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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Photopolarimetry of scattering surfaces and their interpretation by computer model

Wolff's computer model of a rough planetary surface was simplified and revised. Close adherence to the actual geometry of a pitted surface and the inclusion of a function for diffuse light resulted in a quantitative model comparable to observations by planetary satellites and asteroids. A function is also derived to describe diffuse light emitted from a particulate surface. The function is in terms of the indices of refraction of the surface material, particle size, and viewing angles. Computer-generated plots describe the observable and theoretical light components for the Moon, Mercury, Mars and a spectrum of asteroids. Other plots describe the effects of changing surface material properties. Mathematical results are generated to relate the parameters of the negative polarization branch to the properties of surface pitting. An explanation is offered for the polarization of the rings of Saturn, and the average diameter of ring objects is found to be 30 to 40 centimeters.

Wolff, M.↗
NASA NTRS1981

Determination of crash test pulses and their application to aircraft seat analysis

Deceleration time histories (crash pulses) from a series of twelve light aircraft crash tests conducted at NASA Langley Research Center (LaRC) were analyzed to provide data for seat and airframe design for crashworthiness. Two vertical drop tests at 12.8 m/s (42 ft/s) and 36 G peak deceleration (simulating one of the vertical light aircraft crash pulses) were made using an energy absorbing light aircraft seat prototype. Vertical pelvis acceleration measured in a 50 percentile dummy in the energy absorbing seat were found to be 45% lower than those obtained from the same dummy in a typical light aircraft seat. A hybrid mathematical seat-occupant model was developed using the DYCAST nonlinear finite element computer code and was used to analyze a vertical drop test of the energy absorbing seat. Seat and occupant accelerations predicted by the DYCAST model compared quite favorably with experimental values.

Alfaro-Bou, E.↗
NASA NTRS1981

Modeling the human controller in environments that include continuous and discrete tasks

In complex environments where the human operator is a supervisor, he must allocate his attention between different kinds of tasks for satisfactory overall performance. When a portion of the future reference trajectory for a continuous control task is available for preview, scheduling various other discrete activities is possible. A model has been developed for this situation using dynamic programming to solve an optimal control problem. An experiment was conducted where subjects controlled an airplane symbol over a map, shown a fixed distance into the future. Discrete tasks were introduced as data entry tasks. Results showed that the model compared favorably with experimental results.

Govindaraj, T.↗
NASA NTRS1983

Ponderomotive force in a warm two-fluid plasma

A general expression for the ponderomotive force of electromagnetic field with slowly varying amplitude in a warm plasma is obtained in the collisionless two-fluid model. Compared to the existing expression obtained for a cold and stationary plasma, this result contains two additional terms for the general case. One of them is involved with the density gradient and the wavenumber derivative of the dielectric tensor of a plasma. Thus, for an inhomogeneous plasma, it accounts for the effects of spatial dispersion, hence of finite temperature of the plasma. The other term is proportional to the time derivative of the density and the frequency derivative of the dielectric tensor, and is nonzero unless the plasma is stationary or nondispersive. Inclusion of these new terms in the ponderomotive force will enable self-consistent analysis of the general behavior of nonlinear waves with finite wavelengths in a warm plasma.

Lee, N. C.↗
NASA NTRS1983

Physics of the interaction of the solar wind with the ionosphere of Venus - Flow/field models

Venus deflects incident solar wind by means of thermal and magnetic pressure, both of which are produced by ionospheric currents that are in turn driven by pressure gradients and electric fields induced in the ionosphere by the flowing magnetized solar wind plasma. The ionospheric currents connect across the ionopause into the ionosheath region, and close on, and behind, the bow shock. The computed ionospheric plasma velocity distribution is found to be spatially asymmetric, with the fastest flow velocities lying perpendicular to the ionospheric magnetic fields. Computations also indicate that the ionosphere is susceptible to an MHD shear instability whose extent and location is a function of external solar wind conditions. The present Venus ionosphere model compares well with in situ Pioneer Venus Orbiter observations.

Cloutier, P. A.↗
NASA NTRS1985

Cosmological density fluctuations and large-scale structure From N-point correlation functions to the probability distribution

Knowledge of N-point correlation functions for all N allows one to invert and obtain the probability distribution of mass fluctuations in a fixed volume. The hierarchical sequence of higher order is applied to correlations with dimensionless amplitudes suggested by the BBGKY equations. The resulting distribution is significantly non-Gaussian, even for quite small mean square fluctuations. The qualitative and to some degree quantitative results are to a large degree independent of the exact sequence of amplitudes. An ensemble of such models compared with N-body simulations fails in detail to account for the low-density frequency distribution.

