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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Large space structures assembly simulation

Simulation requirements for the evaluation of Large Space Structure assembly techniques have been developed by analysis of a typical large space structure through its entire life cycle. Applications of neutral buoyancy, air bearing simulators, computer driven rendezvous and docking simulators, and other simulation modes were studied. Several demonstrations of assembly techniques in neutral buoyancy have been conducted at MSFC using large scale hardware. Experience from those demonstrations and subsequent simulations can be used to select viable approaches for orbital application in operational vehicles. Demonstration results are presented which illustrate techniques and hardware designs that can significantly simplify construction of early large space structures.

Hall, S. B.↗
NASA NTRS1979

The promise of space processing

The long exposure to a virtually zero-gravity environment is a truly unique situation that cannot be duplicated or even approximated for any length of time on earth. The topics discussed are: (1) low gravity; (2) atmospheric effects; (3) potential advantages of materials processing in space; (4) the absence of buoyancy-driven convection; (5) the absence of hydrostatic pressure; (6) the absence of sedimentation and buoyancy; (7) containerless processing; and (8) ultrahigh vacuum processing.

Source record↗
NASA NTRS1981

Comparative evaluation operability of large space structure connectors

The evaluation of several connector concepts in the neutral buoyancy simulator is discussed. Parameters for evaluating the fasteners included subject comments and hardware damage. Evaluation results include a rank ordering of the candidates with descriptions of the acceptable and unacceptable points of each. General design recommendations established as a result of neutral buoyancy testing are defined. Recommendations include detailed hardware and operational design requirements.

Stokes, J. W.↗
NASA NTRS1982

Tunnel-to-tunnel correlation

Flow quality is discussed. Incremental comparisons of: (1) the angle of attack, (2) the axial force coefficient, and (3) the base cavity axial force coefficient against the normal force coefficient are presented. Relative blockage determination, relative buoyancy corrections, and boundary layer transition length are discussed. Blockage buoyancy caused by tunnel model wall dynamic interaction is discussed in terms of adaptive walls. The effect of 'transonic turbulence factor' is considered.

Steinle, F. W., Jr.↗
NASA NTRS1982

Secondary effects in combustion instabilities leading to flashback

The secondary effects in turbulent combustion instabilities leading to flashback are investigated, including those due to buoyancy and contraction at the combustor outlet. Experiments were conducted in an oblong, rectangular cross-section combustion tunnel, where the effects of a bluff-body flames holder were generated by a rear-facing step behind a streamlined inlet nozzle. The results of experiments leading to flashback with the step mounted at the bottom of the combustion chamber were compared to those of experiments in which it was located at the top. Irrespective of the flow obstructions introduced downstream, the critical equivalence ratio for flashback was consistently lower with the step at the bottom, indicating that buoyancy was enhancing the growth of the recirculation zone that pushed the flame upstream and caused flashback. The contraction at the end of the combustion chamber had a promoting influence on the process of vortex pairing, re-enforcing the influence of the trailing vortices over that of the recirculation vortex system, and thereby curbing the tendency to flashback. Provided that the flow velocity was low, however, the characteristic features of combustion instabilities leading to flashback in the absence of contraction could still be established in its presence.

Vaneveld, L.↗
NASA NTRS1982

Low-gravity fluid flows

The behavior of fluids in micro-gravity conditions is examined, with particular regard to applications in the growth of single crystals. The effects of gravity on fluid behavior are reviewed, and the advent of Shuttle flights are noted to offer extended time for experimentation and processing in a null-gravity environment, with accelerations resulting solely from maneuvering rockets. Buoyancy driven flows are considered for the cases stable-, unstable-, and mixed-mode convection. Further discussion is presented on g-jitter, surface-tension gradient, thermoacoustic, and phase-change convection. All the flows are present in both gravity and null gravity conditions, although the effects of buoyancy and g-jitter convection usually overshadow the other effects while in a gravity field. Further work is recommended on critical-state and sedimentation processes in microgravity conditions.

Ostrach, S.↗
NASA NTRS1980

The fluid mechanics of continuous flow electrophoresis in perspective

Buoyancy alters the flow in continuous flow electrophoresis chambers through the mechanism of hydrodynamic instability and, when the instability is supressed by careful cooling of the chamber boundaries, by restructuring the axial flow. The expanded roles of buoyancy follow upon adapting the size of the chamber and the electric field so as to fractionate certain sorts of cell populations. Scale-up problems, hydrodynamic stability and the altered flow fields are discussed to show how phenomena overlooked in the design and operations of narrow-gap devices take on an overwhelming importance in wide-gap chambers

Saville, D. A.↗
NASA NTRS1982

The curvature of radio jets and tails in the intracluster media of Abell 1446 and 2220

