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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

SINDA-NASTRAN interfacing program theoretical description and user's manual

The task of converting SINDA finite difference thermal model temperature results into NASTRAN finite element model thermal loads can be very labor intensive if there is not one node-to-one element, or systematic node-to-element. correlation between models. This paper describes the SINDA-NASTRAN Interfacing Program (SNIP), a FORTRAN computer code that generates NASTRAN structural model thermal load cards given by SINDA (or similar thermal model) temperature results and thermal model geometric data. SNIP generates NASTRAN thermal load cards for NASTRAN plate, shell, bar, and beam elements. The paper describes the interfacing procedures used by SNIP, and discusses set-up and operation of the program. Sample cases are included to demonstrate use of the program and show its performance under a variety of conditions. SNIP can provide structural model thermal loads that accurately reflect thermal model results while reducing the time required to interface thermal and structural models when compared to other methods.

Winegar, Steven R.↗
NASA NTRS1989

Numerical investigation of the control of the secondary instability process in boundary layers

A numerical model has been developed for investigating boundary layer transition control for a flat plate boundary layer. Active control of a periodically forced boundary layer in an incompressible fluid is studied using surface heating techniques. The spatially evolving boundary layer is simulated. The Navier-Stokes and energy equations are integrated using a fully implicit finite difference/spectral method. Temperature perturbations are introduced locally along finite heater strips to directly attenuate the instability waves in the flow. A feedback control loop is employed in which a downstream sensor is used to monitor wall shear stress fluctuations. Active control of small amplitude two-dimensional and three-dimensional disturbances is numerically simulated. With proper phase control, in-phase reinforcement and out-of-phase attenuation are demonstrated. A receptivity study of the localized temperature perturbations is made. It is shown that narrow heater strips are more receptive in that they maximize the amplitude level of the disturbances in the flow. Active control of the early stages of the fundamental breakdown process is also numerically simulated. Control is achieved with either two-dimensional or three-dimensional control inputs.

Kral, L. D.↗
NASA NTRS1984

Ion viscous effects on magnetosonic shock

Profiles of magnetic field and plasma bulk velocity associated with perpendicular shocks are investigated theoretically by solving magnetohydrodynamic equations. The main dissipation mechanism is assumed to be due to ion viscosity. Shock structures with zero temperatures and finite pressure are examined separately. The numerical solutions indicate that ion viscosity tends to smooth profiles of magnetic field and bulk velocity for high Mach number shocks. For smaller ion viscosity and lower Mach number, a damped wave train would appear downstream. The results suggest that kinetic effects such as ion reflections and microturbulence are necessary in order to explain magnetic field overshoots as well as oscillatory magnetic-field structure in high Mach number shocks.

Lin, C. S.↗
NASA NTRS1988

Linear stability of the H(+) - O(+) two-stream interaction in a magnetized plasma

The Bergman and Lotko (1986) analysis of the linear stability of the H(+) - O(+) two-stream interactions for parallel propagating wave modes is extended to an analysis of the stability of wave modes that propagate at an angle to the terrestrial magnetic field. The analaysis provides an insight into the manner in which the oblique modes linearly couple and become unstable; the conditions under which the particular couplings are important; the qualitative behavior of the frequency, wave vector, and growth rate of the most unstable mode; and the changes to these introduced by finite ion temperatures. Consideration is also given to the relative importance of obliquely propagating and parallel propagating waves for the condition when the velocity difference between two ion beams is below the upper limit.

Bergmann, R.↗
NASA NTRS1990

Three-dimensional simulation study of ionospheric plasma clouds

The results of fully three-dimensional numerical simulations of ionospheric plasma cloud evolution are presented. The evolution of the plasma cloud considered by Drake and Huba (1987) in the limit of vanishingly small ion compressibility is discussed. Simulations support the results of the analytical theory: finite plasma temperature, combined with fully three-dimensional plasma dynamics, is a stabilizing influence on plasma cloud evolution. This stability is associated with sheared azimuthal ion flows in the vicinity of the cloud surface. Cloud evolution using realistic values of ion compressibility show that the cloud rapidly diffuses to a state in which the sheared azimuthal flow is substantially reduced; subsequently, the cloud becomes unstable and structures.

