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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 523 records · Page 29
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NASA NTRS1984

Photovoltaic power conditioning subsystem: State of the art and development opportunities

Photovoltaic systems, the state of the art of power conditioning subsystem components, and the design and operational interaction between photovoltaic systems and host utilities are detailed in this document. Major technical issues relating to the design and development of power conditioning systems for photovoltaic application are considered; these include: (1) standards, guidelines, and specifications; (2) cost effective hardware design; (3) impact of advanced components on power conditioning development; (4) protection and safety; (5) quality of power; (6) system efficiency; and (7) system integration with the host utility. Theories of harmonic distortion and reactive power flow are discussed, and information about power conditioner hardware and manufacturers is provided.

Krauthamer, S.↗
NASA NTRS1984

Freezing of Martian streams under climatic conditions

The valley networks of Mars are widely believed to have formed at a time when climatic conditions on the planet were significantly different from those that currently prevail. This view arises from the following observations: (1) the valleys form integrated branching networks which suggests fluid drainage, and water is the most plausible fluid, (2) the present atmosphere contains only minute amounts of water, (3) the networks appear to be more akin to terrestrial valleys that are eroded by streams of modest discharges than features that form by catastrophic floods, and (4) small streams of water will rapidly freeze under present climatic conditions. Climatic conditions at the time of formation of the valleys are studied based on the assumption that they were cut by running water.

Carr, M. H.↗
NASA NTRS1983

Boundary conditions for the solution of compressible Navier-Stokes equations by an implicit factored method

A method is presented for formulating the boundary conditions in implicit finite-difference form needed for obtaining solutions to the compressible Navier-Stokes equations by the Beam and Warming implicit factored method. The usefulness of the method was demonstrated (a) by establishing the boundary conditions applicable to the analysis of the flow inside an axisymmetric piston-cylinder configuration and (b) by calculating velocities and mass fractions inside the cylinder for different geometries and different operating conditions. Stability, selection of time step and grid sizes, and computer time requirements are discussed in reference to the piston-cylinder problem analyzed.

Shih, T. I.-P.↗
NASA NTRS1984

A rule for selecting analytical boundary conditions for the conservative quasi-one-dimensional nozzle flow equations

For the one-dimensional nozzle flow equations, the number of analytical boundary conditions that may be applied corresponds to the number of exterior characteristics reaching the boundary points. Since the slopes of the characteristics are the characteristic speeds u, u+c, and u-c, one concludes that two analytical boundary conditions are required at a subsonic inflow boundary and one at a subsonic outflow boundary. However, using this guideline alone can lead to nonunique solutions in subsonic cases. This paper presents a rule for selecting analytical boundary conditions for the dependent variables which will yield unique solutions if they exist.

Wornom, S. F.↗
NASA NTRS1984

The effect of closed boundary conditions on a stationary dynamo

One of two boundary conditions generally assumed in solutions of the dynamo equation is related to the disappearance of the azimuthal field at the boundary. Parker (1984) points out that for the realization of this condition the field must escape freely through the surface. Escape requires that the field be detached from the gas in which it is embedded. In the case of the sun, this can be accomplished only through reconnection in the tenuous gas above the visible surface. Parker concludes that the observed magnetic activity on the solar surface permits at most three percent of the emerging flux to escape. He arrives at the conclusion that, instead of B(phi) = 0, the partial derivative of B(phi) to r is equal to zero. The present investigation is concerned with the effect of changing the boundary condition according to Parker's conclusion. Implications for the solar convection zone are discussed.

Choudhuri, A. R.↗
NASA NTRS1984

Video compression using conditional replenishment and motion prediction

A study of a low-rate monochrome video compression system is presentd in this paper. This system is a conditional-replenishment coder that uses two-dimensional Walsh-transform coding within each video frame. The conditional-replenishment algorithm works by transmitting only the portions of an image that are changing in time. This system is augmented with a motn-prediction algorithm that measures spatial dispalcement parameters from frame to frame, and codes the data using these parameters. A comparison is made between the conditional-replenishment system with, and without, the motion-predictinalgorthm. Subsampling in time is ued to maintain the data rate rate at a fixed value. Average bit rates of 1 bit/picture element (pel) to 1/16 bit/pel are considered. The resultant performance of the compression simulations is presented in terms of the average frame rates produced.

Hein, D. N.↗
NASA NTRS1984

Characteristic boundary conditions for three-dimensional transonic unsteady aerodynamics

Characteristic far-field boundary conditions for the three-dimensional unsteady transonic small disturbance potential equation have been developed. The boundary conditions were implemented in the XTRAN3S finite difference code and tested for a flat plate rectangular wing with a pulse in angle of attack; the freestream Mach number was 0.85. The calculated force response shows that the characteristic boundary conditions reduce disturbances that are reflected from the computational boundaries.

