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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Analytical study of twin-jet shielding

An analytical model a three-dimensional model, of twin-jet shielding, consisting of a point noise source impinging on a cylinder of heated flow in which the temperature and flow velocity are uniform across the cross-section is discussed. Wave equations are given for the regions outside the flow and within the flow cylinder and solutions are matched at the jet boundary under the conditions of continuity of pressure and continuity of the vortex sheet. The model was analyzed to identify mechanisms of transmission and diffraction which control sheilding in the shadow of the shielding jet. It was found that in the zone of the shadow region dominates, shielding is relatively insensitive to variations of such parameters as Mach Number and spacing ratio, but in the zone in which diffraction dominates; shielding is more sensitive to variations in Mach Number, jet temperature and spacing ratio.

Gerhold, C. H.↗
NASA NTRS1983

Modified shielding jet model for twin-jet shielding analysis

An analytical model to estimate the shielding of noise emitted from a point noise source has been developed assuming the shielding jet to be a cylinder of constant radius with uniform flow across the cross section. Comparison to experiment indicated that the model overestimates diffraction of sound around the jet in the far downstream region. The shielding jet model is modified to include widening downstream of the nozzle exit. This not only represents a more realistic model of the jet, but is also expected to improve the shielding estimate downstream. The modified jet model incorporates a Mach number dependent widening rate, a corresponding decrease in flow velocity downstream and an equivalent slug flow evaluation to retain the locally parallel flow approximation of the model development. The shielding analysis with modified jet model is compared to measured data for a subsonic isothermal air jet and a simulated hot subsonic jet. Improvement of the shielding estimate is discussed.

Gerhold, C. H.↗
NASA NTRS1985

Reversal ion source - A new source of negative ion beams

A new type of ion source utilizing beams of electrons and target molecules, rather than a diffuse, volume plasma, is described. The source utilizes an electrostatic electron 'mirror' which reverses trajectories in an electron beam, producing electrons at their turning point having a distribution of velocities centered at zero velocity. A gas which attaches zero-velocity electrons is introduced at this turning point. Negative ions are produced by an attachment or dissociative attachment process. For many of the thermal electron-attaching molecules the cross sections can be quite large, varying as the inverse square root of the electron energy or just the s-wave threshold law. The efficiency and current density of the ion source for production of Cl(-) through the large, thermal energy attachment process is estimated. It is argued that the source can be used for the production of negative ions through attachment resonances located at higher energies as well.

Orient, O. J.↗
NASA NTRS1986

Separated flows near the nose of a body of revolution

The solution of the Navier-Stokes equations for the problem of cross-flow separataion about a deforming cylinder was achieved by iteration. It was shown that the separation starts at the rear stagnation point and the point of primary separation moves upstram along the cylinder surface. A general method of linear stability analysis for nonparallel external flows was constructed, which consists of representing the eigenfunctions with complete orthogonal sets and forms characteristic equations with the Galerkin method. The method was applied to the Kovasznay flow which is an exact solution of the Navier-Stokes equation. The results show that when the critical parameter is exceeded, there are only a few isolated unstable eigen-frequencies. Another exact solution is shown to be absolutely and monotonically stable with respect to infinitesimal disturbances of all frequencies. The flow is also globally, asymptotically, and monotonically stable in the mean with respect o three-dimensional disturbances. This result forms the sound foundation of rigorous stability analysis for nonparallel flows, and provides an invaluable test ground for future studies of nonparallel flows in which the basic states do not posses exact solutions. The application of this method to the study of the formation of spiral vorticies near the nose of a rotating body of revolution is underway. The same method will be applied to the stability analysis of reversed flow over a plate with suction.

Lin, S. P.↗
NASA NTRS1987

NASA Ames Research Center's parabolized Navier-Stokes code - A critical evaluation of heat-transfer predictions

The viscous supersonic/hypersonic flow over a biconic configuration was numerically simulated using the NASA Ames Research Center's Parabolized Navier-Stokes (PNS) code, and results obtained for the effects of various computational parameters on the heat transfer are compared with experimental results. The PNS code is found to provide accurate results for heat transfer parameters using low smoothing coefficient values, assuming that the radial spacing is at some small value and that the body is simple in cross section. For the cases considered, the nominal value of the spatial step size was 0.05, and the grid density was 45 points in the meridional direction and 30 points in the radial direction.

