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At least 343 records · Page 19

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NASA NTRS1992

Numerical simulation of plasma transport driven by the Io torus

The Rice convection model (RCM) has been modified to a form suitable for Jupiter (RCM-J) to study plasma interchange motion in and near the Io plasma torus. The net result of the interchange is that flux tubes, heavily loaded with torus plasma, are transported outward, to be replaced by tubes containing little low-energy (less than 1 keV) plasma. The process is numerically simulated in terms of time evolution from an initial torus that is longitudinally asymmetric and with gradually decreasing density outward from Io's orbit. In the simulations, the nonlinear stage of the instability characteristically exhibits outreaching fingers of heavily-loaded flux tubes that lengthen at an accelerating rate. The principal finding is that the primary geometrical form of outward transport of torus plasma in Jupiter's magnetosphere is through long, outward-moving fingers of plasma. In the simulations, the fingers mainly form in the active sector of the Io torus (the heavier side of the asymmetric torus), and they are spaced longitudinally roughly 20 deg apart.

Yang, Y. S.↗

NASA NTRS1992

Solar prediction analysis

Solar Activity prediction is essential to definition of orbital design and operational environments for space flight. This task provides the necessary research to better understand solar predictions being generated by the solar community and to develop improved solar prediction models. The contractor shall provide the necessary manpower and facilities to perform the following tasks: (1) review, evaluate, and assess the time evolution of the solar cycle to provide probable limits of solar cycle behavior near maximum end during the decline of solar cycle 22, and the forecasts being provided by the solar community and the techniques being used to generate these forecasts; and (2) develop and refine prediction techniques for short-term solar behavior flare prediction within solar active regions, with special emphasis on the correlation of magnetic shear with flare occurrence.

Smith, Jesse B.↗

NASA NTRS1992

Monte Carlo modeling of the resurfacing of Venus

We have developed a three-dimensional model of venusian resurfacing that employs Monte Carlo simulations of both impact cratering and volcanism. The model simulates the production of craters on Venus by using the observed mass distributions of Earth- and Venus-crossing asteroids and comets. Lava flows are modeled by an energy minimization technique to simulate the effects of local topography on the shape and extent of flows. The model is run under a wide range of assumptions regarding the scale and time evolution of volcanism on Venus. Regions of the parameter space that result in impact crater distributions and modifications that are currently observed will be explored to place limits on the possible volcanic resurfacing history of Venus.

Bullock, M. A.↗

NASA NTRS1992

Homogeneous quantum electrodynamic turbulence

The electromagnetic field equations and Dirac equations for oppositely charged wave functions are numerically time-integrated using a spatial Fourier method. The numerical approach used, a spectral transform technique, is based on a continuum representation of physical space. The coupled classical field equations contain a dimensionless parameter which sets the strength of the nonlinear interaction (as the parameter increases, interaction volume decreases). For a parameter value of unity, highly nonlinear behavior in the time-evolution of an individual wave function, analogous to ideal fluid turbulence, is observed. In the truncated Fourier representation which is numerically implemented here, the quantum turbulence is homogeneous but anisotropic and manifests itself in the nonlinear evolution of equilibrium modal spatial spectra for the probability density of each particle and also for the electromagnetic energy density. The results show that nonlinearly interacting fermionic wave functions quickly approach a multi-mode, dynamic equilibrium state, and that this state can be determined by numerical means.

Shebalin, John V.↗

NASA NTRS1993

Methodological problems with gamma-ray burst hardness/intensity correlations

The hardness and intensity are easily measured quantities for all gamma-ray bursts (GRBs), and so, many past and current studies have sought correlations between them. This Letter presents many serious methodological problems with the practical definitions for both hardness and intensity. These difficulties are such that significant correlations can be easily introduced as artifacts of the reduction procedure. In particular, cosmological models of GRBs cannot be tested with hardness/intensity correlations with current instrumentation and the time evolution of the hardness in a given burst may be correlated with intensity for reasons that are unrelated to intrinsic change in the spectral shape.

Schaefer, Bradley E.↗

NASA NTRS1993

Two-dimensional viscous accretion disk models. II - On viscous overstability

Accretion disks have been shown to be susceptible to pulsational overstability when the viscosity coefficient increases upon compression. In this study, the stability of an axisymmetric accretion disk with vertical structure is considered. 2D axisymmetric calculations were performed by solving the set of fully nonlinear hydrodynamic equations, including radiation transport. The time evolution of a representative section (ring) of a thin radiative accretion disk is studied. The existence of the overstability in 2D is confirmed. It is found that the presence of periodic oscillations near the outer boundary of a real disk might account for some of the low-amplitude flickering observed in dwarf novae.

