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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 199 records · Page 11
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NASA NTRS1984

Two-dimensional wind-tunnel interference from measurements on two contours

This paper describes how wall-induced velocities near a model in a two-dimensional wind tunnel can be estimated from upwash distributions measured along two contours surrounding a model. The method is applicable to flows that can be represented by linear theory. It was derived by applying the Schwarz Integral Formula separately to the two contours and by exploiting the free-air relationship between upwashes along the contours. Advantages of the method are that only one flow quantity need by measured and no representation of the model is required. A weakness of the method is that it assumes streamwise interference velocity vanishes far upstream of the model. This method was applied to a simple theoretical model of flow in a solid-wall wind tunnel. The theoretical interference velocities and the velocities computed using the method were in excellent agreement. The method was then used to analyze experimental data acquired during adaptive-wall experiments at Ames Research Center. This analysis confirmed that the wall adjustments reduced wall-induced velocities near the model.

Schairer, E. T.↗
NASA NTRS1972

Composition dependence of ion diffusion coefficients in gas mixtures at arbitrary field strengths

Expressions for the diffusion coefficient of ions in gas mixtures are obtained from momentum transfer theory, and are given in terms of the diffusion coefficients and drift velocities of the ions in the pure component gases. Blanc's law holds exactly at all field strengths if the mean free time between collisions is independent of velocity (Maxwell model), but otherwise there may be either positive or negative deviations from Blanc's law at high fields. Such deviations are of comparable magnitude for the diffusion coefficients and the mobility, but are not identical. Specific cases of inverse-power potentials are treated in further detail, and some numerical examples are given for rigid-sphere interactions.

Whealton, J. H.↗
NASA NTRS1972

Analysis and testing of compressible flow ejectors with variable area mixing tubes.

An analytical model has been developed to predict the flow behavior within axisymmetric single-nozzle ejectors employing variable-area mixing tubes. The primary flow may be supersonic or subsonic and may have a different stagnation temperature from the subsonic secondary flow. Tests were performed on an ejector with an 800 F supersonic (M = 2.72) primary jet to evaluate the analytical model. Measured velocity profiles, temperature profiles, and wall static pressure distributions are presented and compared to the analytical predictions. Agreement is generally good.

Hickman, K. E.↗
NASA NTRS1973

Nonstationary ascent wind analysis.

A statistical approach for computing launch vehicle response to inflight winds is presented. In the response analysis the zonal component of the ascent wind velocity is modeled as a nonstationary random process. Model parameters are computed by processing available measured wind data. Pertinent pitch plane response statistics are computed and their value in constructing fatigue load spectra for a reusable launch vehicle is demonstrated. A method for using the response statistics to estimate structural reliability is also discussed. The accuracy and the cost saving potential of the probabilistic structural dynamics approach are assessed by performing comparisons with digital simulation results.

Howell, L. J.↗
NASA NTRS1976

Wind-enhanced escape with application to terrestrial helium

We derive a simple expression for the escape flux which takes into account horizontal winds at the exobase. The escape flux is shown to increase with the wind velocity and becomes much larger than the Jeans escape flux for velocities approaching the thermal velocity. This model is applied to terrestrial He-4, where we determine the conditions required for wind-enhanced escape to contribute to the overall He-4 budget. For completeness, we also derive an expression for the escape flux when only a vertical wind is present at the exobase.

Hartle, R. E.↗
NASA NTRS1976

Higher harmonic rotor blade pitch control

Tests of a model 'Reverse Velocity Rotor' system at high advance ratios and with twice-per-revolution cyclic pitch control were made under joint Navy-NASA sponsorship in the NASA, Ames 12 ft. pressure tunnel. The results showed significant gains in rotor performance at all advance ratios by using twice-per-revolution control. Detailed design studies have been made of alternative methods of providing higher harmonic motion including four types of mechanical systems and an electro-hydraulic system. The relative advantages and disadvantages are evaluated on the basis of stiffness, weight, volume, reliability and maintainability.

Ewans, J. R.↗
NASA NTRS1977

The origin of the broad line emission from Seyfert galaxies

Mass loss from an accretion disk around a supermassive (of the order of 100 million solar masses) black hole is proposed as the origin of the broad permitted lines of Seyfert galaxies and QSOs. The material is expelled by ionizing radiation emitted near the center of the disk and striking it at a radius of approximately 10 to the 16.5 power cm. The escaping gas initially has a density of about 1 trillion per cu cm, high enough for radiation pressure to accelerate the gas to high radial velocities. The model leads to line emission from tiny dense filaments moving at about 10,000 km/s in a region roughly 10 to the 17th power cm across. This agrees with the observed broad line widths, variability time scales, and the absence of broad forbidden-line emission.

Shields, G. A.↗
NASA NTRS1980

The concept of wave overreflection and its application to baroclinic instability

The paper reviews the concept of wave overreflection and its application to baroclinic instability. The instabilities associated with overreflection are likely to be identical to critical layer instabilities; the baroclinic instability is examined in terms of overreflection of vertically propagating Rossby waves which leads to rapid estimates of growth rates and phase speeds of unstable modes for arbitrary distributions of zonal velocities in models with and without lids. It was found that Charney and Green modes are essentially critical-layer instabilities and that baroclinic instability can be eliminated by stretching the transition region from zero shear at the ground to the interior shear. Finally, it is shown that the potential vorticity flux of baroclinically unstable modes is confined to a layer between the ground and the neighborhood of the steering level.

