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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 NTRS1989

Wave propagation effects of broadband electrostatic noise in the magnetotail

An analysis of the wave and particle data from ISEE 1 for 1978 yielded several examples of crossings between the lobe and the plasma sheet boundary layer (PSBL) which exhibited the signatures predicted by the theoretical analysis of the generation of broadband electrostatic noise (BEN) by ion beam instabilities. These signatures are a gradual rise in the upper frequency of BEN as the spacecraft approaches the plasma boundary layer, and a very rapid rise in the upper frequency near the crossing into the PSBL from the lobe. Several examples of crossings are presented that exhibit both signatures, as well as a case of crossings in which the gradual frequency rise signature is absent but the rapid rise is present. This case exhibits a BEN in the range expected for the low-frequency ion-ion two-stream and the high-frequency Buneman instability.

Grabbe, Crockett↗
NASA NTRS1990

Piecewise linear approximation for hereditary control problems

This paper presents finite-dimensional approximations for linear retarded functional differential equations by use of discontinuous piecewise linear functions. The approximation scheme is applied to optimal control problems, when a quadratic cost integral must be minimized subject to the controlled retarded system. It is shown that the approximate optimal feedback operators converge to the true ones both in the case where the cost integral ranges over a finite time interval, as well as in the case where it ranges over an infinite time interval. The arguments in the last case rely on the fact that the piecewise linear approximations to stable systems are stable in a uniform sense.

Propst, Georg↗
NASA NTRS1990

Simulation studies of plasma waves in the electron foreshock - The generation of downshifted oscillations

The generation of waves with frequencies downshifted from the plasma frequency, as observed in the electron foreshock, is analyzed by particle simulation. Wave excitation differs fundamentally from the familiar excitation of the plasma eigenmodes by a gentle bump-on-tail electron distribution. Beam modes are destabilized by resonant interaction with bulk electrons, provided the beam velocity spread is very small. These modes are stabilized, starting with the higher frequencies, as the beam is broadened and slowed down by the interaction with the wave spectrum. Initially a very cold beam is also capable of exciting frequencies considerably above the plasma frequency, but such oscillations are quickly stabilized. Low-frequency modes persist for a long time, until the bump in the electron distribution is completely 'ironed' out. This diffusion process also is quite different from the familiar case of well-separated beam and bulk electrons. A quantitative analysis of these processes is carried out.

Dum, C. T.↗
NASA NTRS1990

Application of vortex embedding to aircraft flows

A combined Eulerian/Lagrangian 'vortex embedding' method was applied to two cases. The first case involved compressible flow over a realistic fighter aircraft (X-29 fighter) at low angle of attack, for both subsonic and transonic speeds, with shock. Good agreement was obtained for computed wing surface pressures when compared to flight test data. The second case involved incompressible flow over a delta wing with round leading edges at moderate angles of attack. Strong, concentrated vortices were shown to be shed from the leading edge. The vortex embedding method appears to be an efficient solution procedure for cases where well-defined vortex sheets are shed from surfaces.

Steinhoff, John↗
NASA NTRS1990

Transient analysis of chilldown in a cryogenic transfer line

A numerical model was developed, with the SINDA'85/FLUINT program, for calculating the thermal and hydrodynamic transients that occur during the chilldown of a cryogenic transfer line, using a well documented test case to validate the modeling process. Using this model, a total of ten cases were analyzed to evaluate the effects of variable inlet valve position, inlet pressures, and the use of an internal flow liner to promote nucleate boiling. It was found that an efficient transfer line cooldown can be achieved if the inlet flow is throttled, to reduce the flow rate and quality, and an internal flow liner such as Teflon is used.

Martin, T.↗
NASA NTRS1990

A four-lidar view of Cirrus from the FIRE IFO: 27-28 October 1986

The four ground-based lidar systems that participated in the 1986 FIRE IFO were configured in a diamond-shaped array across central and southern Wisconsin. Data were generally collected in the zenith pointing mode, except for the Doppler lidar, which regularly operated in a scanning mode with intermittent zenith observations. As a component of the cirrus case study of 27 and 28 October 1986 selected for initial analysis, data collected by the remote sensor ensemble from 1600 (on the 27th) to 2400 UTC (on the 28th) is described and compared. In general, the cirrus studied on the 27th consisted of intermittent layers of thin and subvisual cirrus clouds. Particularly at Wausau, subvisual cirrus was detected from 11.0 to 11.5 km MSL, just below the tropopause. At lower levels, occasional cirrus clouds between approx. 8.0 to 9.5 km were detected from all ground sites. Preliminary analysis of the four-lidar dataset reveals the passage of surprisingly consistent cloud features across the experiment area. A variety of types and amounts of middle and high level clouds occurred, ranging from subvisual cirrus to deep cloud bands. It is expected that the ground-based lidar measurements from this case study, as well as the airborne observations, will provide an excellent data base for comparison to satellite observations.