Kley, W.↗

NASA NTRS1993

The soliton transform and a possible application to nonlinear Alfven waves in space

The inverse scattering transform (IST) based on the derivative nonlinear Schroedinger (DNLS) equation is applied to a complex time series of nonlinear Alfven wave data generated by numerical simulation. The IST describes the long-time evolution of quasi-parallel Alfven waves more efficiently than the Fourier transform, which is adapted to linear rather than nonlinear problems. When dissipation is added, so the conditions for the validity of the DNLS are not strictly satisfied, the IST continues to provide a compact description of the wavefield in terms of a small number of decaying envelope solitons.

Hada, T.↗

NASA NTRS1992

Particle acceleration in solar flares - Observations

Contrary to our historical understanding, the energetic particles in most major solar proton events do not come from the flare itself. The particle abundances, ionization states, time evolution, and longitude distributions all indicate that the particles are accelerated from the ambient plasma by a shock wave driven by a coronal mass ejection in these events. In contrast, the particles that do come from impulsive solar flares are unique in character. These particles are electron rich, have He-3/He-4 enhancements of up to 10,000, and enhancements in heavy elements such as Fe/C by factors of 10. The high ionization state of Fe, +20 indicates that the material has been heated to temperatures of about 2 x 10 exp 7 K. It is generally believed that preferential heating by selective absorption of plasma waves is combined with stochastic acceleration in these events. Recent studies of the broad gamma-ray lines emitted by energetic particles within the flare loops indicate that they are also Fe-rich, He-3 rich and proton-poor like the particles seen at 1 AU. In large impulsive events, particles from the impulsive phase may be reaccelerated by a coronal blast-wave shock.

Reames, Donald V.↗

NASA NTRS1993

On the tidal interaction between protostellar disks and companions

Formation of protoplanets and binary stars in a protostellar disk modifies the structure of the disk. Through tidal interactions, energy and angular momentum are transferred between the disk and protostellar or protoplanetary companion. We summarize recent progress in theoretical investigations of the disk-companion tidal interaction. We show that low-mass protoplanets excite density waves at their Lindblad resonances and that these waves are likely to be dissipated locally. When a protoplanet acquires sufficient mass, its tidal torque induces the formation of a gap in the vicinity of its orbit. Gap formation leads to the termination of protoplanetary growth by accretion. For proto-Jupiter to attain its present mass, we require that (1) the primordial solar nebula is heated by viscous dissipation; (2) the viscous evolution time scale of the nebula is comparable to the age of typical T Tauri stars with circumstellar disks; and (3) the mass distribution in the nebula is comparable to that estimated from a minimum-mass nebula model.

Lin, D. N. C.↗

NASA NTRS1993

Collective effects of temperature gradients and gravity on droplet coalescence

The interaction and coalescence of small spherical drops in dilute, homogeneous dispersions are considered theoretically under conditions, where drop motion results from gravity settling and thermocapillary migration acting simultaneously. A trajectory analysis is used to predict pairwise collision rates, and population dynamics equations are solved to predict the time evolution of the droplet size distribution. The rate of droplet collisions and growth may be reduced dramatically by antiparallel alignment of the gravitational and thermocapillary velocities. For such antiparallel alignment with the gravitational relative velocity exceeding the thermocapillary relative velocity for two widely separated drops, there is a 'collision-forbidden region' in parameter space. This occurs because the gravitational relative velocity decays more rapidly with decreasing separation distance between the drops than does the thermocapillary relative velocity, and so the resultant relative velocity along the line-of-centers from these two sources combined becomes zero at a finite separation and the drops are unable to collide. As a result, small drops which initially collide and coalesce due to thermocapillary motion will only grow until they reach a critical size for which the oppositely directed gravitational motion balances the thermocapillary motion.

Zhang, Xiaoguang↗

NASA NTRS1993

Navier-Stokes calculations for vortex rings in an unbounded domain

A computational code for the vorticity-potential method is developed for a three-dimensional bounded vorticity field. The evaluation of the boundary data for the vector potential in the code is improved so that the numerical solution simulates that in an unbounded domain to a high order. The time evolution of two vortex rings and that of an elliptic ring are investigated with this code. The cut-and-connect phenomena of vortex rings are successfully captured. The results are compared with those of asymptotic theory and the experiment. They also highlight the need for additional theoretical and numerical investigations.

