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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Trajectory optimization for an asymmetric launch vehicle

A numerical optimization technique is used to fully automate the trajectory design process for an symmetric configuration of the proposed Advanced Launch System (ALS). The objective of the ALS trajectory design process is the maximization of the vehicle mass when it reaches the desired orbit. The trajectories used were based on a simple shape that could be described by a small set of parameters. The use of a simple trajectory model can significantly reduce the computation time required for trajectory optimization. A predictive simulation was developed to determine the on-orbit mass given an initial vehicle state, wind information, and a set of trajectory parameters. This simulation utilizes an idealized control system to speed computation by increasing the integration time step. The conjugate gradient method is used for the numerical optimization of on-orbit mass. The method requires only the evaluation of the on-orbit mass function using the predictive simulation, and the gradient of the on-orbit mass function with respect to the trajectory parameters. The gradient is approximated with finite differencing. Prelaunch trajectory designs were carried out using the optimization procedure. The predictive simulation is used in flight to redesign the trajectory to account for trajectory deviations produced by off-nominal conditions, e.g., stronger than expected head winds.

Sullivan, Jeanne Marie↗
NASA NTRS1990

Formation of ozone by irradiation of oxygen at 248 nanometers

While Slanger et al. (1988) have reported that the 248-nm KrF laser radiation generates ozone from oxygen, despite this wavelength's exceeding of the conventionally accepted photodissociation threshold of 242.4 for the ground electronic state, the initiating mechanism for this ozone formation remains obscure. It is presently suggested that the initiating step is the absorption of the 248-nm radiation by O2. In a reply to the present authors, Slanger et al. indicate that their original experiment should have been performed by introducing pure O2 into a baked cell, with the start time defined by the unblocking of the 248-nm laser.

Freeman, D. E.↗
NASA NTRS1990

Modelling the solution growth of TGS crystals in low gravity

The experimental growth of triglycine sulfate (TGS) crystals from aqueous solution is modeled here in two dimensions using the PHOENICS finite volume code. Simulations are carried out for steady, impulsive, and periodic accelerations in order to determine tolerable acceleration levels. Scaling arguments are used to estimate the times required for thermal and solutal variations from the initial equilibrium state to be diffusively transported throughout the system, and to obtain order of magnitude information on the relative magnitudes of diffusive and convective transport. The computed concentration fields reflect the features of the concentration distributions found experimentally during experiments conducted aboard Spacelab 3 in 1985.

Nadarajah, Arunan↗
NASA NTRS1991

The thermal stability of coronal loops - Numerical simulations

The radiative stability of thermally isolated coronal loops with free-flow boundary conditions by nonlinear numerical simulation are studied. A chromosphere-to-corona loop equilibrium (including the option of a deep chromosphere) is first established by following the nonlinear evolution from an initial isothermal state with rigid boundaries. The end conditions are then changed, to allow free flow and to fix the temperature, and investigate the response to nonisobaric perturbations. Within a family of loops of the same pressure, long hot loops to be stable and short cool loops to be unstable to the thermal chromospheric-expansion mode are found. The stable cases remain so, even when long chromospheric ends and/or gravity are added. In those cases which are unstable, the subsequent nonlinear evolution which exhibits swelling of the chromosphere until the entire loop becomes cool and dense are followed.

Mok, Y.↗
NASA NTRS1991

Dynamical evolution and molecular abundances of interstellar clouds

Dynamical models are presented that start with interstellar gas in an initial diffuse state and consider their gravitational collapse and the formation of dense cores. Frozen-in tangled magnetic fields are included to mimic forces that might oppose gravitational contraction and whose effectiveness may increase with increasing core densities. Results suggest the possibility that dense cloud cores may be dynamically evolving ephemeral objects, such that their lifespan at a given core density decreases as that density increases.