Chaussee, Denny S.↗
NASA NTRS1999

Determination of Jet Noise Radiation Source Locations using a Dual Sideline Cross-Correlation/Spectrum Technique

The goal of our efforts is to extrapolate nearfield jet noise measurements to the geometric far field where the jet noise sources appear to radiate from a single point. To accomplish this, information about the location of noise sources in the jet plume, the radiation patterns of the noise sources and the sound pressure level distribution of the radiated field must be obtained. Since source locations and radiation patterns can not be found with simple single microphone measurements, a more complicated method must be used.

Allen, C. S.↗
NASA NTRS1988

Calculation of symmetric and asymmetric vortex seperation on cones and tangent ogives based on discrete vortex models

An inviscid discrete vortex model, with newly derived expressions for the tangential velocity imposed at the separation points, is used to investigate the symmetric and asymmetric vortex separation on cones and tangent ogives. The circumferential locations of separation are taken from experimental data. Based on a slender body theory, the resulting simultaneous nonlinear algebraic equations in a cross-flow plane are solved with Broyden's modified Newton-Raphson method. Total force coefficients are obtained through momentum principle with new expressions for nonconical flow. It is shown through the method of function deflation that multiple solutions exist at large enough angles of attack, even with symmetric separation points. These additional solutions are asymmetric in vortex separation and produce side force coefficients which agree well with data for cones and tangent ogives.

Chin, S.↗
NASA NTRS1999

Point Spread Function and Transmittance Analyses for Conical and Hexapod Secondary Mirror Support Towers for the Next Generation Space Telescope (NGST)

Different secondary mirror support towers were modeled on the CODE V optical design/analysis program for the NGST Optical Telescope Assembly (OTA) B. The vertices of the NGST OTA B primary and secondary mirrors were separated by close to 9.0 m. One type of tower consisted of a hollow cone 6.0 m long, 2.00 m in diameter at the base, and 0.704 m in diameter at its top. The base of the cone was considered attached to the primary's reaction structure through a hole in the primary. Extending up parallel to the optical axis from the top of this cylinder were eight blades (pyramidal struts) 3.0 m long. A cross section of each these long blades was an isosceles triangle with a base of 0.010 m and a height of 0.100 m with the sharpest part of each triangle pointing inward. The eight struts occurred every 45 deg. The other type of tower was purely a hexapod arrangement and had no blades or cones. The hexapod consisted simply of six, very thin, circular struts, leaving in pairs at 12:00, 4:00, and 8:00 at the primary and traversing to the outer edge of the back of the secondary mount. At this mount, two struts arrived at each of 10:00, 2:00, and 6:00. The struts were attached to the primary mirror in a ring 3.5 m in diameter. They reached the back of the secondary mount, a circle 0.704 m in diameter. Transmittance analyses at two levels were performed on the secondary mirror support towers. Detailed transmittances were accomplished by the use of the CODE V optical design/analysis program and were compared to transmittance calculations that were almost back-of-the-envelope. Point spread function (PSF) calculations, including both diffraction and aberration effects, were performed on CODE V. As one goes out from the center of the blur (for a point source), the two types of support towers showed little difference between their PSF intensities until one reaches about the 3% level. Contours can be delineated on CODE V down to about 10 (exp -8) times the peak intensity, fine detailing the differences in the wings of the PSF. Between the first type of tower and the hexapod tower, below 3% of the peak intensity, the PSF contour maps well delineated significant differences in the outer portions of the PSFS. Such differences in the wings of the PSF significantly impact the resolution of brown dwarfs or extra solar planets from their star. The hexapod tower provided the best PSF.

Wilkerson, Gary W.↗
NASA NTRS1984

The reflection of airborne UV laser pulses from the ocean

It is experimentally shown here for the first time that the normalized laser backscatter cross-section of the sea surface is a function of elevation or height position on teh ocean wave. All data were taken off-nadir, resulting in incidence angles of about 6.5 deg measured relative to the normal to mean sea level (MSL). In the limited data sets analyzed to date, the normalized backscatter cross-section was found to be higher in wave crest regions and lower in wave troughs for a swell-dominated sea over which the wind speed was 5 m/s. The reverse was found to be the case for a sea that was driven by a 14 m/s wind. These isolated results show that the MSL, as measured by an off-nadir and/or multibeam type satellite laser altimeter, will be found above, at, or below the true MSL, depending on the local sea conditions existing in the footprint of the altimeter. Airborne nadir-pointed laser altimeter data for a wide variety of sea conditions are needed before a final determination can be made of the effect of sea state on the backscatter cross-section as measured by a down-looking satellite laser system.