Ishii, Katsuya↗

NASA NTRS1993

Energy considerations in computational aeroacoustics

A finite-volume multistage time-stepping Euler code is used to investigate the use of CFD algorithms for the direct calculation of acoustics. The 2D compressible inviscid flow about an accelerating or decelerating circular cylinder is used as a model problem. The time evolution of the energy transfer from the cylinder to the fluid, as the cylinder is moved from rest to some nonnegligible velocity, is clearly seen. By examining the temporal and spatial characteristics of the numerical solution, a distinction can be made between the propagating acoustic energy, the convecting energy associated with the entropy change in the fluid, and the energy contained in the local aerodynamic field. Systematic variation of the cylinder acceleration shows that the radiated acoustic energy depends strongly upon the rate of acceleration or deceleration. The computational grid has a large effect on the ratio of acoustic energy to nonphysical entropy associated energy, while the role of the explicit artificial viscosity seems to be of second order. The entropy term was nearly negligible in all cases the cylinder was started slowly.

Brentner, Kenneth S.↗

NASA NTRS1992

A numerical model including PID control of a multizone crystal growth furnace

This paper presents a 2D axisymmetric combined conduction and radiation model of a multizone crystal growth furnace. The model is based on a programmable multizone furnace (PMZF) designed and built at NASA Lewis Research Center for growing high quality semiconductor crystals. A novel feature of this model is a control algorithm which automatically adjusts the power in any number of independently controlled heaters to establish the desired crystal temperatures in the furnace model. The control algorithm eliminates the need for numerous trial and error runs previously required to obtain the same results. The finite element code, FIDAP, used to develop the furnace model, was modified to directly incorporate the control algorithm. This algorithm, which presently uses PID control, and the associated heat transfer model are briefly discussed. Together, they have been used to predict the heater power distributions for a variety of furnace configurations and desired temperature profiles. Examples are included to demonstrate the effectiveness of the PID controlled model in establishing isothermal, Bridgman, and other complicated temperature profies in the sample. Finally, an example is given to show how the algorithm can be used to change the desired profile with time according to a prescribed temperature-time evolution.

Panzarella, Charles H.↗

NASA NTRS1993

Mixing in a stratified shear flow: Energetics and sampling

Direct numerical simulations of the time evolution of homogeneous stably stratified shear flows have been performed for Richardson numbers from 0 to 1 and for Prandtl numbers between 0.1 and 2. The results indicate that mixing efficiency R(sub f) varies with turbulent Froude number in a manner consistent with laboratory experiments performed with Prandtl numbers of 0.7 and 700. However, unlike the laboratory results, for a particular Froude number, the simulations do not show a clear dependence on the magnitude of R(sub f) on Pr. The observed maximum value of R(sub f) is 0.25. When averaged over vertical length scales of an order of magnitude greater than either the overturning or Ozmidov scales of the flow, the simulations indicate that the dissipation rate epsilon is only weakly lognormally distributed with an intermittency of about 0.01 whereas estimated values in the ocean are 3 to 7.

Ivey, G. N.↗

NASA NTRS1993

On the large eddy simulation of turbulent flows in complex geometry

Application of the method of Large Eddy Simulation (LES) to a turbulent flow consists of three separate steps. First, a filtering operation is performed on the Navier-Stokes equations to remove the small spatial scales. The resulting equations that describe the space time evolution of the 'large eddies' contain the subgrid-scale (sgs) stress tensor that describes the effect of the unresolved small scales on the resolved scales. The second step is the replacement of the sgs stress tensor by some expression involving the large scales - this is the problem of 'subgrid-scale modeling'. The final step is the numerical simulation of the resulting 'closed' equations for the large scale fields on a grid small enough to resolve the smallest of the large eddies, but still much larger than the fine scale structures at the Kolmogorov length. In dividing a turbulent flow field into 'large' and 'small' eddies, one presumes that a cut-off length delta can be sensibly chosen such that all fluctuations on a scale larger than delta are 'large eddies' and the remainder constitute the 'small scale' fluctuations. Typically, delta would be a length scale characterizing the smallest structures of interest in the flow. In an inhomogeneous flow, the 'sensible choice' for delta may vary significantly over the flow domain. For example, in a wall bounded turbulent flow, most statistical averages of interest vary much more rapidly with position near the wall than far away from it. Further, there are dynamically important organized structures near the wall on a scale much smaller than the boundary layer thickness. Therefore, the minimum size of eddies that need to be resolved is smaller near the wall. In general, for the LES of inhomogeneous flows, the width of the filtering kernel delta must be considered to be a function of position. If a filtering operation with a nonuniform filter width is performed on the Navier-Stokes equations, one does not in general get the standard large eddy equations. The complication is caused by the fact that a filtering operation with a nonuniform filter width in general does not commute with the operation of differentiation. This is one of the issues that we have looked at in detail as it is basic to any attempt at applying LES to complex geometry flows. Our principal findings are summarized.

