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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Middle atmosphere modeling

Breaking gravity waves generate and maintain a background level of turbulence which is capable of producing substantial cooling and/or heating in the upper mesosphere and lower thermosphere. The net thermodynamic effect of breaking gravity waves is critically dependent on the eddy Prandt number (P sub t) applicable to mesospheric turbulence. When P sub t is approximately 1, the calculations of the heat budget for the mesopause region imply that the globally averaged eddy or turbulent diffusion coefficient cannot exceed .000001 sq cm/s. This upper limit on turbulant diffusion applies to both potential temperature transport and chemically inert tracer transport when radiative damping is neglible. For chemically active species larger diffusion coefficients are permitted, because the effective eddy diffusion coefficient is increased by an additive term L/2 gamma (sup 2), where L is the chemical loss rate and gamma is the vertical wavenumber. For P sub t is approximately 4 to 6, the turbulent diffusion of momentum (D sub M) is sufficiently greater than the turbulent diffusion of heat (D sub H) that the conversion of gravity wave energy to heat with high efficiency nearly balances the divergence of the downward eddy heat flux in the wave breaking zone. Therefore the heat budget of the mesopause region would no longer provide a powerful and useful constraint on D sub H. If P sub t exceeds 6 with high efficiency for energy conversion to heat, gravity waves would heat the mesosphere throughout the wave breaking region.

Strobel, D. F.↗
NASA NTRS1964

Radiation Effects of Protons and Electrons in Silicon Diffused-Junction Detectors

Diffused n-on-p silicon particle detectors were irradiated with 1-Mev electrons, and with 31-Mev and 8.2-Mev protons. The detector capacitance was observed to increase with electron irradiation and to decrease with proton irradiation. A physical interpretation is given in terms of defect energy levels known to be introduced in silicon by irradiation. Defect ionization states are taken to be characteristic of the space-charge region, rather than of bulk silicon. The relation to other studies of damage in semiconductor particle detectors is pointed out, considering differences in the types of detector and irradiation used. The significance of these results is discussed for detector applications where the damage threshold is exceeded.

Scott, R. E.↗
NASA NTRS1988

Response of convective-diffusive transport to spatial and temporal variations in effective gravity

The response of heat, mass, and momentum transport to conditions characteristic of the microgravity environment is studied. It is found that the orientation of the experiment with respect to the steady component of the residual gravity is a crucial factor in determining the suitability of the spacecraft as a means of suppressing or eliminating unwanted effects caused by buoyant fluid motion. The process is highly sensitive to transient disturbances.

Alexander, J. Iwan D.↗
NASA NTRS1985

Application of computational fluid dynamics to complex inlet ducts

A three-dimensional parabolic Navier-Stokes code, PEPSIG, was used to analyze the flow in the subsonic diffuser section of a typical modern inlet design. The effect of curvature of the diffuser centerline and transitioning cross sections was studied to determine the primary cause of flow distortion in the duct. Total pressure values at the engine compressor face are reported.

Towne, C. E.↗
NASA NTRS1985

Application of computational fluid dynamics to complex inlet ducts

A three-dimensional parabolic Navier-Stokes code, PEPSIG, was used to analyze the flow in the subsonic diffuser section of a typical modern inlet design. The effect of curvature of the diffuser centerline and transitioning cross sections was studied to determine the primary cause of flow distortion in the duct. Total pressure values at the engine compressor face are reported.

Towne, C. E.↗
NASA NTRS1999

The Effect of Velocity on the Extinction Behavior of a Diffusion Flame during Transient Depressurization

Current fire suppression plans for the International Space Station include the use of venting (depressurization) as a method for extinguishing a fire. Until recently this process had only been examined as part of a material flammability experiment performed on Skylab in the early 1970's. Due to the low initial pressure (0.35 Atm) and high oxygen concentration (65%), the Skylab experimental results are not applicable for understanding the effects of venting on a fire in a space station environment (21%O2, 1 Atm). Recent research examined the extinction behavior of a diffusion flame over a polymethyl methacrylate (PMMA) cylinder during a transient depressurization in low-gravity. The numerical model was used to examine extinction limits as a function of depressurization rate, forced flow velocity, and initial solid phase temperature. The experimental and numerically predicted extinction data indicated that as the solid phase temperature increased the pressure required to extinguish the flame decreased. The numerical model was also used to examine conditions not obtainable in the low-gravity experiments. From these simulations, a series of extinction boundaries were generated that showed a region of increased flammability existed at a forced flow of 10 cm/s. Analysis of these extinction boundaries indicated that they were quasi-steady in nature, and that the final extinction conditions were independent of the transient process. The velocity range in the previous study was limited and thus the results did not examine the effects of velocities less than 1 cm/s or greater than 20 cm/s. This study utilized low-gravity experiments performed on NASA's Reduced-gravity Research Aircraft Laboratory and numerical simulations to examine conditions applicable to the Space Station environment. This paper extends the analysis of the previous study to a comprehensive examination of the effect of increased velocity on extinction behavior and extinction limits during a transient depressurization in low-gravity. This is achieved by examining extinction data from buoyant (normal-gravity) and low-buoyant (low-gravity) depressurization. experiments, as well as from numerical predictions of flame behavior during depressurization in a non-buoyant (zero-gravity) environment.