X-ray observations are reported for two rich clusters which contain prominent tailed radio galaxies. These X-ray data are used as probes of the cluster gas to explore mechanisms which may distort radio jets and tails. A nonrelativistic hydrodynamic flow model is invoked to examine the plasma beam parameters needed to bend the radio streams in a manner consistent with the radio and X-ray data. Within the context of this model, the wide-angle tailed radio structure of 1159 + 583 in A1446 is explained by a combination of a buoyancy pressure gradient in the cluster gas and dynamic pressure arising from slow galaxy motion (200 km/sec). The second cluster observed, A2220, contains a clumped X-ray structure with peaks near two dominant galaxies. The strongly bent radio jets of 1638 + 538 (associated with one of the bright galaxies) are consistent with about 1000 km/sec motion, possibly in a binary orbit, through the X-ray emitting cluster gas; a curved feature in the northern tail, however, may be attributable to buoyancy forces. These X-ray data also suggest that the radio jets and tails are thermally confined by the hot intracluster media.

Burns, J. O.↗
NASA NTRS1983

Nonlinear anelastic modal theory for solar convection

Solar envelope models are developed using single-mode anelastic equations as a description of turbulent convection which provide estimates for the variation with depth of the largest convective cellular flows, with horizontal sizes comparable to the total depth of the convection zone. These models can be used to describe compressible motions occurring over many density scale heights. Single-mode anelastic solutions are obtained for a solar envelope whose mean stratification is nearly adiabatic over most of its vertical extent because of the enthalpy flux explicitly carried by the big cell, while a subgrid scale representation of turbulent heat transport is incorporated into the treatment near the surface. It is shown that the single-mode equations allow two solutions for the same horizontal wavelength which are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is found that the upward directed flow experiences large pressure effects which can modify the density fluctuations so that the sense of the buoyancy force is changed, with buoyancy braking actually achieved near the top of the convection zone. It is suggested that such dynamical processes may explain why the amplitudes of flows related to the largest scales of convection are so weak in the solar atmosphere.

Latour, J.↗
NASA NTRS1982

Single-mode theory of diffusive layers in thermohaline convection

A two-layer configuration of thermohaline convection is studied, with the principal aim of explaining the observed independence of the buoyancy-flux ratio on the stability parameter when the latter is large. Temperature is destabilizing and salinity is stabilizing, so diffusive interfaces separate the convecting layers. The convection is treated in the single-mode approximation, with a prescribed horizontal planform and wavenumber. Surveys of numerical solutions are presented for a selection of Rayleigh numbers R, stability parameters lambda and horizontal wavenumbers. The solutions yield a buoyancy flux ratio chi that is insensitive to lambda, in accord with laboratory experiments. However chi increases with increasing R, in contradiction to laboratory observations.

Gough, D. O.↗
NASA NTRS1982

The effect of small temperature gradients on flow in a continuous flow electrophoresis chamber

Continuous flow electrophoresis employs an electric field to separate biological cells suspended in a flowing liquid buffer solution. Good separations based on differences in electrophoretic mobility are obtained only when a unidirectional flow is maintained. The desired flow has a parabolic structure in the narrow dimension of the chamber and is uniform acros the width, except near the edges where the no-slip condition prevails. However, because of buoyancy, very small laterall or axial temperature gradients deform the flow significantly. The results of experiments conducted with a specially instrumented chamber show the origin and structure of the buoyancy-driven perturbations. It is found that very small temperature gradients can disturb the flow significantly, as was predicted by earlier theoretical work.

Rhodes, P. H.↗
NASA NTRS1983

Existence of a persistent background of turbulence

A plausible scenario for the existence of a persistent back-ground of turbulence in the free atmosphere is described. The MST radar technique is the only existing technique that can be used to describe the morphology of occurrence of turbulence as a function of altitude, wind speed, shear, weather conditions, geographical location, etc. This technique was used also to assess the degree of universality of shape and amplitude of the buoyancy wave spectrum and the relation between the buoyancy wave spectrum and turbulence.

Vanzandt, T. E.↗
NASA NTRS1984

On the stability of the ionopause of Venus

The stability of the Venus ionopause is examined in light of the importance of gravitation and curvature. Using a one-fluid approximation for the equation of motion of the plasma, and ignoring the effects of neutrals, a dispersion relation is obtained that includes the effects of the magnetic field, sheared plasma flow, buoyancy, centrifugal force and magnetic tension due to boundary curvature. It is found that buoyancy acts to neutralize the flute instability. As expected, the Kelvin-Helmholtz mode is the dominant instability over most of the dayside ionopause. The expected growth times of this mode are short in comparison with the wave-convection time over the boundary; the waves can grow and saturate quickly, producing a turbulent boundary that may affect electrodynamic coupling between the solar wind and ionospheric plasmas.