Hoge, F. E.↗
NASA NTRS2000

A Dual Polarized Quasi-Optical SIS Mixer at 550-GHz

We describe the design, fabrication, and the performance of a low-noise dual-polarized quasi-optical superconductor insulator superconductor (SIS) mixer at 550 GHz. The mixer utilizes a novel cross-slot antenna on a hyperhemispherical substrate lens, two junction tuning circuits, niobium trilayer junctions, and an IF circuit containing a lumped element 180 deg hybrid. The antenna consists of an orthogonal pair of twin-slot antennas, and has four feed points, two for each polarization. Each feed point is coupled to a two-junction SIS mixer. The 180 deg IF hybrid is implemented using a lumped element/microstrip circuit located inside the mixer block. Fourier transform spectrometer (FTS) measurements of the mixer frequency response show good agreement with computer simulations. The measured co-polarized and cross-polarized patterns for both polarizations also agree with the theoretical predictions. The noise performance of the dual-polarized mixer is excellent, giving uncorrected receiver noise temperature of better than 115 K (DSB) at 528 GHz for both the polarizations.

Chattopadhyay, Goutam↗
NASA NTRS2013

Circularly Polarized Microwave Antenna Element with Very Low Off-Axis Cross-Polarization

The goal of this work was to improve off-axis cross-polarization performance and ease of assembly of a circularly polarized microwave antenna element. To ease assembly, the initial design requirement of Hexweb support for the internal circuit part, as well as the radiating disks, was eliminated. There is a need for different plating techniques to improve soldering. It was also desirable to change the design to eliminate soldering as well as the need to use the Hexweb support. Thus, a technique was developed to build the feed without using solder, solving the lathing and soldering issue. Internal parts were strengthened by adding curvature to eliminate Hexweb support, and in the process, the new geometries of the internal parts opened the way for improving the off-axis cross-polarization performance as well. The radiating disks curvatures were increased for increased strength, but it was found that this also improved crosspolarization. Optimization of the curvatures leads to very low off-axis cross-polarization. The feed circuit was curved into a cylinder for improved strength, eliminating Hexweb support. An aperture coupling feed mechanism eliminated the need for feed pins to the disks, which would have required soldering. The aperture coupling technique also improves cross-polarization performance by effectively exciting the radiating disks very close to the antenna s central axis of symmetry. Because of the shape of the parts, it allowed for an all-aluminum design bolted together and assembled with no solder needed. The advantage of a solderless design is that the reliability is higher, with no single-point failure (solder), and no need for special plating techniques in order to solder the unit together. The shapes (curved or round) make for a more robust build without extra support materials, as well as improved offaxis cross-polarization.

Greem. David↗
NASA NTRS1973

Comparison of results obtained with various sensors used to measure fluctuating quantities in jets.

An experimental investigation has been conducted to compare the results obtained with six different instruments that sense fluctuating quantities in free jets. These sensors are typical of those that have recently been used by various investigators who are engaged in experimental studies of jet noise. Intensity distributions and two-point correlations with space separation and time delay were obtained. The static pressure, density, and velocity fluctuations are well correlated over the entire cross section of the jet and the cross-correlations persist for several jet diameters along the flow direction. The eddies appear to be flattened in the flow direction by a ratio of 0.4.

Parthasarathy, S. P.↗
NASA NTRS1975

Dual frequency scatterometer measurement of ocean wave height

A technique for remotely measuring wave height averaged over an area of the sea surface was developed and verified with a series of aircraft flight experiments. The measurement concept involves the cross correlation of the amplitude fluctuations of two monochromatic reflected signals with variable frequency separation. The signal reflected by the randomly distributed specular points on the surface is observed in the backscatter direction at nadir incidence angle. The measured correlation coefficient is equal to the square of the magnitude of the characteristic function of the specular point height from which RMS wave height can be determined. The flight scatterometer operates at 13.9 GHz and 13.9 - delta f GHz with a maximum delta f of 40 MHz. Measurements were conducted for low and moderate sea states at altitudes of 2, 5, and 10 thousand feet. The experimental results agree with the predicted decorrelation with frequency separation and with off-nadir incidence angle.

