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At least 325 records · Page 18

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NASA NTRS1973

Analysis of gas flow through a multilayer insulation system.

A theoretical investigation of gas flow inside a multilayer insulation system has been made for the case of the broadside pumping process. A set of simultaneous first-order differential equations for the temperature and pressure of the gas mixture was obtained by considering the diffusion mechanism of the gas molecules through the perforations on the insulation layers. A modified Runge-Kutta method was used for numerical experiment. The numerical stability problem was investigated. It has been shown that when the relaxation time is small compared with the time period over which the gas properties change appreciably, the set of differential equations can be replaced by a set of algebraic equations for solution. Numerical examples were given and comparison with experimental data was made.

Lin, J. T.↗

NASA NTRS1973

Radiative damping of trapped gravity waves in the solar atmosphere.

The radiative damping of trapped gravity waves in an optically thin atmosphere is studied for a stratified Boussinesq fluid. The character of the atmospheric eigenmodes depends on the distribution of the Brunt-Vaisala frequency N and the radiative relaxation time tau. The calculations for simple layer models show that if N x tau is large over some finite fraction of the trapping region, then modes of long lifetime can exist. In order to suppress gravity waves entirely, it is necessary that N x tau be less than or equal to 1 over the entire trapping region. Qualitative application of the results to the solar atmosphere leads to the conclusion that gravity wave eigenmodes of the solar atmosphere, although damped, are by no means eliminated by radiative effects.

Clark, P. A.↗

NASA NTRS1973

Liquid lubricants - functions and requirements

The state of liquid lubricant technology is discussed. The limit of present knowledge, especially in the boundary regime, is analyzed. Factors such as colloid behavior, viscoelastic effects, and relaxation times are investigated. Specific applications problems and some lubricant requirements which have not been met are reported.

Lansdown, A. R.↗

NASA NTRS1973

Thermal engineering research

Shih, C. C.↗

NASA NTRS1973

Effect of photochemical models on calculated equilibria and cooling rates in the stratosphere

The determination of the relaxation time of a temperature perturbation in the stratosphere must take into account the effects of the absorption of solar energy by ozone, while the ozone density itself is dependent on temperature. A photochemical model, consisting of continuity equations for each of 29 constituents with reaction rates and adjustment times, is used to obtain the vertical distribution of ozone. The altitude range from 35 to 60 km in non-Arctic regions is shown to be in approximate joint radiative-photochemical equilibrium. The temperature and ozone distributions are thus well buffered. Modest changes in ozone and temperature were calculated on the basis of this model from large changes in cooling rates and reaction rates, and the results are shown to be more in line with actual observations. The importance of estimating the mixing ratios for NO and H2O is emphasized.

Blake, D.↗

NASA NTRS1974

Four possible types of pulses for self-induced transparency

Four types of steady-state solutions were derived for the coupled Maxwell-Bloch equations which describe highly intense pulse propagation in a resonant medium. Essential in the derivation procedures is the replacement of the usual slowly varying envelope approximation with an alternative procedure, the omission of possible nonresonant losses, and the assumption that the relaxation times are infinite.

Lee, C. T.↗

NASA NTRS1974

Irregular bilayer structure in vesicles prepared from Halobacterium cutirubrum lipids

Fluorescent probes were used to study the structure of the cell envelope of Halobacterium cutirubrum, and, in particular, to explore the effect of the heterogeneity of the lipids in this organism on the structure of the bilayers. The fluorescence polarization of perylene was followed in vesicles of unfractionated lipids and polar lipids as a function of temperature in 3.4 M solutions of NaCl, NaNO3, and KSCN, and it was found that vesicles of unfractionated lipids were more perturbed by chaotropic agents than polar lipids. The dependence of the relaxation times of perylene on temperature was studied in cell envelopes and in vesicles prepared from polar lipids, unfractionated lipids, and mixtures of polar and neutral lipids.