Elphic, R. C.↗
NASA NTRS1982

Natural convection in melt crystal growth - The influence of flow pattern on solute segregation

The results of two lines of research aimed at calculating the structure of the flows driven by buoyancy in small-scale crystal growth systems and at understanding the coupling between these flows, the shape of the solidification interface, and dopant segregation in the crystal are reviewed. First, finite-element methods are combined with computer-aided methods for detecting multiple steady solutions to analyze the structure of the buoyancy-driven axisymmetric flows in a vertical cylinder heated from below. This system exhibits onset of convection, multiple steady flows, and loss of the primary stable flow beyond a critical value of the Rayleigh number. Second, results are presented for calculations of convection, melt/solid interface shape, and dopant segregation within a vertical ampoule with thermal boundary conditions that represent a prototype of the vertical Bridgman growth system.

Brown, R. A.↗
NASA NTRS1984

Buoyant production and consumption of turbulence kinetic energy in cloud-topped mixed layers

It is pointed out that studies of the entraining planetary boundary layer (PBL) have generally emphasized the role of buoyancy fluxes in driving entrainment. The buoyancy flux is proportional to the rate of conversion of the potential energy of the mean flow into the kinetic energy of the turbulence. It is not unusual for conversion to proceed in both directions simultaneously. This occurs, for instance, in both clear and cloudy convective mixed layers which are capped by inversions. A partitioning of the net conversion into positive parts, generating turbulence kinetic energy (TKE), and negative parts (TKE-consuming), would make it possible to include the positive part in the gross production rate, and closure would be achieved. Three different approaches to partitioning have been proposed. The present investigation is concerned with a comparison of the three partitioning theories. Particular attention is given to the cloud-topped mixed layer because in this case the differences between two partitioning approaches are most apparent.

Randall, D. A.↗
NASA NTRS1984

Analytic modeling of a spray diffusion flame

A detailed model for a spray diffusion flame is described. The model is based on the boundary layer form of the equations of motion, with droplet transport accounted for using a discretized droplet size distribution function. Interphase transport of mass and energy are accounted for, with a flame-sheet model used to describe the combustion process on a droplet scale. Near dynamic equilibrium is assumed for the description of droplet transport; droplets can diffuse relative to the gas phase. Gas-phase mixing is accounted for using a two-equation turbulence model; buoyancy effects are included, with a temperature fluctuation equation used to account for buoyancy effects on turbulence structure. Thermal radiation from gas-phase CO2 and H2O is included. Gas-phase chemical kinetics are modeled using a 20-reaction, 10-species version of the advanced quasi-global chemical kinetics formulation. Results are compared with data for a vaporizing Freon spray and a pentane spray flame. It is shown that the computational approach provides a reasonably valid picture of the overall development of a spray diffusion flame, and, furthermore, provides a useful tool for the parametric examination of the spray combustion process.

Harsha, P. T.↗
NASA NTRS1983

Cumulus clouds - Interactions between laboratory experiments and observations as foundations for models

Early Woods Hole cumulus observations conducted with the aid of an aircraft suggested that buoyancy dilution by entrainment was a major brake upon tropical cumulus growth. The mechanism by which entrainment occurred, however, was not well understood. Ludlam and Scorer (1953) postulated that buoyant bubbles were the building blocks of cumulus clouds and that it was aerodynamic drag which caused the tops to cease rising. The present investigation is concerned with laboratory experiments and analyses which have been conducted to clarify remaining questions. Attention is given to bubbles in water of uniform density, bubbles released into stably stratified fluids, bubbles released into a two-layer fluid, some observational questions, and buoyant plumes and thermals. The considered experiments provide some insight concerning the mechanism involved in the conversion of buoyancy into motion, taking into account a simpler fluid situation.

Simpson, J.↗
NASA NTRS1984

Effects of the pressure perturbation field in numerical models of unidirectionally sheared thunderstorm convection - Two versus three dimensions

The physical roles of 'buoyant' and 'dynamic' pressure components, and the distinction between buoyant and hydrostatic pressure perturbations, are aspects of the pressure perturbation field in strongly sheared convective storms studied by means of two- and three-dimensional anelastic numerical modeling experiments with common environmental profiles. The pressure analysis clarifies the differences between two- and three-dimensional storms. In the main updraft, strong midlevel thermal buoyancy is partly opposed by a downward-perturbed vertical pressure gradient force. This, however, occurs to a much greater extent in two dimensions than in three, contributing to smaller net upward accelerations. While the buoyant and hydrostatic fields are intimately related to the total buoyancy distribution, the buoyant pressure perturbation is smoother and of lower amplitude than its hydrostatic counterpart. For the model experiments, this distinction is far greater in three dimensions than in two, in association with the smaller scale of the active convection in three dimensions.

Schlesinger, R. E.↗