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At least 235 records · Page 13

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NASA NTRS1984

Geoid anomalies in a dynamic earth

Potential and surface deformation Love numbers for internal loading have been calculated in order to obtain a dynamically consistent relationship between the geoid and the earth's response to internal buoyancy forces. These quantities depend on the depth and harmonic degree of loading, and can be integrated as Green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts. Constructing a series of spherically symmetric, self-gravitating flow models for a variety of radial Newtonian viscosity variations and flow configurations, and calculating relaxation times for spherically symmetric viscous earth models, it is demonstrated that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound; this is much less time than the time scale for significant change in the convective flow pattern.

Richards, M. A.↗

NASA NTRS1984

Thermal detectors as X-ray spectrometers

Sensitive thermal detectors should be useful for measuring very small energy pulses, such as those produced by the absorption of X-ray photons. The measurement uncertainty can be very small, making the technique promising for high resolution nondispersive X-ray spectroscopy. The limits to the energy resolution of such thermal detectors are derived and used to find the resolution to be expected for a detector suitable for X-ray spectroscopy in the 100 eV to 10,000 eV range. If there is no noise in the thermalization of the X-ray, resolution better than 1 eV full width at half maximum is possible for detectors operating at 0.1 K. Energy loss in the conversion of the photon energy to heat is a potential problem. The loss mechanisms may include emission of photons or electrons, or the trapping of energy in long lived metastable states. Fluctuations in the phonon spectrum could also limit the resolution if phonon relaxation times are very long. Conceptual solutions are given for each of these possible problems.

Moseley, S. H.↗

NASA NTRS1985

Numerical analysis of flow non-uniformity in the hot gas manifold of the Space Shuttle main engine

Three-dimensional viscous flow in a conceptual hot gas manifold (HGM) for the Space Shuttle Main Engine High Pressure Fuel Turbopump (SSME HPFTP) was numerically analyzed. A finite difference scheme was used to solve the Navier-Stokes equations. The exact geometry of the SSME HGM was modeled using boundary fitted curvilinear coordinates and the General Interpolants Method (GIM) code. Slight compressibility of the subsonic flow was modeled using a linearized equation of state with artificial compressibility. A time relaxation method was used to obtain a steady state solution. The feasibility and potential usefulness of computational methods in assisting the design of SSME components which involves the flow of fluids within complex geometrical shapes is demonstrated.

Thoenes, J.↗

NASA NTRS1984

The suppression of convective wavebreaking by radiative transfer processes, part 1.3C

Schoeberl et al. (1983) suggested that convective wavebreaking of monochromatic gravity waves might be suppressed by radiative transfer processes if the vertical wavelength waves were sufficiently short. As the vertical wavelength or the gravity wave decreases, radiative transfer between adjacent vertical layers becomes increasingly important. This exchange can increase the radiative relaxation time scale so that the wave will no longer grow with altitude. Thus, very short vertical wavelength waves may dissipate radiatively rather than become convectively unstable. Apruzese and Strobel (1984) have revised the exchange coefficients used in Schoeberl et al. (1983). Also, Chao and Schoeberl (1984) pointed out that the computation made by LINDZEN (1981) of the convective diffusion rate may be a factor of two too low as the convective adjustment processes tends to minimize the thermal transport by the wave. The purpose here is to revise the values given in Schoeberl et al. (1983). These results also suggest that the very thin turbulent layers observed by mesosphere-stratosphere-troposphere (MST) radars (e.g., WOODMAN, 1980) cannot be produced by the convective instability of monochromatic gravity waves with large horizontal scales.

Schoeberl, M. R.↗

NASA NTRS1984

Errors in radiometric remote sensing of sea-surface temperature and salinity

Techniques for remote measurement of sea-surface physical temperature and salinity using radiometric measurements from aircraft or satellite are reviewed. Studies have been conducted to determine the sensitivity of the errors in surface temperature and salinity to errors in the measured brightness temperatures using combinations of UHF, L, S, and C-band measurements. These investigations were made using values of conductivity, static dielectric constant, and relaxation time derived from the regression equations of Klein and Swift (1977). Results of the error sensitivity study are presented in the form of error contour plots which permit the calculation of errors in the estimation of the physical parameters for given errors in the raw radiometric measurements.