Sassen, Kenneth↗
NASA NTRS1991

Versatile, low-cost, computer-controlled, sample positioning system for vacuum applications

A versatile, low-cost, easy to implement, microprocessor-based motorized positioning system (MPS) suitable for accurate sample manipulation in a Second Ion Mass Spectrometry (SIMS) system, and for other ultra-high vacuum (UHV) applications was designed and built at NASA LeRC. The system can be operated manually or under computer control. In the latter case, local, as well as remote operation is possible via the IEEE-488 bus. The position of the sample can be controlled in three linear orthogonal and one angular coordinates.

Vargas-Aburto, Carlos↗
NASA NTRS1987

Comparison of smooth pursuit and combined eye-head tracking in human subjects with deficient labyrinthine function

The effects of deficient labyrinthine function on smooth visual tracking with the eyes and head were investigated, using ten patients with bilateral peripheral vestibular disease and ten normal controls. Active, combined eye-head tracking (EHT) was significantly better in patients than smooth pursuit with the eyes alone, whereas normal subjects pursued equally well in both cases. Compensatory eye movements during active head rotation in darkness were always less in patients than in normal subjects. These data were used to examine current hypotheses that postulate central cancellation of the vestibulo-ocular reflex (VOR) during EHT. A model that proposes summation of an integral smooth pursuit command and VOR/compensatory eye movements is consistent with the findings. Observation of passive EHT (visual fixation of a head-fixed target during en bloc rotation) appears to indicate that in this mode parametric gain changes contribute to modulation of the VOR.

Leigh, R. J.↗
NASA NTRS1991

Vorticity-streamfunction formulation of unsteady incompressible flow past a cylinder - Sensitivity of the computed flow field to the location of the outflow boundary

The influence of the location of the outflow computational boundary on the unsteady incompressible flow past a circular cylinder at Reynolds number 100 is examined. The vorticity-streamfunction formulation of the Navier-Stokes equations is used in all computations. Two types of outflow boundary conditions are subjected to a series of tests in which the domain length is gradually reduced. The traction-free condition performs well in most cases and allows the outflow boundary to be located as close as 6.5 cylinder diameters from the body. The other boundary condition type is not as forgiving, but has the advantage of being simpler to implement and can still provide reasonably accurate solutions. It is also observed that both condition types can influence the flow field strongly and globally when the boundary is brought closer than 2.5 diameters from the body. In such cases the temporal periodicity of the solution is lost.

Behr, M.↗
NASA NTRS1992

Unusually distant bow shock encounters at Venus

The location of the Venus bow shock on two days (July 6, 1981 and August 4, 1971) when the IMF was strong and the Mach number low is examined. When the solar wind Mach number approaches unity the bow shock appears to be greatly detached from the obstacle. Thus inferences from observations of shocks near planets should be treated cautiously, especially if the Mach number is small or not well known. The case of the Phobos mission to Mars is examined as an example of such a situation.

Russell, C. T.↗
NASA NTRS1990

Manned Mars System Study (MMSS): Mars transportation and facility study. Volume 1: Executive Summary

The purpose of the study was to design and analyze systems for conducting human missions to Mars and the moon, with special emphasis on the transportation and facility infrastructure. This study was conducted by Martin Marietta Astronautics Group, with an important teaming role by Science Applications International Corporation (SAIC). This work included the studies and separate reports of the FY-1988 and 1989 case studies as well as special analyses and parametric studies.

Source record↗
NASA NTRS1993

Mg II and Ly-alpha fluxes in M dwarfs - Evaluation of an acoustic model

Surface fluxes of Mg II and Ly-alpha from a sample of M dwarfs have been reported by Panagi and Mathioudakis (PM, 1992). In order to determine if acoustic effects can reproduce the fluxes in the least active ('basal') stars in the PM sample, we have constructed steady state models of plane-parallel atmospheres with T(eff) = 3000 and 4000 K including heating by shock dissipation. The models incorporate radiative losses in the chromosphere by solving the transfer equation for two strong lines (Mg II k and Ly-alpha). Predictions of standard weak shock theory are expected to overestimate the rate of energy deposition in the chromospheres of M dwarfs compared to the rate of deposition in F, G, or K dwarfs. In order to incorporate this effect in M dwarfs, we compute a series of atmospheric models in which, for a given T(eff), the standard weak shock dissipation rate is reduced by a series of factors F(r). Although the input acoustic flux F(mech) may in principle be determined uniquely by T(eff) and log g, the correct value to use in any particular case is not well known. We therefore use F(mech) and F(r) as parameters to construct a family of M dwarf models at various T(eff) and g. Comparing with the observed fluxes in the least active M dwarfs of the PM sample, we find that our models reproduce the observed fluxes of both Mg II and Ly-alpha best if F(mech) is close to the value predicted by Bohn (1984) and if F(r) is approximately equal to 10-20.

Mullan, D. J.↗
NASA NTRS1941

Two-dimensional potential flow past a smooth wall with partly constant curvature

The speed of a two-dimensional flow potential flow past a smooth wall, which evinces a finite curvature jump at a certain point and approximates to two arcs in the surrounding area, has a vertical tangent of inflection in the critical point as a function of the arc length of the boundary curve. This report looks at a general theorem of the local character of the conformal function at the critical point as well as the case of the finite curvature jump.