Prasad, Sheo S.↗
NASA NTRS1991

Transient response of a high-capacity heat pipe for Space Station Freedom

High-capacity heat pipe radiator panels have been proposed as the primary means of heat rejection for Space Station Freedom. In this system, the heat pipe would interface with the thermal bus condensers. Changes in system heat load can produce large temperature and heat load variations in individual heat pipes. Heat pipes could be required to start from an initially cold state, with heat loads temporarily exceeding their low-temperature transport capacity. The present research was motivated by the need for accurate prediction of such transient operating conditions. In this work, the cold startup of a 6.7-meter long high-capacity heat pipe is investigated experimentally and analytically. A transient thermohydraulic model of the heat pipe was developed which allows simulation of partially-primed operation. The results of cold startup tests using both constant temperature and constant heat flux evaporator boundary conditions are shown to be in good agreement with predicted transient response.

Ambrose, J. H.↗
NASA NTRS1991

Planetary origin, evolution, and structure

Three areas of recent and ongoing research are presented. The first area is giant planet heatflows. Conventional wisdom attributes the heatflow of the giant planets to the gradual loss of primordial heat, except in the case of Saturn where helium separation is evidently occurring. There are two problems with this picture: (1) the observed helium abundance of Saturn's atmosphere is so low that Jupiter must also be differentiating helium since its internal entropy cannot be much higher than Saturn; and (2) the heatflow of Neptune (not to mention Uranus) is too high to be consistent with adiabatic cooling from an initial hot state. A self-consistent solution to these two problems is presented. The second area covered is that of the despinning protogiant planets. Modeling of the possible despinning of these protoplanets by hydromagnetic torques was performed and the model results are discussed. The third area covered is how Titan hides its ocean. Until recently, the favored picture of Titan's surface was a roughly kilometer-thick ethane/methane ocean, presumably global in extent with at most a few outcroppings of dry land. The depth of the ocean is well constrained by observed atmospheric properties, and the constraints on subaerial topography are obtained indirectly from tidal considerations. A different picture of Titan's surface was pursued which was motivated by the perspective that methane on Titan should more properly be considered as a magmatic fluid. In this picture, methane is stored subsurface in magma chambers fed from deep-seated sources of methane, most probably due to the high pressure breakdown of methane clathrate. Other aspects of this model of Titan are presented.

Stevenson, David J.↗
NASA NTRS1992

A burst from a thermonuclear runaway on an ONeMg white dwarf

Studies which examine the consequences of accretion, at rates of 10(exp -9) solar mass/yr and 10(exp -10) solar mass/yr, onto an ONeMg white dwarf with a mass of 1.35 solar masses are performed. In these studies, a Lagrangian, hydrodynamic, one-dimensional computer code was used. The code now includes a network with 89 nuclei up to Ca-40, elemental diffusion, new opacities, and new equation of state. The initial abundance distribution corresponded to a mixture that was enriched to either 25, 50, or 75 percent in products of carbon burning. The remaining material in each case is assumed to have a solar composition. The evolution of the thermonuclear runaway in the 1.35 solar mass white dwarf, with M = 10(exp -9) solar mass, produced peak temperatures in the shell source exceeding 300 million degrees. The sequence produced significant amounts of Na-22 from proton captures onto Ne-20 and significant amounts of Al-26 from proton captures on Mg-24. This sequence ejected 5.2 x 10(exp -6) solar mass moving with speeds from approximately 100 km/s to 2300 km/s. When the mass accretion rate was decreased to 10(exp -10) solar mass, the resulting thermonuclear runaway produced a shock that moved through the outer envelope of the white dwarf and raised the surface luminosity to L greater than 10(exp 7) solar luminosity and the effective temperature to values exceeding 10(exp 7) K. The interaction of the material expanding from off of the white dwarf with the accretion disk should produce a burst of gamma-rays.

Starrfield, S.↗
NASA NTRS1992

Unsteady Thermocapillary Migration of Isolated Drops in Creeping Flow

The problem of an isolated immiscible drop that slowly migrates due to unsteady thermocapillary stresses is considered. All physical properties except for interfacial tension are assumed constant for the two Newtonian fluids. Explicit expressions are found for the migration rate and stream functions in the Laplace domain. The resulting microgravity theory is useful, e.g., in predicting the distance a drop will migrate due to an impulsive interfacial temperature gradient as well as the time required to attain steady flow conditions from an initially resting state.