Britt, C. L., Jr.↗

NASA NTRS1985

Electron-impact vibrational excitation rates in the flow field of aeroassisted orbital transfer vehicles

This paper examines the vibrational excitation rate processes expected in the flow field of aeroassisted orbital transfer vehicles (AOTVs). An analysis of the multiple-quantum vibrational excitation processes by electron impact is made to predict the vibrational excitation cross sections, rate coefficients, and relaxation times which control vibrational temperature. The expression for the rate of electron-vibration energy transfer is derived by solving the system of master equations which account for the multiple-level transitions. The vibrational excitation coefficients, which are the prerequisite physical quantities in solving the obtained vibrational equation, are calculated based on the theoretically predicted cross sections. These cross sections are obtained from quantum mechanical calculations, based on the concept that vibrational excitation of molecules by electron impact occurs through formation of an intermediate negative ion state. Finally, the modified Landau-Teller-type rate equation, which is suitable for the numerical calculations for the AOTV flow fields, is suggested.

Lee, J.-H.↗

NASA NTRS1985

Critical Transport Phenomena in Fluid Helium Under Low Gravity

The feasibility of carrying out measurements of certain critical transport properties of pure fluid under conditions of low gravity was studied. These properties are the thermal conductivity, kappa, the shear viscosity zeta and the diffusive relaxation time tau, which are predicted to diverge (tend to infinity) as the liquid-vapor critical point is approached. However, in this critical region, the Earth's gravity effect becomes very important. As the critical point is approached, the gravity effects increasingly distort the results. The reason for this is that the compressibility of the fluid also diverges and under the influence of gravity causes a vertical density gradient in the fluid, which is significant even when very thin fluid layers (typically 1 mm high) are being used. The result is that the temperature dependence of kappa, zeta, and tau tends to flatten off as T sub c is approached instead of continuing to increase, and therefore the predictions from the renormalization group and mode coupling theories cannot be subjected to a satisfactory test.

Meyer, H.↗

NASA NTRS1985

Characterization of the relationship of the cure cycle chemistry to cure cycle processing properties

Dynamic dielectric analysis (DDA) is used to study curing polymer systems and thermoplastics. Measurements are made over a frequency range of six decades. This wide range of frequencies increases the amount of information which can be obtained. The data is analyzed in terms of the frequency dependence of the complex permittivity epsilon sup *, specific conductivity sigma (ohm/cm) and the relaxation time tau, parameters which are characteristic of the cure state of the material and independent of the size of the sample.

Kranbuehl, D. E.↗

NASA NTRS1985

Recovery in skeletal muscle contractile function after prolonged hindlimb immobilization

The effect of three-month hindlimb immobilization (IM) in rats on contractile properties of slow-twitch soleus (SOL), fast-twitch extensor digitorum longus, and fast-twitch superficial region of the vastus lateralis were measured after 0, 14, 28, 60, and 90 days of recovery on excized, horizontally suspended muscles stimulated electrically to maximal twitch tension. IM caused decreases in muscle-to-body weight ratios for all muscles, with no complete recovery even after 90 days. The contractile properties of the fast-twitch muscles were less affected by IM than those of the slow-twitch SOL. The SOL isometric twitch duration was shortened, due to reduced contraction and half-relaxation time, both of which returned to control levels after 14 days of recovery. The peak tetanic tension, P(O), g/sq cm,, decreased with IM by 46 percent in the SOL, but recovered by the 28th day. The maximum shortening velocity was not altered by IM in any of the muscles. Thus, normal contractile function could recover after prolonged limb IM.

Fitts, R. H.↗

NASA NTRS1986

Dynamic dielectric analysis - A nondestructive cure process monitoring method

The cure processes of TGDDM-based epoxy thermosets and polysulfone thermoplastic resins were monitored by means of computer-controlled dynamic dielectric measurements made at frequencies 5 Hz to 5 million Hz. The values of capacitance and conductance were recorded, and the data were used to calculate the complex permittivity, an intensive property which is characteristic of the material's state of cure and independent of the amount of material measured. The frequency dependence of the complex permittivity value was used to determine the conductivity and mean relaxation time. The conductivity which is also an intensive variable and is independent of sample size, was correlated with the viscosity in both the epoxy and polysulfone systems.