Koppenfels, Werner Von↗
NASA NTRS1993

Composition of the Martian aerosols through near-IR spectroscopy

Near-infrared spectroscopy is a powerful technique to study the composition of planetary surfaces, as the main minerals exhibit absorption bands in this spectral range. It gave important information on the mineralogy and petrology of Mars in the past twenty years although in this case it is well known that a large fraction of light is scattered by the airborne particles before reaching the surface. The measured signal is thus the sum of two different contributions that should be studied separately: One from the surface and one from the aerosols that depends on their density, size distribution and composition. Data from the ISM imaging spectrometer are used here to derive the aerosols spectrum. They consist in sets of spectra (from 0.76 to 3.16 microns) of approximately 3000 pixels approximately 25x25 sq km in size. The resulting spectrum exhibits both water-ice and clay mineral features superimposed on a scattering continuum.

Erard, Stephane↗
NASA NTRS1992

Ultraviolet observations of clusters of Wolf-Rayet stars in the SBm3 galaxy NGC 4214 and Ultraviolet and optical observations of LINER's

The purpose of the grant was to obtain and analyze IUE (UV) and ground-based (optical) spectra of the central bar of NGC 4214, which contains several bright H II regions, in order to further explore the properties of the Wolf-Rayet stars in this galaxy. Several spatially distinct regions, with widely different equivalent widths of optical Wolf-Rayet lines, could be sampled by the large IUE entrance aperture. By using newly developed extraction techniques, the spectra of these H II regions could be isolated, and differences in their stellar populations would be systematically studied. Data were obtained with IUE in late February and early March, 1992. Some of the shifts were successful, but a few were not -- apparently the blind offset from the nearby star did not work equally well in all cases. Thus, the signal-to-noise ratio is somewhat lower than we had hoped. This necessitated a more careful extraction of the spectra of individual H II regions from the two-dimensional spectra. (A program that models the point spread function in the spatial direction was used to deblend the distinct H II regions.) The IUE data are currently being analyzed in conjunction with ground-based optical spectra. There appear to be obvious variations in the stellar population over angular scales of only a few arc seconds. The second part of the research performed under this grant was a continuation of a project that uses IUE (UV) and ground-based (optical) spectra to infer the physical conditions in Low-Ionization Nuclear Emission-Line Regions (LINER's). We have obtained spectra of a few key objects that cover a representative range in LINER continuum and emission-line properties. The overall goals are to (1) separate the emission into spatially distinct components, (2) establish whether the observed nuclear ultraviolet continua indicate sufficient photoionizing fluxes to account for the emission lines, (3) determine whether the nuclear emission can be explained by hot stars alone, (4) detect and measure the strengths of UV emission lines, and (5) search for systematic differences in the UV spectra of LINER's whose other properties differ in some respects.

Filippenko, Alexei V.↗
NASA NTRS1993

Meteorological analysis of the December 4-6 FIRE cirrus-2 case

During the period 4-6 December 1991, three distinct and separate cirrus cloud systems were observed to move over the FIRE Cirrus-II field site at Coffeyville (COF) in southeastern Kansas. Meteorological analyses of the first and third events are presented here. These cases were well-sampled by the regional rawinsonde network, highly instrumented aircraft, and extensive ground-based remote sensors at COF. Our analyses are primarily based on the rawinsonde observations. Unfortunately, the intervening event was not well-observed by the regional rawinsonde network.

Starr, David↗
NASA NTRS1993

Time-dependent grid adaptation for meshes of triangles and tetrahedra

This paper presents in viewgraph form a method of optimizing grid generation for unsteady CFD flow calculations that distributes the numerical error evenly throughout the mesh. Adaptive meshing is used to locally enrich in regions of relatively large errors and to locally coarsen in regions of relatively small errors. The enrichment/coarsening procedures are robust for isotropic cells; however, enrichment of high aspect ratio cells may fail near boundary surfaces with relatively large curvature. The enrichment indicator worked well for the cases shown, but in general requires user supervision for a more efficient solution.

Russ D Rausch↗
NASA NTRS1994

Validation of the RPLUS3D Code for Supersonic Inlet Applications Involving Three-Dimensional Shock Wave-Boundary Layer Interactions

A three-dimensional computational fluid dynamics code, RPLUS3D, which was developed for the reactive propulsive flows of ramjets and scramjets, was validated for glancing shock wave-boundary layer interactions. Both laminar and turbulent flows were studied. A supersonic flow over a wedge mounted on a flat plate was numerically simulated. For the laminar case, the static pressure distribution, velocity vectors, and particle traces on the flat plate were obtained. For turbulent flow, both the Baldwin-Lomax and Chien two-equation turbulent models were used. The static pressure distributions, pitot pressure, and yaw angle profiles were computed. In addition, the velocity vectors and particle traces on the flat plate were also obtained from the computed solution. Overall, the computed results for both laminar and turbulent cases compared very well with the experimentally obtained data.

Kapoor, Kamlesh↗