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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Time-dependent dusty gasdynamical flow near cometary nuclei

This paper presents time-dependent solutions to the coupled dusty hydrodynamics equations describing the spherically symmetric expansion of cometary neutral gas in the vicinity of a cometary nucleus. The sublimation process is repressented by gas outflow from a dust-covered reservoir containing stationary gas whose pressure and density values are determined by the sublimating (Ts) and surface (T0) temperatures. The model resolves earlier ambiguities in determining gas production rates and provides analytic relations between Ts, T0, and the gas parameters at the sonic point. The time evolution of a cometary outburst was modeled. It was found that, as a result of the strong gas-dust interaction in the inner coma region, a 'slow' disturbance in both the dust and gas parameters will be created in addition to the familiar gas blast-wave solution. This new 'slow' disturbance, which propagates with a velocity of about 0.2 km/s, might be responsible for some of the observed slowly expanding cometary halos, such as the one which was recently identified using 1910 Mount Wilson high-resolution comet Halley photographs.

Gombosi, T. I.↗
NASA NTRS1991

Thermal maps of Jupiter - Spatial organization and time dependence of stratospheric temperatures, 1980 to 1990

The spatial organization and time dependence of Jupiter's stratospheric temperatures have been measured by observing thermal emission from the 7.8-micrometer CH4 band. These temperatures, observed through the greater part of a Jovian year, exhibit the influence of seasonal radiative forcing. Distinct bands of high temperature are located at the poles and midlatitudes, while the equator alternates between warm and cold with a period of approximately 4 years. Substantial longitudinal variability is often observed within the warm midlatitude bands, and occasionally elsewhere on the planet. This variability includes small, localized structures, as well as large-scale waves with wavelengths longer than about 30,000 kilometers. The amplitudes of the waves vary on a time scale of about 1 month; structures on a smaller scale may have lifetimes of only days. Waves observed in 1985, 1987, and 1988 propagated with group velocities less than + or - 30 meters/sec.

Orton, Glenn S.↗
NASA NTRS1974

A measurement technique of time-dependent dielectric breakdown in MOS capacitors

The statistical nature of time-dependent dielectric breakdown characteristics in MOS capacitors was evidenced by testing large numbers of capacitors fabricated on single wafers. A multipoint probe and automatic electronic visual display technique are introduced that will yield statistical results which are necessary for the investigation of temperature, electric field, thermal annealing, and radiation effects in the breakdown characteristics, and an interpretation of the physical mechanisms involved. It is shown that capacitors of area greater than 0.002 sq cm may yield worst-case results, and that a multipoint probe of capacitors of smaller sizes can be used to obtain a profile of nonuniformities in the SiO2 films.

Li, S. P.↗
NASA NTRS1982

Solution of three-dimensional time-dependent viscous flows

A procedure for solving three-dimensional, time-dependent turbulent flows is presented. The consistently split Linearized Block Implicit (LBI) scheme is used in conjunction with the QR Operator scheme to solve an approximate form of the Navier-Stokes equations in generalized nonorthogonal coordinates employing physical velocity components. Results of computations for both second order finite differences and the fourth order generalized Operator Compact Implicit (OCI) schemes are presented.

Weinberg, B. C.↗
NASA NTRS1984

Time-dependent models of single- and double-layer mantle convection

Time-dependent numerical models of two-dimensional convection which simulate mantle convection are presented. Starting from a fluid initially at rest with a purely conductive temperature profile, it is found that for a Rayleigh number of ten million, the model experiences a transient period of double-layer convection which lasts on the order of hundreds of millions of years when scaled to the earth's mantle. It is suggested that transient periods of double-layer convection in the earth's mantle may have existed in the past, whether or not such a period exists today.

Boss, A. P.↗
NASA NTRS1985

Numerical simulation in three space dimensions of time-dependent thermal convection in a rotating fluid

Three-dimensional time-dependent convection in a plane layer of fluid, uniformly heated from below and subject to vertical shear and to rotation about an axis tilted from the vertical, was simulated by the numerical solution of the Boussinesq equations, including all Coriolis terms. Rotation about a vertical axis produces smaller convection cells with diminished heat fluxes and considerable vorticity. When the rotation axis is tilted from the vertical to represent tropical latitudes, the convection cells become elongated in a N-S direction. Imposed flows with constant vertical shear produce convective rolls aligned with the mean flow. When the rotation vector is tilted from the vertical, the competing effects due to rotation and shear can stabilize the convective motions.

Hathaway, D. H.↗
NASA NTRS1992

Time-dependent three dimensional compressible convection with depth-dependent properties

Using a spectral code, we have studied the time-dependent regime of three-dimensional anelastic compressible convection with depth-dependent thermal expansivity, viscosity and thermal conductivity in a wide box of size 5 x 5 x 1. Surface Rayleigh numbers up to 5 x 10 exp 6 have been considered. Very few cylindrical plumes are developed at the bottom but they join up collectively to form strong upwellings, which pulsate chaotically. Major descending flows occur in sheets which form rectangular planform at the top. The thermal and flow fields are dominated by large-scale features. The bottom 20 percent of the convecting layer is found to be superadiabatic.

Balachandar, S.↗
NASA NTRS1993

Central difference TVD schemes for time dependent and steady state problems

A Runge-Kutta formula in time is presently used to advance schemes in which central differences are used to solve the time-dependent Euler equations; a second difference is added near shocks as an artificial viscosity to reduce the given scheme to a first-order upwind one at shocks. A matrix-valued dissipation is introduced and compared with the scalar viscosity; a connection is shown between this artificial viscosity and flux limiters. The use of various flux limiters for this central difference scheme is compared.