Kranbuehl, D. E.↗

NASA NTRS1986

Radiative heating and cooling rates in the middle atmosphere

One of the limitations to the accurate calculation of radiative heating and cooling rates in the stratosphere and mesosphere has been the lack of accurate data on the atmospheric temperature and composition. Data from the LIMS experiment on Nimbus-7 has been extended to the South Pole with the aid of other observations. The data have been used as input to codes developed by Ramanathan and Dickinson to calculate the individual components and the net radiative heating rates from 100-0.1 mb. Solar heating due to ozone, nitrogen dioxide, carbon dioxide, water vapor and oxygen is shown to be nearly balanced by cooling in the thermal infrared spectral region due to carbon dioxide, ozone and water vapor. In the lower stratosphere, infrared transfer by ozone leads to heating that is sensitive to the distribution of tropospheric ozone, clouds and water vapor. The heating and cooling rates are adjusted slightly in order to satisfy the global mass balance. The results are in qualitative agreement with earlier calculations, but show additional detail. There is as strong temporal and vertical variation of cooling in the tropics. Radiative relaxation times are as short as 7 days or less at the stratopause.

Gille, John C.↗

NASA NTRS1987

Assessment of two-temperature kinetic model for ionizing air

A two-temperature chemical-kinetic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock-tubes, ballistic ranges, and flight experiments. In the model, named the TTv model, one temperature (T) is assumed to characterize the heavy-particle translational and molecular rotational energies, and another temperature (Tv) to characterize the molecular vibrational, electron translational, and electronic excitation energies. The theoretical results for nonequilibrium air flow in shock tubes are obtained using the computer code STRAP (Shock-Tube Radiation Program), and for flow along the stagnation streamline in the shock layer over spherical bodies using the newly developed code STRAP (Stagnation-Point Radiation Program). Substantial agreement is shown between the theoretical and experimental results for relaxation times and radiative heat fluxes. At very high temperatures the spectral calculations need further improvement. The present agreement provides strong evidence that the two-temperature model characterizes principal features of nonequilibrium air flow. New theoretical results using the model are presented for the radiative heat fluxes at the stagnation point of a 6-m-radius sphere, representing an aeroassisted orbital transfer vehicle, over a range of free-stream conditions. Assumptions, approximations, and limitations of the model are discussed.

Park, Chul↗

NASA NTRS1987

Viscous relaxation of a compressible spherical shell

Analytical solutions for analyzing viscous relaxation of a compressible mantle to surface loading are developed. Two different models are considered: one in which the density step of the core-mantle boundary is held fixed and the other in which the mass of the earth remains invariant with respect to variations of equation of state parameters. For thermodynamic parameters characteristic of the mantle, relaxation times of the fundamental and first overtone modes change by no more than about 30 percent for degree less than four. For a given initial depression, the ratio between the velocity in the lower mantle and that at the surface is bigger for incompressible models than for compressible models. Eigenfunctions between the two models also differ in appearance throughout the mantle, with the incompressible ones being larger for the second model. These results would suggest that the resolution of mantle viscosity may be impaired by the smaller excitation in the lower mantle due to effects of mantle compressibility.

Li, Guoying↗

NASA NTRS1988

Stratospheric responses to solar ultraviolet variations - Comparisons with photochemical models

At latitudes and altitudes where transport effects on ozone mixing ratio can be neglected, ozone and temperature responses to short-term changes in solar ultraviolet flux can be used to calculate implied bounds on two basic photochemical parameters: the odd oxygen photochemical relaxation time and the chemical sensitivity of odd oxygen to local temperature changes. Seasonally averaged values of these parameters are derived from response measurements and are compared with model calculations between 0.7 and 4 mbar at tropical latitudes. Results are in agreement within measurement and model uncertainties at all pressure levels. Model values of mean ozone and of the response of ozone to perturbations are functions of the same photochemical data. Therefore this agreement provides a constraint which should be considered when changes are proposed in model input data for the purpose of improving agreement between measured and calculated high-altitude ozone.