Lanyi, J. K.↗

NASA NTRS1975

Anelastic deformation of boron fibers

The flexural deformation behavior of vapor-deposited boron fibers was examined from 100 to 1100 K by stress-relaxation and internal friction techniques. Only strong thermally-activated anelasticity was observed with no evidence of plasticity up to surface strains of 0.006. The parameters governing the relaxation processes within the anelastic spectra of as-received and annealed fibers were determined. These parameters were correlated with X-ray structure studies to develop preliminary models for the sources of boron's anelasticity. The large relaxation strengths of the dominant Ia processes coupled with their relaxation times and energies suggest a sliding mechanism between certain basic structural subunits common to both the beta-rhombohedral and vapor-deposited boron structures.

Dicarlo, J. A.↗

NASA NTRS1975

A B-B-G-K-Y framework for fluid turbulence

A kinetic theory for fluid turbulence is developed from the Liouville equation and the associated BBGKY hierarchy. Real and imaginary parts of Fourier coefficients of fluid variables play the roles of particles. Closure is achieved by the assumption of negligible five-coefficient correlation functions and probability distributions of Fourier coefficients are the basic variables of the theory. An additional approximation leads to a closed-moment description similar to the so-called eddy-damped Markovian approximation. A kinetic equation is derived for which conservation laws and an H-theorem can be rigorously established, the H-theorem implying relaxation of the absolute equilibrium of Kraichnan. The equation can be cast in the Fokker-Planck form, and relaxation times estimated from its friction and diffusion coefficients. An undetermined parameter in the theory is the free decay time for triplet correlations. Some attention is given to the inclusion of viscous damping and external driving forces.

Montgomery, D.↗

NASA NTRS1974

Magnetic environment of hydrogen in Fe from muon precession measurements

Polarized positive muon radiation was stopped in an ellipsoidal iron target and its precession was observed in a transverse magnetic field. Results indicate that the conduction electron polarization in the 77 K-Fe Curie point region is less than expected, and that the relaxation time of the muon polarization is dominated by the static inhomogeneity to 900 K, at which point magnetization fluctuations become important.

Heiman, N.↗

NASA NTRS1975

Anelastic deformation of boron fibers

The flexural deformation behavior of vapor-deposited boron fibers is investigated in the temperature range from 100 to 1100 K by stress-relaxation and internal friction methods. Strong thermally-activated anelasticity is uniquely observed, and no evidence of plasticity is found up to surface strains of 0.006. The parameters governing the relaxation processes within the anelastic spectra of untreated and annealed fibers are determined and correlated with the results of X-ray structural studies to develop preliminary models for the possible causes of boron's anelasticity. The large relaxation strengths of the dominant Ia processes together with their relaxation times and energies are shown to suggest a sliding mechanism between certain basic structural subunits common to both the beta-rhombohedral and vapor-deposited boron structures.

Dicarlo, J. A.↗

NASA NTRS1974

Ferromagnetic resonance studies of thermal effects on lunar metallic Fe phases

Ferromagnetic resonance results of annealing experiments are discussed which illustrate the thermal effects on lunar metallic iron phases already present in a lunar fines sample. Spectral features of ferromagnetic resonance produced in the sample by heat treatments at temperatures between 600 and 1025 C are described which resemble those detected in lunar breccias and crystalline rocks. A correlation is shown to exist between these features and the degree of thermal metamorphism. It is noted that this correlation can be used as a built-in geothermometer or probe to investigate the thermal history and degree of metamorphism of a lunar sample containing metallic iron phases. The thermal history of a metaclastic rock is analyzed in this way, and it is shown that thermal metamorphism is an effective process for increasing relaxation times or the stability of the natural remanent magnetization carried by single-domain metallic iron particles.

Tsay, F.-D.↗

NASA NTRS1975

The effect of adiabatic focusing upon charged particle propagation in random magnetic fields

Charged particles propagating along the diverging lines of force of a spatially inhomogeneous guiding field were considered as they are scattered by random fields. Their longitudinal transport is described in terms of the eigenfunctions of a Sturm-Liouville operator incorporating the effect of adiabatic focussing along with that of scattering. The relaxation times and characteristic velocities are graphed and tabulated. The particle density is evaluated as a function of space and time for two different regimes. In the first regime (relatively weak focussing), a diffusive mode of propagation is dominant but coherent modes are also dominant. In the second regime (strong focussing), diffusion does not occur and the propagation is purely coherent. This supercoherent mode corresponds exactly to the so-called scatter-free propagation of kilovolt solar flare electrons. On a larger scale, focussed transport provides an interpretation of many observed characteristics of extragalactic radio sources.