Fry, J. N.↗
NASA NTRS1985

Curie isotherm surfaces inferred from high-altitude magnetic anomaly data

Two-dimensional Curie depth models along two sections across the western United States are derived using an equivalent layer magnetization model derived from Magsat data, and the results are used to constrain finite element thermal models developed by Mayhew and Majer (1980). Regional heat flow variations predicted by the models compare favorably with those inferred from direct measurements. The methodology is applied to upward-continued aeromagnetic data, and the results are found to be in good agreement with previous Curie depth estimates and average measured heat flow.

Mayhew, M. A.↗
NASA NTRS1984

Multidisciplinary optimization applied to a transport aircraft

Decomposition of a large optimization problem into several smaller subproblems has been proposed as an approach to making large-scale optimization problems tractable. To date, the characteristics of this approach have been tested on problems of limited complexity. The objective of the effort is to demonstrate the application of this multilevel optimization method on a large-scale design study using analytical models comparable to those currently being used in the aircraft industry. The purpose of the design study which is underway to provide this demonstration is to generate a wing design for a transport aircraft which will perform a specified mission with minimum block fuel. A definition of the problem; a discussion of the multilevel composition which is used for an aircraft wing; descriptions of analysis and optimization procedures used at each level; and numerical results obtained to date are included. Computational times required to perform various steps in the process are also given. Finally, a summary of the current status and plans for continuation of this development effort are given.

Giles, G. L.↗
NASA NTRS1988

Surficial textures of the Galilean satellites

The observations of the Galilean satellites of Jupiter obtained with the IUE have been analyzed as a function of solar phase angle. By fitting the measurements to a shadowing model, comparative descriptions of the microtextures of the optically active portion of the surfaces of the satellites are derived. Important differences among the satellites and between leading and trailing hemispheres of individual satellites result from the different processes of meteoritic bombardment, magnetospheric interaction, and geological resurfacing that operate in the Jovian system. Io and Callisto have the most tenuous upper regoliths, whereas the surface of the leading side of Europa is the most compact.

Buratti, Bonnie J.↗
NASA NTRS1988

An analytic-geometric model of the effect of spherically distributed injection errors for Galileo and Ulysses spacecraft - The multi-stage problem

In previous work the problem of injecting the Galileo and Ulysses spacecraft from low earth orbit into their respective interplanetary trajectories has been discussed for the single stage (Centaur) vehicle. The central issue, in the event of spherically distributed injection errors, is what happens to the vehicle? The difficulties addressed in this paper involve the multi-stage problem since both Galileo and Ulysses will be utilizing the two-stage IUS system. Ulysses will also include a third stage: the PAM-S. The solution is expressed in terms of probabilities for total percentage of escape, orbit decay and reentry trajectories. Analytic solutions are found for Hill's Equations of Relative Motion (more recently called Clohessy-Wiltshire Equations) for multi-stage injections. These solutions are interpreted geometrically on the injection sphere. The analytic-geometric models compare well with numerical solutions, provide insight into the behavior of trajectories mapped on the injection sphere and simplify the numerical two-dimensional search for trajectory families.

Longuski, James M.↗
NASA NTRS1989

Predicting Flutter Of A Propfan

Report discusses theoretical model and associated computer program for prediction of subsonic bending-torsion cascade flutter in propfans. Predictions of model compared with results of experiments. Additional parametric studies illustrate effects upon flutter speed of steady aeroelastic deformations, angle at which blade set, speed of rotation, structural damping, and number of vibrational modes. Calculations performed by parts of ASTROP computer code: ASTROP 2, based on two-dimensional, subsonic, unsteady aerodynamics; and ASTROP 3, based on three-dimensional, subsonic, steady and unsteady aerodynamics.

Kaza, K. R. V.↗
NASA NTRS1990

The source charge and isotopic abundances of cosmic rays with Z = 9-16 - A study using new fragmentation cross sections

The cosmic ray source charge and isotopic abundances for charges with Z = 9-16 are reexamined using newly measured fragmentation cross sections in a standard Galactic propagation model. Compared with earlier studies, the cosmic-ray data are now consistent with no excess of Si-29 and Si-30 in the source relative to the solar coronal abundances. The excess of Mg-25 and Mg-26 is now about 1 sigma or less relative to solar coronal isotopic abundances, leaving Ne-22 as the only clearly established neutron-rich isotopic excess in the cosmic ray source. Better estimates of the source abundances of elements obtained using the new cross sections permit the conclusion that high first ionization potential (FIP) elements have a wide spread of compositional differences in the cosmic-ray source relative to solar coronal abundances, whereas elements with a low FIP have a composition similar to the solar corona.