Dill, Loren H.↗
NASA NTRS1994

General properties of quantum optical systems in a strong field limit

We investigate the dynamics of an arbitrary atomic system (n-level atoms or many n-level atoms) interacting with a resonant quantized mode of an em field. If the initial field state is a coherent state with a large photon number then the system dynamics possesses some general features, independently of the particular structure of the atomic system. Namely, trapping states, factorization of the wave function, collapses and revivals of the atomic energy oscillations are discussed.

Chumakov, S. M.↗
NASA NTRS1994

Gasdynamic models of the solar wind/interstellar medium interaction

The interaction between the solar wind and the interstellar medium is modeled self-consistently using numerical solutions of the time-dependent gasdynamic equations in spherical and cylindrical coordinates. For the results presented here it is assumed that the solar system moves through the surrounding medium with a supersonic velocity. After an initial (nonequilibrium) state has been specified, the numerical solution follows the evolution in time until the interaction relaxes to a dynamic equilibrium. As would be expected, the solutions show the formation of a bow shock upstream of the traveling solar system to deflect the interstellar plasma around the cavity created by the solar wind. A terminiation shock also forms to slow and compress the solar wind plasma. For the simulation in spherical coordinates, the downstream portion of the termination shock reaches equilibrium more than three times further from the Sun than the equilibrium distance to the termination shock on the upstream side.

Steinolfson, R. S.↗
NASA NTRS1995

Shear-induced opening of the coronal magnetic field

This work describes the evolution of a model solar corona in response to motions of the footpoints of its magnetic field. The mathematics involved is semianalytic, with the only numerical solution being that of an ordinary differential equation. This approach, while lacking the flexibility and physical details of full MHD simulations, allows for very rapid computation along with complete and rigorous exploration of the model's implications. We find that the model coronal field bulges upward, at first slowly and then more dramatically, in response to footpoint displacements. The energy in the field rises monotonically from that of the initial potential state, and the field configuration and energy appraoch asymptotically that of a fully open field. Concurrently, electric currents develop and concentrate into a current sheet as the limiting case of the open field is approached. Examination of the equations shows rigorously that in the asymptotic limit of the fully open field, the current layer becomes a true ideal MHD singularity.

Wolfson, Richard↗
NASA NTRS1994

An experiment in hurricane track prediction using parallel computing methods

The barotropic model is used to explore the advantages of parallel processing in deterministic forecasting. We apply this model to the track forecasting of hurricane Elena (1985). In this particular application, solutions to systems of elliptic equations are the essence of the computational mechanics. One set of equations is associated with the decomposition of the wind into irrotational and nondivergent components - this determines the initial nondivergent state. Another set is associated with recovery of the streamfunction from the forecasted vorticity. We demonstrate that direct parallel methods based on accelerated block cyclic reduction (BCR) significantly reduce the computational time required to solve the elliptic equations germane to this decomposition and forecast problem. A 72-h track prediction was made using incremental time steps of 16 min on a network of 3000 grid points nominally separated by 100 km. The prediction took 30 sec on the 8-processor Alliant FX/8 computer. This was a speed-up of 3.7 when compared to the one-processor version. The 72-h prediction of Elena's track was made as the storm moved toward Florida's west coast. Approximately 200 km west of Tampa Bay, Elena executed a dramatic recurvature that ultimately changed its course toward the northwest. Although the barotropic track forecast was unable to capture the hurricane's tight cycloidal looping maneuver, the subsequent northwesterly movement was accurately forecasted as was the location and timing of landfall near Mobile Bay.

