is discussed.

Stern, R. A.↗
NASA NTRS1987

MPD thruster plasma instability studies

Results of an ongoing experimental/theoretical effort to understand the influence of plasma waves and instabilities upon coaxial plasma thruster performance and longevity are discussed. An experiment which permits measurement of the plasma dispersion relation plus a suite of plasma parameters for each one millisecond quasi-steady discharge is described. This investigation is guided by a linearized Boltzmann-Poisson kinetic numerical model which can accommodate the effects of collisions, inhomogeneities and the magnetic field. Preliminary tests reveal the presence of the low-frequency branch of an ion cyclotron wave. Although the stability of these waves still has to be established through spatial growth rate measurements, it is well known that they are prone to 'current-driven' instabilities. 'Current-driven' instabilities in the collisional tensor conducting plasma of the MPD thruster are analytically discussed and their relevance to the MPD plasma is illustrated by estimating a critical total current at which their onset occurs.

Kelly, A. J.↗
NASA NTRS1987

A possible explanation of the electron temperature enhancement in the wake of a satellite

A numerical simulation was performed in an attempt to explain the enhancement of electron temperature in the wake of an ionospheric satellite in terms of counterstreaming plasma expansions. The simulation was conducted using full kinetic treatments (Vlasov-Poisson code) for both the plasma electrons and ions. The calculations indicated that at a distance of a few Debye lengths from the surface of the body the wake/ambient electron-temperature ratio reaches a value of about 2. This result is in good agreement with some in situ and laboratory experimental results.

Singh, Nagendra↗
NASA NTRS1987

A systematic search for long-term variability in a large sample of X-ray sources

A method which uses the correct measurement error distribution (Poisson) and which naturally takes into account detections as well as upper limits has been developed to search for flux variability in X-ray sources. The method is applied to 256 sources observed at least twice with the Einstein Observatory Imaging Proportional Counter. These sources have, in the very large majority of cases, an X-ray flux in the range 1 x 10 to the -13th to 6 x 10 to the -12th erg/sq cm/s (0.2-3.5 keV). Thirty-one sources show flux variations with amplitude ranging from about 20 percent to a factor of 6. Flux variations of small amplitude seem to be fairly common, but can be detected only in bright sources or in sources observed with very long exposures. Variations of a factor of 2 or more are seen in about 15 percent of the sources for which variations of such an amplitude are detectable.

Maccacaro, Tommaso↗
NASA NTRS1986

A synchronization technique for optical PPM signals

A technique for maintaining synchronization between optical PPM (pulse-position modulation) pulses and a receiver clock by means of a delay-tracking loop is described and analyzed. The tracking loop is driven by a doubly stochastic Poisson process that contains information about the location of the desired slot boundaries. The slot boundaries are subject to slowly varying random delays that are ultimately tracked by the loop. The concept of fractional rms delay error is introduced to quantify the effects of signal and background induced shot noise on the performance of the delay-tracking loop.

Vilnrotter, V. A.↗
NASA NTRS1987

Cell model and elastic moduli of disordered solids - Low temperature limit

The cell theory has been previously employed to compute the equation of state of a disordered condensed system. It is now generalized to include anisotropic stresses. The condition of affine deformation is adopted, transforming an orginally spherical into an ellipsoidal cell. With a Lennard-Jones n-m potential between nonbonded centers, the formal expression for the deformational free energy is derived. It is to be evaluated in the limit of the linear elastic range. Since the bulk modulus in this limit is already known, it is convenient to consider a uniaxial deformation. To begin with, restrictions are made to the low-temperature limit in the absence of entropy contributions. Young's modulus and Poisson's ratio then follow.