Webber, W. R.↗
NASA NTRS1990

X-ray spectral signatures of photoionized plasmas

Plasma emission codes have become a standard tool for the analysis of spectroscopic data from cosmic X-ray sources. However, the assumption of collisional equilibrium, typically invoked in these codes, renders them inapplicable to many important astrophysical situations, particularly those involving X-ray photoionized nebulae. This point is illustrated by comparing model spectra which have been calculated under conditions appropriate to both coronal plasmas and X-ray photoionized plasmas. It is shown that the (3s-2p)/(3d-2p) line ratios in the Fe L-shell spectrum can be used to effectively discriminate between these two cases. This diagnostic will be especially useful for data analysis associated with AXAF and XMM, which will carry spectroscopic instrumentation with sufficient sensitivity and resolution to identify X-ray photoionized nebulae in a wide range of astrophysical environments.

Liedahl, Duane A.↗
NASA NTRS1990

Energy coupling in the magnetospheres of earth and Mercury

The mechanisms involved in the dissipation of solar-wind energy during magnetospheric substorms are considered theoretically, comparing models for earth and Mercury. In the model for terrestrial substorms, IMF lines interconnect with terrestrial field lines near the front of the magnetosphere and are dragged back, carrying plasma and energy, to form tail lobes; a magnetic neutral region is then formed by reconnection of the open lines as the plasma sheet thins, and reconnective heating and acceleration of tail plasma lead to plasma inflow at the poles and formation of a plasmoid flowing down the tail at high velocity. Analogous phenomena on Mercury could produce precipitation of particles carrying 10-1000 GW of power into 'auroral zones' on the dark side of the planet. The feasibility of remote or in situ observations to detect such processes is discussed.

Baker, D. N.↗
NASA NTRS1990

Matrix fatigue crack development in a notched continuous fiber SCS-6/Ti-15-3 composite

In this study the extensive matrix fatigue cracking that has been observed in notched SCS-6/Ti-15-3 composites is investigated. Away from the notch a uniform spacing of the fatigue cracks develops. Closer to the notch, fiber-matrix debonding which occurs increases the crack spacing. Crack spacing and debond length determined from shear-lag cylinder models compare favorably with experimental observations. Scanning electron microscope (SEM) fractography showed that the principal fatigue crack initiation occurred around the zero degree fibers. Interface failure in the 90 degree plies does not lead to the development of the primary fatigue cracking.

Hillberry, B. M.↗
NASA NTRS1990

A model for the space shuttle main engine high pressure oxidizer turbopump shaft seal system

A simple static model is presented which solves for the flow properties of pressure, temperature, and mass flow in the Space Shuttle Main Engine pressure Oxidizer Turbopump Shaft Seal Systems. This system includes the primary and secondary turbine seals, the primary and secondary turbine drains, the helium purge seals and feed line, the primary oxygen drain, and the slinger/labyrinth oxygen seal pair. The model predicts the changes in flow variables that occur during and after failures of the various seals. Such information would be particularly useful in a post flight situation where processing of sensor information using this model could identify a particular seal that had experienced excessive wear. Most of the seals in the system are modeled using simple one dimensional equations which can be applied to almost any seal provided that the fluid is gaseous. A failure is modeled as an increase in the clearance between the shaft and the seal. Thus, the model does not attempt to predict how the failure process actually occurs (e.g., wear, seal crack initiation). The results presented were obtained using a FORTRAN implementation of the model running on a VAX computer. Solution for the seal system properties is obtained iteratively; however, a further simplified implementation (which does not include the slinger/labyrinth combination) was also developed which provides fast and reasonable results for most engine operating conditions. Results from the model compare favorably with the limited redline data available.

Paxson, Daniel E.↗
NASA NTRS1990

A continuous proportional low-thrust propulsion system

Helium gas flows continuously from a low-temperature dewar supplying propellant for satellite thrusters. Thrust is obtained by movement of a spool located between two opposing nozzles, thus differentially restricting the helium flow. The design procedure and test results of a high bandwidth, low power, linear electromagnetic actuator for the pool movement are presented. Two mathematical models are used to describe the helium flow through the thruster under different flow conditions. The experimental data measured in the vacuum chamber and the computational results from the mathematical models are compared. The models agree with the experiment to 3 percent except near a Knudsen number of 1.

Chen, J.-H.↗
NASA NTRS1991

Evaluation of an efficient statistical transport model for turbulent droplet dispersion in dilute combusting sprays

Evaluation of an efficient statistical transport model for turbulent droplet dispersion is made for a dilute spray of methanol droplets injected into a turbulent, axisymmnetric methane-fuelled diffusion flame burning in stagnant air. In the dispersion model, a computational parcel representing a group of physical particles (droplets) is considered to have a normal probability density function (pdf) in space. The mean is determined by Lagrangian tracking through a sequence of stochastically generated turbulent eddies and the variance is evaluated from a statistical formulation based on the linearized particle equations of motion. The basic validity of this model is established through comparison with available experimental data and with theoretical predictions using a conventional stochastic direct modeling approach. The conclusion of the evaluation is that the proposed dispersion model compares favorably with experimental data and provides a valid technique for simulating turbulent combusting sprays with significant computational savings over conventional methods.

Litchford, Ron J.↗