Song, Chang G.↗
NASA NTRS1996

Forced Oscillations of Supported Drops

Oscillations of supported liquid drops are the subject of wide scientific interest, with applications in areas as diverse as liquid-liquid extraction, synthesis of ceramic powders, growing of pure crystals in low gravity, and measurement of dynamic surface tension. In this research, axisymmetric forced oscillations of arbitrary amplitude of viscous liquid drops of fixed volume which are pendant from or sessile on a rod with a fixed or moving contact line and surrounded by an inviscid ambient gas are induced by moving the rod in the vertical direction sinusiodally in time. In this paper, a preliminary report is made on the computational analysis of the oscillations of supported drops that have 'clean' interfaces and whose contact lines remain fixed throughout their motions. The relative importance of forcing to damping can be increased by either increasing the amplitude of rod motion A or Reynolds number Re. It is shown that as the ratio of forcing to damping rises, for drops starting from an initial rest state a sharp increase in deformation can occur when they are forced to oscillate in the vicinity of their resonance frequencies, indicating the incipience of hysteresis. However, it is also shown that the existence of a second stable limit cycle and the occurrence of hysteresis can be observed if the drop is subjected to a so-called frequency sweep, where the forcing frequency is first increased and then decreased over a suitable range. Because the change in drop deformation response is abrupt in the vicinity of the forcing frequencies where hysteresis occurs, it should be possible to exploit the phenomenon to accurately measure the viscosity and surface tension of the drop liquid.

Wilkes, Edward D.↗
NASA NTRS2001

Human Dimensions of Deforestation and Regrowth in the Brazilian Amazon: Integrating Data from Satellites, Demographic Censuses, and Field Surveys

This report summarizes research activities and products from a collaborative project on the "Human Dimensions of Deforestation and Regrowth in the Brazilian Amazon," awarded to Charles H. Wood (PI; Department of Sociology, University of Texas at Austin, now in the Center for Latin American Studies, University of Florida), Steven E. Sanderson (Co-PI; Department of Political Science, University of Florida, now Dean of Emory College, Emory University) and David L. Skole (Co-PI; Institute for Earth, Oceans and Space, University of New Hampshire, now in the Department of Geography and Basic Science Remote Sensing Initiative, Michigan State University).

Wood, Charles H.↗
NASA NTRS2001

The Transient Dendritic Solidification Experiment (TDSE)

Dendritic solidification is a common mode of solidification. It is also an important model problem in nonequilibrium physics and pattern formation physics. Current theories couple the transfer of latent heat with selection mechanisms at the interface. Measurements of succinonitrile (SCN) dendrites in microgravity show reasonable agreement between heat transfer predictions and experiment. However, data and analysis for assessing interfacial physics theories are less definitive. We are studying, and will present data on, transient effects in dendritic growth of SCN. We employ the Clapeyron pressure/melting temperature effect to make a rapid change in a sample's hydrostatic pressure, and thereby rapidly change the specimen's melting temperature, forcing the dendrite to select a new steady-state. These initial measurements show some surprising and non-intuitive effects.

Koss, M. B.↗
NASA NTRS1995

Brazil Fire Characterization and Burn Area Estimation Using the Airborne Infrared Disaster Assessment (AIRDAS) System

Remotely sensed estimations of regional and global emissions from biomass combustion have been used to characterize fire behavior, determine fire intensity, and estimate burn area. Highly temporal, low resolution satellite data have been used to calculate estimates of fire numbers and area burned. These estimates of fire activity and burned area have differed dramatically, resulting in a wide range of predictions on the ecological and environmental impacts of fires. As part of the Brazil/United States Fire Initiative, an aircraft campaign was initiated in 1992 and continued in 1994. This multi-aircraft campaign was designed to assist in the characterization of fire activity, document fire intensity and determine area burned over prescribed, agricultural and wildland fires in the savanna and forests of central Brazil. Using a unique, multispectral scanner (AIRDAS), designed specifically for fire characterization, a variety of fires and burned areas were flown with a high spatial and high thermal resolution scanner. The system was used to measure flame front size, rate of spread, ratio of smoldering to flaming fronts and fire intensity. In addition, long transects were flown to determine the size of burned areas within the cerrado and transitional ecosystems. The authors anticipate that the fire activity and burned area estimates reported here will lead to enhanced information for precise regional trace gas prediction.

Brass, J. A.↗