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At least 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Oxidation and hot corrosion of coated and bare oxide dispersion strengthened superalloy MA-755E

Cyclic hot corrosion and oxidation of an experimental oxide dispersion strengthened (ODS) superalloy MA-755E were conducted in a hot gas stream at Mach 0.3. The response of the ODS alloy, bare or with protective coatings, was similar to that of a conventional cast alloy, IN-792, in hot corrosion at 900 C. However, during oxidation at 1100 and 1150 C the ODS alloy differed from the cast alloy by developing a greater amount of subsurface porosity. Compared with a diffused aluminide coating, an electron beam vapor deposited NiCrAlY coating offered superior oxidation protection and decreased porosity formation. In additional testing, the tendency to form porosity was associated with the large grains of recrystallized powder metallurgy alloys but was independent of the presence of an oxide dispersion.

Glasgow, T. K.↗
NASA NTRS1981

Drift and dispersion studies of ocean-dumped waste using Landsat imagery and current drogues

The drift and dispersion of industrial acid wastes dumped 64 km off the Delaware coast were investigated using 16 Landsat images. Waste plume drift velocities and spread rates were obtained, and principal component analysis was performed to discriminate the acid waste from other pollutants. Waste plumes averaged drifting rates from 0.59 km/hr to 3.39 km/hr into the southwest quadrant, and remained above the thermocline, which formed at depths ranging from 13 m to 24 m. Rapid waste movement toward shore occurred primarily during storms, although plumes were rapidly dispersed and diluted at spread rates in excess of 4 cm/sec. Landsat data analysis also indicates plume width increase of about 1.5 cm/sec during calm sea conditions, which is also in agreement with Falk's (1974) estimates of plume dilution.

Klemas, V.↗
NASA NTRS1982

Barriers and dispersal surfaces in minimum-time interception

Minimum time interception of a target moving in a horizontal plane is analyzed as a one-player differential game. Dispersal points and points on the barrier are located for a class of pursuit evasion and interception problems. These points are determined by constructing cross sections of the isochrones and hence obtaining the barrier, dispersal, and control level surfaces. The game solution maps the controls as a function of the state within the capture region.

Rajan, N.↗
NASA NTRS1979

Formation of metallic and metal hydrous oxide dispersions

The formation, via hydrothermally induced precipitation from homogeneous solution, of a variety of well-defined dispersions of metallic and hydrous metal in the conditions under which the particles are produced (e.g., pH and composition of the growth medium, aging temperature, rate of heating, or degree of agitation) can be readily discerned by following changes in the mass, composition, and morphology of the final solid phase. The generation of colloidal dispersions in the absence of gravity convection or sedimentation effects may result in the appearance of morphological modifications not previously observed in terrestrially formed hydrosols.

Matijevic, E.↗
NASA NTRS1981

Dispersive Doppler measurement of the electron content of the torus of Io

As Voyager 1 made its swing-by of Io, it passed through and behind the satellite's plasma torus. The phase paths of the coherent 13 cm and 3.6-cm wavelength signals transmitted from the spacecraft were shortened differentially by the plasma, resulting in the observation of a dispersive Doppler signature in the signals received at the NASA/Jet Propulsion Laboratory (deep space network) stations. Ray path integration through three different models of the electron distribution of the torus of Io (Warwick et al. 1979; Birmingham et al. 1981; Bagenal et al., 1981) have been performed. The results of the integrations are compared with the dispersive Doppler data.

Levy, G. S.↗
NASA NTRS1981

Analytic approximations to the modon dispersion relation

Three explicit analytic approximations are given to the modon dispersion relation developed by Flierl et al. (1980) to describe Gulf Stream rings and related phenomena in the oceans and atmosphere. The solutions are in the form of k(q), and are developed in the form of a power series in q for small q, an inverse power series in 1/q for large q, and a two-point Pade approximant. The low order Pade approximant is shown to yield a solution for the dispersion relation with a maximum relative error for the lowest branch of the function equal to one in 700 in the q interval zero to infinity.

Boyd, J. P.↗
NASA NTRS1982

Measurement of the dispersive and refractive indices of germanium using diode lasers

Germanium Fabry-Perot etalons are commonly used in diode laser spectroscopy to establish relative frequency calibration scales for molecular absorption spectra. Typically the channel spectrum of the etalon is recorded simultaneously with the molecular spectrum as the laser injection current is tuned linearly, and the etalon free spectral range (fringe spacing) is used to measure the frequency separations among lines in the molecular spectrum. For this purpose, the etalon free spectral range must be known for the measurement wavelength, and this in turn relies on knowledge of the etalon length and the dispersive index at this wavelength. It is pointed out that the most satisfactory approach is to measure directly the free spectral range or dispersive index of the etalon at the wavelength of interest. A description is presented of measurements of this type, performed near 7.9 and 10.1 micrometers for a 7.7-cm long germanium etalon.