Goldmeer, Jeffrey S.↗
NASA NTRS1993

Effects of thermal-solutal convection on temperature and solutal fields under various gravitational orientations

Semiconductor crystals such as Hg(1-x)CD(x)Te grown by unidirectional solidification Bridgmann method have shown compositional segregations in both the axial and radial directions. Due to the wide separation between the liquidus and the solidus of its pseudobinary phase diagram, there is a diffusion layer of higher HgTe content built up in the melt near the melt-solid interface which gives a solute concentration gradient in the axial direction. The value of effective diffusion coefficient calculated from fitting of the data to 1D model varies with Hg(1-x)Cd(x)Te growth conditions. This indicates that the growth condition of the Hg(1-x)Cd(x)Te is not purely diffusion controlled. Because of the higher thermal conductivity in the melt than that in the crystal in the growth system, there is a thermal leakage through the fused silica crucible wall near the melt-solid interface. This gives a thermal gradient in the radial direction. Hart, and Thorpe, Hutt and Soulsby have shown that under such conditon a fluid will become convectively unstable as a result of different diffusitivities of temperature and solute. It is quite important to understand the effects of this thermosolute convection on the compositonal segregation in both axial and radial directions in the unidirectionally solidified crystals under various gravitational directions. To reach this goal, we start with a simplified problem to study the effects of thermal-solutal convection on the temperature and solutal fields under various gravitional orientations. We begin by reviewing model governing equations.

Wang, Jai-Ching↗
NASA NTRS1997

Structure and Stability of Burke-Schumann Diffusion Flames

The general goal of this NASA Grant is twofold: to improve our understanding of (1) the influence of buoyancy on the stability and structure of Burke-Schumann type diffusion flames, and (2) the effects of buoyancy on vortex-flame interactions in co-flow diffusion flames. A numerical code with a higher order accuracy for spatial discretization is developed in this project for simulation of time-dependent diffusion flames by Sheu and Sheu and Chen, and an extended reduced mechanism is incorporated for prediction of methane oxidation and NO(x)(NO, NO2, and N2O) formation and emission from methane Burke-Schumann diffusion flame (BSDF) as reported in Sheu, and Sheu and Chen. Initial investigation of vortex and flame interaction within the context of fast chemistry is reported. Experiments are conducted in reduced pressure to study the lift-off and stabilization of methane-fueled BSDF in reduced buoyancy environments due to reduced pressure. Measurements of temperature and species concentrations are made in normal and reduced pressure environments to study the effects of buoyancy on the structure of BSDF, and will be reported in this paper. To study the buoyancy effects on the lift-off and stabilization of methane-fueled jet diffusion flames in coflowing air, a glovebox investigation, Enclosed Laminar Flames (ELF), has been proposed and approved for space-based testing on the fourth United States Microgravity Payload (USMP-4) mission, scheduled for October 1997. A brief description of the ELF investigation is also presented.

Lee, Yong G.↗
NASA NTRS1991

Steady spherical hypercritical accretion onto neutron stars

The present study of hypercritical accretion flows onto neutron stars considers steady-state, spherically symmetric flows whose accretion rate range is characterized by the carrying away of gravitational-accretion energy by neutrinos. The models used encompass pair production, radiation diffusion, and general relativistic effects. While pair pressure dominates throughout the accretion envelope when accretion rates above about 100 solar masses/yr, radiation diffusion becomes important when the accretion rate falls below about 0.001 solar masses/yr. At the lower accretion rates, free fall toward the neutron-star surface stops and an extended, quasi-static, radiation pressure-supported envelope emerges which is probably dynamically unstable.

Houck, John C.↗
NASA NTRS2003

Evolution and Transport of Water in the Upper Regolith of Mars

Long standing theoretical predictions [1-3], as well as recent spacecraft observations [4] indicate that large quantities of ice is present in the high latitudes upper decimeters to meters of the Martian regolith. At shallower depths and warmer locations small amounts of H2O, either adsorbed or free, may be present transiently. An understanding of the evolution of water based on theoretical and experimental considerations of the processes operating at the Martian environment is required. In particular, the porosity, diffusivity, and permeability of soils and their effect on water vapor transport under Mars-like conditions have been estimated, but experimental validation of such models is lacking. Goal: Three related mechanisms may affect water transport in the upper Martian regolith. 1) diffusion along a concentration gradient under isobaric conditions, 2) diffusion along a thermal gradient, which may give rise to a concentration gradient as ice sublimes or molecules desorb from the regolith, and 3) hydraulic flow, or mass motion in response to a pressure gradient. Our combined theoretical and experimental investigation seeks to disentangle these mechanisms and determine which process(es) are dominant in the upper regolith over various timescales. A detailed one-dimensional model of the upper regolith is being created which incorporates water adsorption/ desorption, condensation, porosity, diffusivity, and permeability effects. Certain factors such as diffusivity are difficult to determine theoretically due to the wide range of intrinsic grain properties such as particle sizes, shapes, packing densities, and emergent properties such as tortuosity. An experiment is being designed which will allow us to more accurately determine diffusivity, permeability, and water desorption isotherms for regolith simulants.

Hudson, T. L.↗
NASA NTRS1993

Effects of detailed droplet heating models on turbulent sprays vaporization behavior

The effects of three different droplet heating models on the vaporization history and internal structure of turbulent liquid fuel sprays were investigated. The models considered were the infinite-diffusion, diffusion-limit, and effective-conductivity models. A numerical solution for the models was developed and implemented in the KIVA-II computer code. Low temperature and relatively high temperature numerical studies were conducted. The low temperature computations were compared with existing experimental data. The comparisons showed that while the infinite-diffusion and diffusion-limit models respectively overpredicted and underpredicted the fuel vapor peak concentration and distribution in the combustor, the effective-conductivity model gave results that were in better agreement with measurements. A limited study for the high temperature case was performed due to lack of experimental data and predictions using the three models were compared with each other.

Mawid, M. A.↗