Whitlow, W., Jr.↗
NASA NTRS1985

Development and Maturation of the Neuromuscular Junciton in Cell Culture Under Conditions of Simulated Zero-gravity

Alterations in gravitational conditions which alter the normal development and interactions of nerve and muscle cells grown in culture is examined. Clinostat conditions, similating Og, which produce changes in cell morphology and growth patterns is studied. Data show that rotation of cocultures of nerve and muscle cells results in morphologic changes which are predicted to significantly alter the functional interactions between the elements of a prototypic synapse. It is further predicted that similar alterations may occur in central synapses which may therefore affect the development of the central nervous system when subjected to altered gravitational conditions.

Gruener, R.↗
NASA NTRS1985

Nonreflecting boundary conditions for the complete unsteady transonic small-disturbance equation

Nonreflecting far-field boundary conditions that are consistent with the complete transonic small-disturbance (TSD) equations are derived. They are implemented in a new code for solving the complete TSD equation and are tested for a harmonically oscillating NACA 64A010 airfoil in transonic flow and for a flat-plate airfoil with a pulse in the angle of attack. Using the new boundary conditions on a relatively small grid, solutions for the airfoil that are obtained that agree with large-grid calculations, resulting in a 44 percent savings in computer time. Frequency responses for the flat plate show that most of the disturbances incident on the computational boundaries are absorbed by the boundary conditions.

Whitlow, W., Jr.↗
NASA NTRS1985

On the existence and stability conditions for mixed-hybrid finite element solutions based on Reissner's variational principle

The extensions of Reissner's two-field (stress and displacement) principle to the cases wherein the displacement field is discontinuous and/or the stress field results in unreciprocated tractions, at a finite number of surfaces ('interelement boundaries') in a domain (as, for instance, when the domain is discretized into finite elements), is considered. The conditions for the existence, uniqueness, and stability of mixed-hybrid finite element solutions based on such discontinuous fields, are summarized. The reduction of these global conditions to local ('element') level, and the attendant conditions on the ranks of element matrices, are discussed. Two examples of stable, invariant, least-order elements - a four-node square planar element and an eight-node cubic element - are discussed in detail.

Karlovitz, L. A.↗
NASA NTRS1985

Free Surface Phenomena Under Low- and Zero-Gravity Conditions

In a low- or zero-gravity environment the free surface of a liquid can behave in striking, unexpected ways. For example, in a partially filled container, a free surface that is well behaved under terrestrial conditions can rise to an arbitrarily large height or even fail to exist when gravity is absent. An answer is sought to the central mathematical questions: Under what conditions can free surfaces exist and what are their properties? -- and experimental questions of what means can be devised to observe and to measure the surfaces quantitatively. Current activity focuses on material selection and the design of optical diagnostic methods for in-space experiments. These experiments will test mathematical predictions of discontinuous transition from existence to nonexistence of capillary free surfaces in certain geometric configurations under zero gravity conditions. Liquids and container materials suitable for achieving the required contact-angle range and optical properties are being investigated, as well as the effects of contaminants and fluid motion on contact angle. The optical diagnostic technique investigation concerns laser-induced fluorescence, including emphasis on data on acquisition, sources and detectors, reliability, data management, and computer control.

Concus, P.↗
NASA NTRS1985

Flammability Limits of Gases Under Low Gravity Conditions

The purpose of this combustion science investigation is to determine the effect of zero, fractional, and super gravity on the flammability limits of a premixed methane air flame in a standard 51 mm diameter flammability tube and to determine, if possible, the fluid flow associated with flame passage under zero-g conditions and the density (and hence, temperature) profiles associated with the flame under conditions of incipient extinction. This is accomplished by constructing an appropriate apparatus for placement in NASA's Lewis Research Center Lear Jet facility and flying the prescribed g-trajectories while the experiment is being performed. Data is recorded photographically using the visible light of the flame. The data acquired is: (1) the shape and propagation velocity of the flame under various g-conditions for methane compositions that are inside the flammable limits, and (2) the effect of gravity on the limits. Real time accelerometer readings for the three orthogonal directions are displayed in full view of the cameras and the framing rate of the cameras is used to measure velocities.

Strehlow, R. A.↗
NASA NTRS1985

Laboratory evaluation and application of microwave absorption properties under simulated conditions for planetary atmospheres

Radio absorptivity data for planetary atmospheres obtained from spacecraft radio occultation experiments and Earth-based radio astronomical observations can be used to infer abundances of microwave absorbing atmospheric constituents in those atmospheres, as long as reliable information regarding the microwave absorbing properties of potential constituents is available. The use of theoretically-derived microwave absorption properties for such atmospheric constituents, or laboratory measurements of such properties under environmental conditions which are significantly different than those of the planetary atmosphere being studied, often lead to significant misinterpretation of available opacity data. Steffes and Eshleman showed that under environmental conditions corresponding to the middle atmosphere of Venus, the microwave absorption due to atmospheric SO2 was 50 percent greater than that calculated from Van Vleck-Weiskopff theory. Similarly, the opacity from gaseous H2SO4 was found to be a factor of 7 greater than theoretically predicted for conditions of the Venus middle atmosphere. The recognition of the need to make such measurements over a range of temperatures and pressures which correspond to the periapsis altitudes of radio occultation experiments, and over a range of frequencies which correspond to both radio occultation experiments and radio astronomical observations, has led to the development of a facility at Georgia Tech which is capable of making such measurements.