Jennings, D. E.↗
NASA NTRS1982

The influence of gamma prime on the recrystallization of an oxide dispersion strengthened superalloy - MA 6000E

The requirement of large, recrystallized, highly elongated grains is of primary importance to the development of suitable high temperature properties in oxide dispersion strengthened-superalloys. In the present study the recrystallization behavior of MA 6000E, a recently developed Y2O3 strengthened superalloy produced by mechanical alloying, was examined using transmission and replication microscopy. Gradient and isothermal annealing treatments were applied to extruded and hot rolled products. It was found that conversion from a very fine (0.2 micron) grain structure to a coarse (approximately 10 mm) grain structure is controlled by the dissolution of the gamma prime phase, while grain shape was controlled primarily by the thermal gradient. The fine uniform oxide dispersion appeared to have only a secondary influence in determining the grain shape as columnar grains could be grown transverse to the working direction by appropriate application of the thermal gradient.

Hotzler, R. K.↗
NASA NTRS1982

Mount St. Helens plume dispersion based on trajectory analyses

The major eruption of Mount St. Helens on 18 May 1980, had sufficient energy to traverse the troposphere (9 km above the mountain top) and to penetrate an additional 10 km into the stratosphere. This plume, initially quasi-vertical, rapidly acquired the horizontal momentum of the environmental winds and suffered differential rotation due to a positive speed shear in the troposphere and a negative shear in the stratosphere. Advected rapidly eastward by the undulating jet stream, the lower stratospheric portion of the plume circled the globe at an average speed of approx. 25 m s/l, reentering North America over California in early June. During the same period, the uppermost portion slowly looped over the northwestern United States and then moved westward over the northern Pacific Ocean. Thus, plume dispersion was initiated by the vertical shears of the horizontal winds which converted a nearly vertical plume to a thin, quasi-horizontal, quasi-zonal lamina. Horizontal shears then dispersed the lamina meridionally while small-scale, wave turbulent motions spread it slowly vertically.

Danielsen, E. F.↗
NASA NTRS1982

Preliminary test results of electrical charged particle generator for application to fog dispersal

A charged particle generator for use in fog dispersal applications was built and preliminary tests were carried out. The parameter used as a measure of performance was the current measured with a needle probe positioned in the charged jet connected to ground through an ammeter. The needle was movable and allowed the current profile throughout the jet to be determined. The measured current is referred to as the current output. The major independent parameters were liquid water injection rate, plenum pressure, and corona voltage. Optimum current output was achieved at the approximate pressure of 30 psig, corona voltage of 5600 volts, and liquid water injection rate of 6 cc/min. The results of the test with the prototype charged particle generator clearly demonstrate that a current on the order of 20 microamperes can be routinely achieved with the system. This measurement of current does not necessarily represent the total issuing from the nozzle current which is expected to be larger. From these results, confidence was established that a charged particle generator which will operate continuously and consistently can be designed, constructed, and operated. Further work is required, however, to better understand the physical mechanisms involved and to optimize the system for fog dispersal application.

Frost, W.↗
NASA NTRS1983

Test results of modified electrical charged particle generator for application to fog dispersal

Modifications to a charged particle generator for use in fog dispersal applications were made and additional testing carried out. The modified nozzle, however, did not work as planned, and reported results are the unmodified nozzle. The addition of a positive displacement pump to supply the liquid water was highly successful. Measurements of the generator output current were made with a cylindrical collector system as well as with the needle probe used in previous studies. Measurements with the cylindrical collector and the needle probe showed identical agreement within the variability of the experiment. A high-voltage prove was purchased, and measurements of the corona voltage as well as the voltage variation in the charged particle jet were made. Electric fields in the vertical direction on the order of 1,000,000 v/m were measured. The voltage distribution along the centerline of the jet was compared with the numerical solutions of the Poisson equation and showed very good agreement. Velocity measurements using a pitot tube were made. The resulting measurements were compared with theoretical and other reported experimental results. The measured data showed the appropriate trends and agreed well with reported results. Based on the measured current-to-mass ratio from the charged particle generator, a calculation of the average droplet size was made. Droplet sizes were estimated to range between 0.8 and 0.4 microns. Using measured data, an analysis of the height to which the droplet can be dispersed by the charged particle generator was made. Although the mathematical model is highly simplified, the results indicated that particles would achieve heights on the order of 80 m.