Hood, L. L.↗

NASA NTRS1988

Synchronously pumped mode-locked Ti:Al2O3 lasers

Synchronously pumped mode-locked laser operation in a Ti:Al2O3 ring laser is demonstrated. The laser pump source was a frequency-doubled Nd:YAG laser-amplifier system producing a 60-micron-long macropulse that comprised a wavetrain of mode-locked 70-ps micropulses with a 10-ns spacing (100 MHz). The Ti:Al2O3 laser consisted of a 1-cm-long crystal with faces cut at the Brewster angle and placed in a ring laser cavity configuration with a 2 percent output coupling mirror. The cavity was adjusted for a 5-ns round trip transit time, which was close to half the temporal spacing of the pump pulse. When the crystal was pumped synchronously at 532 nm with a 30-mJ macropulse, which was approximately an order of magnitude above the laser threshold, mode-locked lasing at approximately 200 MHz was obtained. This macropulse pump energy corresponds to an average micropulse pump energy of about 5 microJ at the Ti:Al2O3 crystal face. The mode-locked operation of the Ti:Al2O3 was observed only after several microseconds into the macropulse, indicating a long build-up process over many cavity trips. During this time, relaxation oscillations were observed. When mode locking started, the relaxation oscillation frequency increased by an order of magnitude. Synchronous pumping has been limited to operation with dye lasers and used either a CW or pulsed mode-locked pump source. Recent work has demonstrated CW mode-locked operation of a Ti:Al2O3 laser. The result raises the possibility that pulsed synchronous pumping may be used to obtain significantly narrowed laser pulses in Ti:Al2O3.

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NASA NTRS1989

The generalized drift flux approach: Identification of the void-drift closure law

The main characteristics and the potential advantages of generalized drift flux models are presented. In particular it is stressed that the issue on the propagation properties and on the mathematical nature (hyperbolic or not) of the model and the problem of closure are easier to tackle than in two fluid models. The problem of identifying the differential void-drift closure law inherent to generalized drift flux models is then addressed. Such a void-drift closure, based on wave properties, is proposed for bubbly flows. It involves a drift relaxation time which is of the order of 0.25 s. It is observed that, although wave properties provide essential closure validity tests, they do not represent an easily usable source of quantitative information on the closure laws.

Boure, J. A.↗

NASA NTRS1989

Spin-lattice relaxation and the calculation of gain, pump power, and noise temperature in ruby

The use of a quantitative analysis of the dominant source of relaxation in ruby spin systems to make predictions of key maser amplifier parameters is described. The spin-lattice Hamiltonian which describes the interaction of the electron spins with the thermal vibrations of the surrounding lattice is obtained from the literature. Taking into account the vibrational anisotropy of ruby, Fermi's rule is used to calculate the spin transition rates between the maser energy levels. The spin population rate equations are solved for the spin transition relaxation times, and a comparison with previous calculations is made. Predictions of ruby gain, inversion ratio, and noise temperature as a function of physical temperature are made for 8.4-GHz and 32-GHz maser pumping schemes. The theory predicts that ruby oriented at 90 deg will have approximately 50 percent higher gain in dB and slightly lower noise temperature than a 54.7-deg ruby at 32 GHz (assuming pump saturation). A specific calculation relating pump power to inversion ratio is given for a single channel of the 32-GHz reflected wave maser.

Lyons, J. R.↗

NASA NTRS1991

A model for the high-temperature transport properties of heavily doped n-type silicon-germanium alloys

A model is presented for the high-temperature transport properties of large-grain-size, heavily doped n-type silicon-germanium alloys. Electron and phonon transport coefficients are calculated using standard Boltzmann equation expressions in the relaxation time approximation. Good agreement with experiment is found by considering acoustic phonon and ionized impurity scattering for electrons, and phonon-phonon, point defect, and electron-phonon scattering for phonons. The parameters describing electron transport in heavily doped and lightly doped materials are significantly different and suggest that most carriers in heavily doped materials are in a band formed largely from impurity states. The maximum dimensionless thermoelectric figure of merit for single-crystal, n-type Si(0.8)Ge(0.2) at 1300 K is estimated at ZT about 1.13 with an optimum carrier concentration of n about 2.9 x 10 to the 20th/cu cm.

Vining, Cronin B.↗