Peng, S. T. J.↗
NASA NTRS1987

Mechanical and electrical properties of graphite fiber-epoxy composites made from pristine and bromine intercalated fibers

The mechanical and electrical properties of pristine and bromine intercalated graphite fiber-epoxy composites were compared. The two types of composite were similar in terms of tensile modulus, tensile strength, and Poisson's Ratio. However, the interlaminar shear strength of the brominated composite was 18 percent greater than its pristine counterpart. Only slight differences were observed in flexural properties. A five-fold decrease was observed in the electrical resistivity of the brominated composite parallel to the axis of the fibers, resulting in a unidirectional resistivity of about 90 microOmega/cm. Transverse resistivity was unaffected. Both types of composite were subjected to a simulated lightning strike of 10 KJ (at a peak current of 150 kA), and the composite with the intercalated graphite exhibited less damage.

Jaworske, Donald A.↗
NASA NTRS1987

Approximating SIR-B response characteristics and estimating wave height and wavelength for ocean imagery

NASA Space Shuttle Challenger SIR-B ocean scenes are used to derive directional wave spectra for which speckle noise is modeled as a function of Rayleigh random phase coherence downrange and Poisson random amplitude errors inherent in the Doppler measurement of along-track position. A Fourier filter that preserves SIR-B image phase relations is used to correct the stationary and dynamic response characteristics of the remote sensor and scene correlator, as well as to subtract an estimate of the speckle noise component. A two-dimensional map of sea surface elevation is obtained after the filtered image is corrected for both random and deterministic motions.

Tilley, David G.↗
NASA NTRS1987

Analysis of counting errors in the phase/Doppler particle analyzer

NASA is investigating the application of the Phase Doppler measurement technique to provide improved drop sizing and liquid water content measurements in icing research. The magnitude of counting errors were analyzed because these errors contribute to inaccurate liquid water content measurements. The Phase Doppler Particle Analyzer counting errors due to data transfer losses and coincidence losses were analyzed for data input rates from 10 samples/sec to 70,000 samples/sec. Coincidence losses were calculated by determining the Poisson probability of having more than one event occurring during the droplet signal time. The magnitude of the coincidence loss can be determined, and for less than a 15 percent loss, corrections can be made. The data transfer losses were estimated for representative data transfer rates. With direct memory access enabled, data transfer losses are less than 5 percent for input rates below 2000 samples/sec. With direct memory access disabled losses exceeded 20 percent at a rate of 50 samples/sec preventing accurate number density or mass flux measurements. The data transfer losses of a new signal processor were analyzed and found to be less than 1 percent for rates under 65,000 samples/sec.

Oldenburg, John R.↗
NASA NTRS1985

Investigation of instability, dynamic forces, and effect of dynamic loading on strength of cages for the bearings in the high pressure oxygen turbopumps for the space shuttle main engine

Experiments were performed to determine the effect of cyclic loading on bearing cage strength. A long term working tensile load of approximately 1300 N (300 lbs) was found to be the likely maximum. Higher loads caused a decrease in cage tensile strength after the 125,000 cycle testing period. Poisson's ratio in compression was found to be highly dependent upon the direction of the fiberglass plies. At room temperature the value was 0.15 with the plies and 0.68 across the plies. At -196 C (-321 F), the value with the plies was 0.20. The results of the analyses conducted have again demonstrated the critical need for improved lubrication in the high pressure oxygen turbopump bearings. Lubricant films with low shear strength and low friction coefficients promote cage stability and decrease ball/cage forces during marginal operating conditions. The analysis of the effect of combined bearing loads on ball/cage loads has identified a radial load of 3600 N (800 lbs) as the maximum for the current clearance of the balls and cage pockets. Liquid oxygen impinging on the cage in the direction of rotation was found to enhance cage stability.

Dufrane, K. F.↗
NASA NTRS1988

Consistent boundary conditions for Reduced Navier-Stokes (RNS) scheme applied to 3-dimensional internal viscous flows

A consistent and efficient set of boundary conditions is developed for the multi-sweep space-marching pressure-elliptic Reduced Navier-Stokes (RNS) scheme as applied for 3-D internal viscous flow problems. No-slip boundary conditions are directly imposed on the solid walls. There is no iteration procedure required in the cross plane to ensure mass conservation across each marching plane. The finite difference equations forming the coefficient matrix are ordered such that the surface normal velocity is specified on all the solid walls; unlike external flows, a pressure boundary condition in the cross plane is not required. Since continuity is directly satisfied at all points in the flow domain, the first order momentum equations can be solved directly for the pressure without the need for a Poisson pressure correction equation. The procedure developed herein can also be applied with periodic boundary conditions. The analysis is given for general compressible flows. Incompressible flow solutions are obtained, for straight and curved ducts of square cross section, to validate the procedure. These solutions are used to demonstrate the applicability of the RNS scheme, with the improved boundary conditions for internal flows with strong interaction, as would be encountered in ducts and turbomachinery geometries.