Frost, W.↗
NASA NTRS1982

A dispersion law for solar oscillations

The simple model of p-mode solar oscillations of Liebacher and Stein (1981), in which the acoustic vibrations are trapped in a resonant cavity taking the form of a spherical shell below the solar surface, is compared with Doppler shift observations of vertical velocities. The model is shown to predict a modified dispersion law in which the sound travel time across the cavity is a function of the ratio of the temporal frequency to the horizontal wavenumber, resulting in a single curve when the temporal frequency is plotted against the wavenumber. Frequencies derived from a two-dimensional power spectrum of velocity observations are found to conform to a modified version of the dispersion relation, and that only when the fundamental mode is excluded. Results thus suggest that all modes with the same frequency/wavenumber ratio are trapped in an identical cavity, or, more plausibly, that the difference in upper boundary conditions for different modes has minimal effect on the resulting frequencies.

Duvall, T. L., Jr.↗
NASA NTRS1983

Inverting a dispersive scene's side-scanned image

Consideration is given to the problem of using a remotely sensed, side-scanned image of a time-variant scene, which changes according to a dispersion relation, to estimate the structure at a given moment. Additive thermal noise is neglected in the models considered in the formal treatment. It is shown that the dispersion relation is normalized by the scanning velocity, as is the group scanning velocity component. An inversion operation is defined for noise-free images generated by SAR. The method is extended to the inversion of noisy imagery, and a formulation is defined for spectral density estimation. Finally, the methods for a radar system are used for the case of sonar.

Harger, R. O.↗
NASA NTRS1983

Evaluation of a stochastic model of particle dispersion in a turbulent round jet

A stochastic model of particle dispersion by turbulence, proposed by Gosman and Ioannides (1981), is evaluated. The method employs a k-epsilon model to estimate turbulence properties. Dispersion is determined by computing particle motion, with random sampling to obtain instantaneous flow properties for a statistically significant number of particle trajectories. The stochastic model yields good results particularly when eddy lifetimes are evaluated. The method allows the effects of large relative velocities between the particles and the flow, drag properties at Reynolds numbers greater than the Stokes flow regime, and the variations of local turbulence properties to be readily handled, at least for boundary layer flows.

Shuen, J.-S.↗
NASA NTRS1983

Stability of the surface layer and its relation to the dispersion of primary pollutants in St. Louis

The effects of atmospheric stability on the dispersion of primary pollutants such as CO, total hydrocarbons (THC), and NO were examined in St. Louis. The pollutant levels were measured at 25 stations, temperature at 12 stations at 5 and 30 m height, and wind speed and direction at the 30 m level at 12 stations. Correlation coefficients were generated for pairs of the vertical temperature differences, the log of the mean wind speed reciprocal, the bulk Richardson number, and specific pollutant concentrations. A high correlation was obtained between the thermal stability and the urban concentration of the primary pollutants in the lowest part of the boundary layer. A restricted nighttime dispersion of the pollutants was observed, indicating near-ground increased concentrations at times when the source emissions actually decrease.

Remsberg, E. E.↗
NASA NTRS1984

Relativistic dispersion and the generation of auroral kilometric radiation

Under the conditions at which auroral kilometric radiation (AKR) is known to be produced by a plasma sheet electron region, relativistic effects of the dispersion of extraordinary-mode waves near the electron cyclotron frequency are found to be very significant. The application of linear theory and computer simulation to a model distribution indicates that an extraordinary mode of a particular Reynolds number dominated the radiation emission due to the cyclotron maser instability, suggesting that hot electrons determine the propagation characteristics of AKR and that larger growth rates are possible for direct amplification of extraordinary waves in the auroral region than had been predicted by calculations employing cold plasma dispersion.

Pritchett, P. L.↗
NASA NTRS1983

Surface roughness effects with solid lubricants dispersed in mineral oils

The lubricating effectiveness of solid-lubricant dispersions are investigated in both point and line contacts using surfaces with both random and directional roughness characteristics. Friction and wear data obtained at relatively low speeds and at room temperature, indicate that the existence of solid lubricants such as graphite, MoS2, and PTFE in a plain mineral oil generally will not improve the effectiveness of the oil as a lubricant for such surfaces. Under boundary lubrication conditions, the friction force, as a function of time, initially depends upon the directional roughness properties of the contacting surfaces irrespective of whether the base oil or dispersions are used as lubricants.

Cusano, C.↗
NASA NTRS1985

Ion and electron energy dispersion features detected by ISEE 1

An energy dispersion feature observed by the ISEE 1 medium energy particle instrument is reported. This feature is characterized by enhancements in both electron and ion fluxes which occur in the 20-200 keV energy range and at radial distances from 5 to 13.5 earth radii. There is a strong energy dependence in the onset times of the enhancements. The time period between the first appearance of an enhancement at high energy to its eventual disappearance in the 20 keV channel can be as much as an hour. The duration of the enhancement in a given energy channel decreases with increasing energy. The species and pitch angle of particles in which the feature is most marked depend on local time. It is shown that the feature is a new manifestation of an old phenomenon, the drift dispersion that frequently produces signatures in particle data from geosynchronous satellites.

Greenspan, M. E.↗