Johnson, J. W.↗
NASA NTRS1981

Use of Magsat anomaly data for crustal structure and mineral resources in the US midcontinent

Magnetic profiles on individual satellites tracks were examined to identify bad (nonterrestrially-based) data points r profiles. Anomaly profiles for the same satellite track, but at different passes were compared for parallel tracks and for tracks that cross. The selected and processed data were plotted and contoured to develop a preliminary magnetic anomaly map. The map is similar in general morphology to NASA's Magsat global scalar anomaly map, but has more detail which is related to crustal properties. Efforts have begun to interpret the satellite magnetic anomalies in terms of crustal character. The correlation of magnetics with crustal petrology may have a much larger tectonic implication. Th possibility of there being an ultramafic lower crust along one zone as a consequence of a continental collision/subduction which helped form the midcontinent craton in Precambrian times is being investigated.

Carmichael, R. S.↗
NASA NTRS1981

Nimbus-7 scanning multichannel microwave radiometer /SMMR/ in-orbit performance appraisal

Calibration and processing techniques enacted during first year of operation of the Nimbus-7 scanning multichannel microwave radiometer (SMMR) are described. It was found that in-orbit calibration was necessary, as was fine-tuning of the geophysical parameter retrieval parameters to account for anomalies such as lower-than-expected polarization differences in ocean radiances. Phase shifts in the scan angles were corrected in order to avoid polarization mixing. Calibration constants to eliminate cross-talk and phase shift effects were established for radiation reflected from the earth, then averaged over data from 300 orbits to fit points on a sine curve to better than 0.2 K accuracy. An iterative approach was determined to be necessary due to signal anomalies caused by antenna dish oscillations. Global ocean and atmosphere parameters used to construct a radiation model of ten latitude bands are presented for use in radiation transfer equations.

Gloersen, P.↗
NASA NTRS1983

Comparison of spherical double probe electric field measurements with plasma bulk flows in plasmas having densities less than 1 cm-3

For a three-hour period in the magnetotail over which plasma density varied from less than 0.1 to about 1/cu cm, comparisons of ISEE-1 spherical double probe (dawn to dusk) electric field measurements and ISEE-2 plasma flows (converted to electric fields) show the zero lag cross correlation coefficient between 768 second averages of the two data sets to have been 0.93. A statistical relative uncertainty between pairs of points in the two data sets, estimated by only including counting statistics in the plasma measurement and time variations of the observed electric field over the measurement interval, is able to account for at least 75 percent of the deviations between the two data sets. In agreement with simple Langmuir probe theory, it has been found that the spacecraft potential measured over the three-hour interval by the double probe instrument varied as the log of the product of the plasma density and the square root of the electron temperature.

Mozer, F. S.↗
NASA NTRS1987

Quantitative use of satellite SAR imagery of sea ice

An automatic technique to obtain ice motion based on maximizing cross-correlation and employing a strategy of nested calculations has been developed and used to obtain displacements of more than 2000 tie-points on a 2 km mesh grid. The mean displacement error is 250 m and is as low as manual data error of 150 m on rigid pieces. The method allows tie-points to be found on a regular grid in a fully automatic fashion. Full resolution data are not needed to maximize the accuracy of displacement measurements, a result which is significant for SAR images, with their large amounts of data.

Fily, M.↗
NASA NTRS1991

Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long-wavelength variations in upper mantle temperature and composition along the Mid-Atlantic Ridge

Measurements were carried out for SS-S differential travel time residuals for nearly 500 paths crossing the northern Mid-Atlantic Ridge, assuming that the residuals are dominated by contributions from the upper mantle near the surface bounce point of the reflected phase SS. Results indicate that the SS-S travel time residuals decrease linearly with square root of age, to an age of 80-100 Ma, in general agreement with the plate cooling model. A joint inversion was formulated of travel time residuals and geoid and bathymetric anomalies for lateral variation in the upper mantle temperature and composition. The preferred inversion solutions were found to have variations in upper mantle temperature along the Mid-Atlantic Ridge of about 100 K. It was calculated that, for a constant bulk composition, such a temperature variation would produce about a 7-km variation in crustal thickness, larger than is generally observed.

Sheehan, Anne F.↗