Reddy, D. R.↗
NASA NTRS1987

Coupling finite element and spectral methods: First results

A Poisson equation on a rectangular domain is solved by coupling two methods: the domain is divided in two squares, a finite element approximation is used on the first square and a spectral discretization is used on the second one. Two kinds of matching conditions on the interface are presented and compared. In both cases, error estimates are proved.

Bernardi, Christine↗
NASA NTRS1988

Computational technique for compressible vortex flows using the integral equation solution

The steady full-potential equation is written in the form of Poisson's equation, and the solution for the velocity field is expressed in terms of an integral equation. The integral solution consists of two surface integrals and one volume integral. The solution is obtained through successive iteration cycles. Each cycle of iteration consists of two sub-cycles, an inner cycle for wake relaxation and an out cycle for the strength of the source distribution integrals representing the flow compressibility. The density gradients in the source distribution is computed by using a type-differencing scheme of the Murman-Cole type. The method is applied to delta wings and the numerical examples show that a curved shock is captured on the wing suction side beneath the leading edge vortex sheet. Recently, a modified version of the scheme was applied to rectangular wings. In this modified scheme, the surface integral terms were computed by using a bilinear distribution of vorticity on triangular vortex panels which represent the wing and its wake. The results were compared with the available experimental data and they are in good agreement.

Kandil, Osama A.↗
NASA NTRS1988

Three-dimensional elliptic grid generation for an F-16

A case history depicting the effort to generate a computational grid for the simulation of transonic flow about an F-16 aircraft at realistic flight conditions is presented. The flow solver for which this grid is designed is a zonal one, using the Reynolds averaged Navier-Stokes equations near the surface of the aircraft, and the Euler equations in regions removed from the aircraft. A body conforming global grid, suitable for the Euler equation, is first generated using 3-D Poisson equations having inhomogeneous terms modeled after the 2-D GRAPE code. Regions of the global grid are then designated for zonal refinement as appropriate to accurately model the flow physics. Grid spacing suitable for solution of the Navier-Stokes equations is generated in the refinement zones by simple subdivision of the given coarse grid intervals. That grid generation project is described, with particular emphasis on the global coarse grid.

Reese L. Sorenson↗
NASA NTRS1988

Vibrational Power Flow Analysis of Rods and Beams

A new method to model vibrational power flow and predict the resulting energy density levels in uniform rods and beams is investigated. This method models the flow of vibrational power in a manner analogous to the flow of thermal power in a heat conduction problem. The classical displacement solutions for harmonically excited, hysteretically damped rods and beams are used to derive expressions for the vibrational power flow and energy density in the rod and beam. Under certain conditions, the power flow in these two structural elements will be shown to be proportional to the energy density gradient. Using the relationship between power flow and energy density, an energy balance on differential control volumes in the rod and beam leads to a Poisson's equation which models the energy density distribution in the rod and beam. Coupling the energy density and power flow solutions for rods and beams is also discussed. It is shown that the resonant behavior of finite structures complicates the coupling of solutions, especially when the excitations are single frequency inputs. Two coupling formulations are discussed, the first based on the receptance method, and the second on the travelling wave approach used in Statistical Energy Analysis. The receptance method is the more computationally intensive but is capable of analyzing single frequency excitation cases. The traveling wave approach gives a good approximation of the frequency average of energy density and power flow in coupled systems, and thus, is an efficient technique for use with broadband frequency excitation.

Wohlever, James Christopher↗