Zalesak, S. T.↗
NASA NTRS1992

Simulation of triglycine sulfate crystal growth in space

Consideration is given to the numerical simulation of the solution growth of triglycine sulfate in space, with heat transfer, mass transfer, buoyancy-driven convection, the dependence of solubility on temperature, and finite interface kinetics taken into account. In the bulk solution, weak thermal convection was established quickly. The thermal convection then becomes dominated by solutionally driven motion. The convection due to the steady background g is predicted to influence both the growth rate and the crystal morphology. The time required for buoyancy-driven convection to influence the growth rate depends on the magnitude of g, i.e., more time for smaller g. Under the same cooling condition, the average growth rate increases as the magnitude of g increases. A steady average growth rate of 1.0 mm/d can be obtained when a properly designed cooling rate is applied to the sting. The most uniform growth is obtained when the g vector is aligned such that the solution flows normal towards the center of the crystal surface.

Sun, Jianhua↗
NASA NTRS1994

A hybrid simulation of contact discontinuity

Contact discontinuities in a collisionless plasma are studied by hybrid simulations, in which ions are treated as particles and electrons are considered as a fluid. It is demonstrated that contact discontinuity with a stable density ramp can exist in cases with a finite electron temperature. An electron pressure gradient is present across the contact discontinuity, leading to the presence of a parallel electric field and hence field-aligned potential increase (Delta Phi (sub parallel)) in the transition region. By reflecting ions at the discontinuity, this parallel electric potential peak reduces the interpenetration between hot and cold ions and maintains a stable density ramp across the contact discontinuity. The ratio of the field-aligned electric potential energy to ion thermal energy, e(Delta) Phi(sub parallel)/kT(sub i), is found to be an increasing function of T(sub e)/T(sub i), where T(sub e) and T(sub i) are respectively the electron and ion temperature.

Wu, B. H.↗
NASA NTRS2011

Large-Amplitude Transmitter-Associated and Lightning-Associated Whistler Waves in the Earth's Inner Plasmasphere at L less than 2

We report observations of very large amplitude whistler mode waves in the Earth fs nightside inner radiation belt enabled by the STEREO Time Domain Sampler. Amplitudes range from 30.110 mV/m (zero ]peak), 2 to 3 orders of magnitude larger than previously observed in this region. Measurements from the peak electric field detector (TDSMax) indicate that these large ]amplitude waves are prevalent throughout the plasmasphere. A detailed examination of high time resolution electric field waveforms is undertaken on a subset of these whistlers at L < 2, associated with pump waves from lightning flashes and the naval transmitter NPM in Hawaii, that become unstable after propagation through the ionosphere and grow to large amplitudes. Many of the waveforms undergo periodic polarization reversals near the lower hybrid and NPM naval transmitter frequencies. The reversals may be related to finite plasma temperature and gradients in density induced by ion cyclotron heating of the plasma at 200 Hz, the modulation frequency of the continuous ]mode NPM naval transmitter signal. Test particle simulations using the amplitudes and durations of the waves observed herein suggest that they can interact strongly with high ]energy (>100 keV) electrons on a time scale of <1 s and thus may be an important previously unaccounted for source of energization or pitch ]angle scattering in the inner radiation belt.

Breneman, A.↗
NASA NTRS2014

Measuring the Coronal Properties of IC 4329A with NuSTAR

We present an analysis of an approximately 160 ks NuSTAR observation of the nearby bright Seyfert galaxy IC 4329A. The highquality broadband spectrum enables us to separate the effects of distant reflection from the direct coronal continuum, and to therefore accurately measure the high-energy cutoff to be E(sub cut) = 178 (+74 / −40) keV. The coronal emission arises from accretion disk photons Compton up-scattered by a thermal plasma, with the spectral index and cutoff being due to a combination of the finite plasma temperature and optical depth. Applying standard Comptonization models, we measure both physical properties independently using the best signal to noise obtained to date in an active galactic nucleus over the 3 - 79 keV band. We derive kT(sub e) = 37(+7 /−6) keV with tau = 1.25(+0.20 / −0.10) assuming a slab geometry for the plasma, and kT(sub e) = 33(+6 / −6) keV with tau = 3.41(+0.58 / −0.38) for a spherical geometry, with both having an equivalent goodness-of-fit.

X-rays: binaries↗
NASA NTRS1981

Transport of infrared radiation in cuboidal clouds

The transport of infrared radiation in a single cuboidal cloud using a vertical two steam approximation was modeled. The emittance of the top face of the model cloud is always less than that for a plane parallel cloud of the same optical depth. The hemisphere flux escaping from the cloud top has a gradient from the center to the edges which brighten when the cloud is over warmer ground. Cooling rate calculations in the 8 to 13.6 micrometer region show that there is cooling from the sides of the cloud at all levels even when there is heating of the core from the ground below. The radiances exiting from model cuboidal clouds were computed by path integration over the source function obtained with the two stream approximation. It is suggested that the brightness temperature measured from finite clouds will overestimate the cloud top temperature.