Jorgenson, P.↗
NASA NTRS1994

Time-dependent collisional-radiative model for capillary discharge plasmas

A time-dependent, zero-dimensional collisional-radiative model has been developed to study capillary discharge plasmas in view of possibilities for obtaining population inversion leading to laser action in the extreme ultraviolet and soft x-ray spectral region, by amplifying the Balmer-alpha (H-alpha) line of different hydrogenlike ions with nuclear charges from Z = 3 to Z = 6. The model is described in detail, and results for the case of a carbon plasma are presented. Limitations of the model are discussed and comparison is made between our calculations and recent experimental results.

Pockl, M.↗
NASA NTRS1985

Selfsimilar time dependent shock structures

Diffusive shock acceleration as an astrophysical mechanism for accelerating charged particles has the advantage of being highly efficient. This means however that the theory is of necessity nonlinear; the reaction of the accelerated particles on the shock structure and the acceleration process must be self-consistently included in any attempt to develop a complete theory of diffusive shock acceleration. Considerable effort has been invested in attempting, at least partially, to do this and it has become clear that in general either the maximum particle energy must be restricted by introducing additional loss processes into the problem or the acceleration must be treated as a time dependent problem (Drury, 1984). It is concluded that stationary modified shock structures can only exist for strong shocks if additional loss processes limit the maximum energy a particle can attain. This is certainly possible and if it occurs the energy loss from the shock will lead to much greater shock compressions. It is however equally possible that no such processes exist and we must then ask what sort of nonstationary shock structure develops. The ame argument which excludes stationary structures also rules out periodic solutions and indeed any solution where the width of the shock remains bounded. It follows that the width of the shock must increase secularly with time and it is natural to examine the possibility of selfsimilar time dependent solutions.

Beck, R.↗
NASA NTRS1986

Time-dependent coronal shock acceleration of energetic solar flare particles

A global time-dependent model is presented for the coronal and interplanetary shock acceleration and propagation of energetic solar flare particles. The calculations are carried out to help prove that coronal shock acceleration of solar flare particles is responsible for energetic solar flare event data gathered in interplanetary space. The model is based on the theory of diffusive shock acceleration, and requires particle speeds to be much greater than bulk velocities. Also, sufficient scattering must occur upstream and downstream of the shock for the particle scattering mean free path to be smaller than the characteristic scale lengths, which causes the same particles to encounter the shock repeatedly. A spherically symmetric shock wave is assumed, which leads to the same emission configuration for impulsively and monoenergetically emitted particles. Consideration is given to acceleration by compression at the shock front, adiabatic deceleration in the divergent downstream flow, the temporal evolution of the shock and the three-dimensional geometry of the corona. The model is used to generate normalized proton omnidirectional distributions at 1 AU and at the shock front. The spectral exhibit trends similar to those in observational data, especially proton acceleration times and the proton distribution profiles at 1 AU.

Lee, M. A.↗
NASA NTRS1993

Time-dependent thermocapillary convection in a Cartesian cavity - Numerical results for a moderate Prandtl number fluid

The present numerical simulation explores a thermal-convective mechanism for oscillatory thermocapillary convection in a shallow Cartesian cavity for a Prandtl number 6.78 fluid. The computer program developed for this simulation integrates the two-dimensional, time-dependent Navier-Stokes equations and the energy equation by a time-accurate method on a stretched, staggered mesh. Flat free surfaces are assumed. The instability is shown to depend upon temporal coupling between large scale thermal structures within the flow field and the temperature sensitive free surface. A primary result of this study is the development of a stability diagram presenting the critical Marangoni number separating steady from the time-dependent flow states as a function of aspect ratio for the range of values between 2.3 and 3.8. Within this range, a minimum critical aspect ratio near 2.3 and a minimum critical Marangoni number near 20,000 are predicted below which steady convection is found.

Peltier, L. J.↗
NASA NTRS1972

Time dependent Hartree-Fock treatment of elastic scattering of electrons by H and He/+/.

Time dependent Hartree-Fock theory, in its coupled and uncoupled forms, is used to calculate the elastic singlet p-wave phase shifts for the scattering of electrons by H and He/+/. On comparison with the best available results it is concluded that the coupled scheme, which contains correlation to at least first order, is superior. Levinson's theorem is confirmed.

Jamieson, M. J.↗
NASA NTRS1974

Recent developments on time-dependent calculation of nonequilibrium flows

It is shown that, in a time-dependent nonequilibrium flow analysis, erroneous and unstable solutions can result from the use of temperature equations and flow variables other than pressure in fitting the bow shock. A formulation is presented that has been critically evaluated by comparison with a previous one and found to yield steady and more accurate solutions in conserving total enthalpy.

Li, C. P.↗
NASA NTRS1987

Measurements of time-dependent boundary layer turbulence in an oscillating flow

Changes in the time-dependent boundary-layer turbulence in an oscillating flow were investigated using the oscillating-flow facility described by Cook et al. (1985). Hot wire measurements were made across the boundary layer for frequencies ranging from 4 to 21 Hz at various positions from the boundary layer origin and with different amplitudes of velocity oscillation. The results clearly demonstrate that the turbulence level is influenced by the imposed oscillation. The variation of the turbulence level is not predicted well by the variation in boundary-layer thickness alone, indicating that the interaction between the imposed oscillation and the turbulence structure is more complex. It is suggested that turbulence models developed for steady flow may not be adequate for periodic turbulent flows.

Cook, William J.↗