Lindzen, R. S.↗
NASA NTRS1981

Numerical simulation of a radially injected barium cloud

Electrostatic two-dimensional numerical simulations of a radially symmetric barium injection experiment demonstrate that ions created by solar UV irradiation are electrostatically bound to the electrons which remain tied to the field lines on which they are created. Two possible instabilities are identified, but neither of them causes the barium plasma cloud to polarize in a way that would permit the plasma to keep up with the neutrals. In a second model, the velocity of the neutrals is allowed to be a function of the azimuthal angle. Here, a portion of the cloud does polarize in a way that allows a portion of the plasma to detach and move outward at the approximate speed of the neutrals. No rapid detachment is found when only the density of the neutrals is given an azimuthal asymmetry.

Swift, D. W.↗
NASA NTRS1983

Stability of electron distributions within the earth's bow shock

The present examination of the linear Vlasov stability of a class of electron velocity distributions modeling those observed within the earth's bow shock is restricted to electrostatic waves propagating parallel to the ambient magnetic field B. Two instabilities are identified as driven by free energy in the direction parallel to B: an ion acoustic wave with real frequency below the ion plasma frequency, and an electron acoustic wave whose real frequency is several times the ion plasma frequency. Unstable wave characteristics are in accord with the trend of the observed electrostatic waves toward polarization parallel to the magnetic field. The instabilities identified may contribute to electron dissipation in collisionless shocks.

Thomsen, M. F.↗
NASA NTRS1983

Excitation of an electrostatic wave by a cold electron current sheet of finite thickness

Calculations for the threshold of current-driven instabilities and the growth rates of ion acoustic and electrostatic ion cyclotron instabilities in a magnetized plasma driven a current sheet with a finite width are presented. Maxwellian equations are employed to model the velocity distributions of electrons and ions in a direction perpendicular to the sheet. A dispersion relation is defined for the regions of instability, and boundary conditions are characterized in order to obtain a set of eigenvalue equations. Thresholds are delineated for various regions, including ducted mode solutions where only ion-acoustic waves are excited in areas where the frequency range significantly exceeds the ion cyclotron frequency. When a constant electron drift velocity is present, a thick current sheet is more unstable than a thin one. Fewer modes become unstable with a thinner sheet.

Hwang, K. S.↗
NASA NTRS1983

Pedersen density drift instabilities

This paper describes the linear kinetic theory of electrostatic-drift instabilities driven by Pedersen and density-drift velocities. The model uses a uniform magnetic field B; a weak, uniform density gradient in the x direction; and a weak, uniform electric field in the y direction. Weak charged-neutral collisions are represented by the addition of BGK model terms to the Vlasov equation. The resulting local dispersion equation is used to study the properties of the associated instabilities at ka(i) greater than about 1 (where k is the wave number and a(i) is the ion gyroradius). Results show that the E x B gradient drift instability at ka(i) = about 1 may grow in the auroral ionosphere primarily in the vicinity of 200 km and only if the electron density is sufficiently small.

Gary, S. P.↗
NASA NTRS1984

Numerical study of terrain-induced mesoscale motions and hydrostatic form drag in a heated, growing mixed layer

Han et al. (1982) have found in a previous numerical study of terrain-induced mesoscale motions that the orography caused a steady-state flow pattern to occur. The study was concerned with a simplified case in which no surface heating occurred. The present investigation considers an extension of this study to the more realistic case of a heated, growing daytime mixed layer containing horizontal variations of potential temperature as well as velocity. The model is also extended to include three layers above the mixed layer. It is found for a heated, growing mixed layer, that the mesoscale form drag is a thermal-anomaly or buoyancy effect associated with horizontal variations of potential temperature within the layer.

Deardorff, J. W.↗
NASA NTRS1985

Steady-state currents in p-n junction filaments or grains in case of large surface recombination velocities at lateral surfaces

Recently it has been pointed out that the saturation current of a semiconductor filament which constitutes part of a p-n junction diverges when the surface recombination velocity at the faces become infinitely large. Here it is pointed out that this is to be expected on physical grounds since, whenever the carrier concentration is kept off equilibrium by an outside agent, and at the same time recombination lifetimes in the bulk or in surface layers tend to zero, concentration gradients tend to infinity. As also previously noted, the situation can be remedied by using realistic (finite) surface recombination velocities in model calculations. However, this procedure leads to mathematical complexities which have been circumvented recently by the introduction of a heuristic approach. It is the aim of this paper to assess the validity of the heuristic approach by means of detailed and exact calculations.

Von Roos, O.↗
NASA NTRS1985

Far-infrared line intensities of H2O and CO from warm molecular clouds

Takahashi et al. (1983) have studied the role of H2O in the overall energetics of warm molecular clouds with large optical depths. It was found that in molecular clouds associated with nearby or embedded luminosity sources, the dust is likely to be warmer than the molecular gas. Thus, the warm dust represents the primary source of heat for the gas. In some cases, H2O was found to cool the gas. Takahashi et al. (1983) have presented the formalism for treating H2O in an optically thick molecular cloud with embedded sources of luminosity. The total H2O heating and cooling rates were also calculated. In the present investigation, the emergent intensities of far-infrared lines of H2O and excited, rotational CO transitions from relatively warm molecular clouds based on the large-velocity gradient model of the previous study are calculated. It is found that the dominant submillimeter CO transitions are related to emission, while the H2O transitions can be either absorption or emission lines.

Takahashi, T.↗