Ghosal, Sandip↗

NASA NTRS1991

Excited state dynamics of thulium ions in yttrium aluminum garnets

The processes that take place in the excited states of a trivalent Thulium (Tm) ion in an Yttrium Aluminum Garnet (YAG) crystal, being relevant to the use of this system for laser applications, have been the object of several studies. We have reexamined this system focusing our attention on the dynamics of Tm following its excitation in the H-3(sub 4) level. Under these conditions the system relaxes through a cross-relaxation process. H-3(sub 4) yields F-3(sub 4), H-3(sub 6) yields F-3(sub 4), whose rate depends upon both the concentration of the Tm ion and the temperature of the crystal. The excitation spectrum obtained by monitoring the 1.8 micron emission of Tm (due to the F-3(sub 4) yields H-3(sub 6) transition) indicates an increase in the contribution to this emission from the H-3(sub 4) level relative to the H-3(sub 5) level as the Tm concentration increases; this shows the increased role played by the H-3(sub 4) level in pumping the infrared emission. Correspondingly, the duration of the luminescence originating in the H-3(sub 4) level is shortened as the concentration of Tm increases. The concentration quenching of this lifetime can be fit to a model which assumes that the cross-relaxation is due to a dipole-dipole interaction; from this fit, the intrinsic Tm lifetime in the absence of cross relaxation can be derived. We have used this lifetime to calculate the rate of the cross-relaxation process. We have evaluated this rate as a function of the temperature and found it to be fastest at 77 K. We have also calculated the microscopic interaction parameters for the cross-relaxation process by using two independent experimental features: (1) the time evolution of the emission from the H-3(sub 4) level; and (2) the spectral overlap between the H-3(sub 4) yields F-3(sub 4) emission and the H-3(sub 6) yields F-3(sub 4) absorption. We have also considered the migration of excitation among the Tm ions in the F-3(sub 4) level and calculated the relevant microparameter by the use of the relevant spectral overlap. The data are consistent with the model in which the Tm ions, once excited into the H-3(sub 4) level decay by cross-relaxation to the F-3(sub 4), and then transfer rapidly their energy to other Tm ions.

Armagan, G.↗

NASA NTRS1994

Observational constraints on black hole accretion disks

We review the empirical constraints on accretion disk models of stellar-mass black holes based on recent multiwavelength observational results. In addition to time-averaged emission spectra, the time evolutions of the intensity and spectrum provide critical information about the structure, stability, and dynamics of the disk. Using the basic thermal Keplerian disk paradigm, we consider in particular generalizations of the standard optically thin disk models needed to accommodate the extremely rich variety of dynamical phenomena exhibited by black hole candidates ranging from flares of electron-positron annihilations and quasiperiodic oscillations in the X-ray intensity to X-ray novae activity. These in turn provide probes of the disk structure and global geometry. The goal is to construct a single unified framework to interpret a large variety of black hole phenomena. This paper will concentrate on the interface between basic theory and observational data modeling.

Liang, Edison P.↗

NASA NTRS1994

On the nonlinear stability of viscous modes within the Rayleigh problem on an infinite flat plate

The stability has been investigated of the unsteady flow past an infinite flat plate when it is moved impulsively from rest, in its own plane. For small times the instantaneous stability of the flow depends on the linearized equations of motion which reduce in this problem to the Orr-Sommerfeld equation. It is known that the flow for certain values of Reynolds number, frequency and wave number is unstable to Tollmien-Schlichting waves, as in the case of the Blasius boundary layer flow past a flat plate. With increase in time, the unstable waves only undergo growth for a finite time interval, and this growth rate is itself a function of time. The influence of finite amplitude effects is studied by solving the full Navier-Stokes equations. It is found that the stability characteristics are markedly changed both by the consideration of the time evolution of the flow, and by the introduction of finite amplitude effects.

Webb, J. C.↗