HARSHVARDHAN↗
NASA NTRS1981

Transport of infrared radiation in cuboidal clouds

The transport of infrared radiation in a single cuboidal cloud is modeled using a variable azimuth two-stream approximation. Computations are made at 10 microns for a Deirmendjian (1969) C-1 water cloud where the single scattering albedo is equal to 0.638 and the asymmetry parameter is 0.865. The results indicate that the emittance of the top face of the model cloud is always less than that for a plane parallel cloud of the same optical depth. The hemispheric flux escaping from the cloud top possesses a gradient from the center to the edges which are warmer when the cloud is over warmer ground. Cooling rate calculations in the 8-13.6 micron region demonstrate that there is cooling out of the sides of the cloud at all levels even when there is heating of the core from the ground below. The radiances exiting from model cuboidal clouds are computed by path integration over the source function obtained with the two-stream approximation. Results indicate that the brightness temperature measured from finite clouds will overestimate the cloud-top temperature.

Harshvardhan, MR.↗
NASA NTRS2000

Simulation of Aluminum Micro-mirrors for Space Applications at Cryogenic Temperatures

Closed form and finite element models are developed to predict the device response of aluminum electrostatic torsion micro-mirrors fabricated on silicon substrate for space applications at operating temperatures of 30K. Initially, closed form expressions for electrostatic pressure arid mechanical restoring torque are used to predict the pull-in and release voltages at room temperature. Subsequently, a detailed mechanical finite element model is developed to predict stresses and vertical beam deflection induced by the electrostatic and thermal loads. An incremental and iterative solution method is used in conjunction with the nonlinear finite element model and closed form electrostatic equations to solve. the coupled electro-thermo-mechanical problem. The simulation results are compared with experimental measurements at room temperature of fabricated micro-mirror devices.

Kuhn, J. L.↗
NASA NTRS1973

The effect of grid transparency and finite collector size on determining ion temperature and density by the retarding potential analyzer

The analysis of ion data from retarding potential analyzers (RPA's) is generally done under the planar approximation, which assumes that the grid transparency is constant with angle of incidence and that all ions reaching the plane of the collectors are collected. These approximations are not valid for situations in which the ion thermal velocity is comparable to the vehicle velocity, causing ions to enter the RPA with high average transverse velocity. To investigate these effects, the current-voltage curves for H+ at 4000 K were calculated, taking into account the finite collector size and the variation of grid transparency with angle. These curves are then analyzed under the planar approximation. The results show that only small errors in temperature and density are introduced for an RPA with typical dimensions; and that even when the density error is substantial for non-typical dimensions, the temperature error remains minimal.

Troy, B. E., Jr.↗
NASA NTRS1986

The use of finite element methods for determining the cross-sectional temperature distribution in heat pipes

A model that is currently used to predict the priming and performance limitations of a monogroove heat pipe is expanded to include the boiling limitation and the cross-sectional temperature distribution as determined from a multidimensional finite element analysis technique. The improved model is verified experimentally and shown to accurately predict the cross-sectional temperature distribution when the heat flux distribution is known. The model provides a way to estimate the level at which nucleate boiling and the associated dryout of the capillary wick occurs.

Peterson, G. P.↗
NASA NTRS1972

Application of finite-element method in the computation of temperature with emphasis on radiative exchanges.

Analyses pertaining to the solution of heat transfer problems in combined modes based on the finite-element method are presented. Two elements - a triangular element employing two spatial variables and a multi-faceted bar element employing one spatial variable - with nonlinear radiation on the boundaries are detailed. The radiative effects considered on the diffuse-gray surface elements include: (1) directional radiant fluxes from distant sources, (2) radiative exchanges including surfaces of prescribed temperatures, and (3) radiative exchanges including elements whose temperatures are not known a priori. The nonlinear part of the boundary condition is treated in two different ways: (1) a consistent linearization method, and (2) a direct energy distribution method. Applications of these elements together with solution algorithms to three sample problems exhibit solution capability and obtainable accuracy.

Lee, H.-P.↗
NASA NTRS1980

The g-modes of white dwarfs

The neutral g-modes of a degenerate fluid at zero temperature are analyzed. The g-modes of a degenerate fluid at finite but small temperatures are then expanded in terms of those of the zero temperature fluid. For nonrelativistic degenerate fluids it is found that (1) the g-eigenvalues are proportional to T mu(6)sub e mu(-1)sub i, where T is the internal temperature of the fluid, mu sub e and mu sub i are the mean molecular weights of electrons and ions, respectively; (2) the ion pressure is solely responsible for driving the g-modes. For white dwarfs of about a solar mass, the periods of the g-oscillations are in the range of a few hundredths of seconds.

Sobouti, Y.↗