Steffes, P. G.↗
NASA NTRS1986

Low density gas dynamic wall boundary conditions

Low density nozzles or large expansion ratio nozzles used in space experience rarefaction effects near their exit in the form of velocity slip and temperature jump at the walls. In addition, the boundary layers become very thick and there is a very strong viscous/inviscid interaction. For these reasons no existing design technique has been found to accurately predict the nozzle flow properties up to the nozzle exit. The objective of this investigation was to examine the slip boundary conditions and formulate them in a form appropriate for use with a full Navier-Stokes numerical code. The viscous/inviscid interaction would automatically be accounted for by using a compressible Navier-Stokes code. Through examination of the interaction of molecules with solid surfaces, a model for the distribution function of the reflected molecules has been determined and this distribution function has been used to develop a new slip boundary condition that can be shown to yield more realistic surface boundary conditions.

Collins, F. G.↗
NASA NTRS1986

The behavior of fuel-lean premixed flames in a standard flammability limit tube under controlled gravity conditions

Fuel-lean flames in methane-air mixtures from 4.90 to 6.20 volume percent fuel and propane-air mixtures from 1.90 to 3.00 volume percent fuel were studied in the vicinity of the limit for a variety of gravity conditions. The limits were determined and the behavior of the flames studied for one g upward, one g downward, and zero g propagation. Photographic records of all flammability tube firings were obtained. The structure and behavior of these flames were detailed including the variations of the curvature of the flame front, the skirt length, and the occurrence of cellular instabilities with varying gravity conditions. The effect of ignition was also discussed. A survey of flame speeds as a function of mixture strength was made over a range of lean mixture compositions for each of the fuels studied. The results were presented graphically with those obtained by other researchers. The flame speed for constant fractional gravity loadings were plotted as a function of gravity loadings from 0.0 up to 2.0 g's against flame speeds extracted from the transient gravity flame histories for corresponding gravity loadings. The effects of varying gravity conditions on the extinguishment process for upward and downward propagating flames were investigated.

Wherley, B. L.↗
NASA NTRS1986

An outflow acoustic boundary condition for internal duct flows

A boundary condition for the linear acoustic equation has been developed that allows the acoustic pressure waves to propagate out of the computational domain boundary, just as they would propagate in an infinitely long duct. The problem is divided into two domains: numerical and analytical. The boundary condition provides a matching of the two domains. Examples show this method works well in an acoustic panel program for a model problem (simple source in a rectangular duct with several propagating modes present). The boundary condition is described so that it can be used with various duct various duct geometries and numerical methods.

Mosher, M.↗
NASA NTRS1986

Laboratory evaluation and application of microwave absorption properties under simulated conditions for planetary atmospheres

After long arduous work with the simulator, measurements of the refractivity and absorptivity of nitrogen under conditions similar to those for Titan were completed. The most significant measurements, however, were those of the microwave absorption from gaseous ammonia under simulated conditions for the Jovian atmospheres over wavelengths from 1.3 to 22 cm. The results of these measurements are critical in that they confirm the theoretical calculation of the ammonia opacity using the Ben-Reuven lineshape. The application of both these results, and results obtained previously, to planetary observations at microwave frequencies were especially rewarding. Applications of the results for ammonia to radio astronomical observations of Jupiter in the 1.3 to 20 cm wavelength range and the application of results for gaseous H2SO4 under simulated Venus conditions are discussed.

Steffes, P. G.↗
NASA NTRS1986

The effect of processing conditions on the GaAs/plasma-grown insulator interface

The effect of processing conditions on the interface state density was evaluated from C-V measurements on metal-oxide-semiconductor capacitors. The optimum processing conditions for the minimum surface state density was found to be related to the postoxidation annealing temperature and time, and was independent of chemical treatments prior to oxidation. Annealing at the optimum condition (i.e., at 350 C for 1 h in either nitrogen or hydrogen gas, with or without an aluminum pattern on the oxide) reduces the fast surface state density by about one order of magnitude. By using a nitrogen/oxygen plasma, the static dielectric constant of the oxide decreased as the N/O ratio was increased, and nitrogen was incorporated into the oxide. In addition, the fast surface state density was reduced as a result of this nitridation process.

Hshieh, F. I.↗