Earl, J. A.↗

NASA NTRS1976

Some applications of equilibrium thermodynamic properties to continuum gasdynamics

The speed of sound for the propagation of isentropic disturbances in a gas is developed, including corrections for chemical reaction. The term zero frequency is used to describe this isentropic limit sound speed; the term signifies that change in the gasdynamic variables are all very slow compared with the chemical rate changes in the gas. A faster, nonisentropic speed of propagation occurs for disturbances where the changes in gasdynamic variables are fast compared with the chemical rate changes. In the limit, this is known as the infinite frequency or frozen sound speed - the former term calling attention to the very high frequency of the disturbance, the latter term calling attention to the frozen character of the chemical reactions under such rapid changes of state. The true sound speed for a disturbance of finite frequency is shown to be between these two limits and is expressed in terms of the chemical relaxation time. The Riemann invariants that are useful in determining the changes in flow speed along characteristic directions in supersonic flow are derived in terms of integrations of acoustic impedance, and example results are given for air.

Source record↗

NASA NTRS1976

Proton magnetic relaxation and internal rotations in tetramethylammonium cadmium chloride

Nuclear magnetic resonance (NMR) and relaxation studies of the proton spin-lattice relaxation time (PSLRT) and proton second moment (PSM) are reported. Tetramethylammonium cadmium chloride (TMCC) was selected as a diamagnetic member of the isomorphic series, and hence proton data relate directly to the motion of the tetramethylammonium ion in the absence of paramagnetic ions. In the model adopted, the correlation time for hindered motion of one of the methyl groups differs from that of the other three groups in the low-temperature phase below 104 K. PSLRT and PSM values agree closely with experimental data with this model. Crystallographic phase transitions in TMCC occur at 104 K and 119 K according to the PSLRT measurements. Dipolar interactions between adjacent protons account for the PSLR rates below 104 K.

Tsang, T.↗

NASA NTRS1976

The effect of adiabatic focusing upon charged-particle propagation in random magnetic fields

The charged particles considered are scattered by random fields while they propagate along the diverging lines of force of a spatially inhomogeneous guiding field. Their longitudinal transport is described in terms of the eigenfunctions of a Sturm-Liouville operator which incorporates the effect of adiabatic focusing along with that of scattering. The relaxation times and characteristic velocities which appear in this matrix formulation of the transport problem are graphed and tabulated. Explicit formulas which describe the particle-density profile that results from a localized impulsive injection are derived for two different regimes. In the first regime, where focusing is relatively weak, a diffusive mode of propagation is dominant, but coherent modes are also present, and they become prominent as the intensity of focusing increases. In the second regime, where focusing is strong and where diffusion does not occur, the propagation is purely coherent. The existence of this supercoherent mode of particle transport opens up many possibilities for the interpretation of astrophysical phenomena.

Earl, J. A.↗

NASA NTRS1976

Vibrational and rotational relaxation in mixtures of water vapor and oxygen

Experimental measurements of sound absorption in air, measurements of ultrasonic absorption in H2O/O2 mixtures at 500 K, and experimental measurements in mixtures of H2O with other impurities have been analyzed to determine the energy transfer rates in mixtures of O2 and H2O as a function of temperature. A set of energy transfer rate coefficients was found that give results consistent with all the experimental data examined and with vibrational energy transfer theory. The rotational relaxation time for H2O/O2 mixtures was determined from the 500 K ultrasonic measurements.

Bass, H. E.↗

NASA NTRS1976

The frequency dependence of the viscous component of the magnetic susceptibility of lunar rock and soil samples

The susceptibility of two lunar samples (a soil and a low metamorphic grade breccia) has been measured in a weak field - 0.001 Oe - and as a function of frequency from 0.032 to 1.0 Hz. The measurements were made using a superconducting magnetometer. The results show that the susceptibility decreases linearly with the log of frequency. This observation is in agreement with a theoretical model for viscous decay based on the Neel theory of single-domain and superparamagnetic grains. The relation derived agrees with a model in which there is a uniform distribution of relaxation times.

Hanneken, J. W.↗