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At least 451 records · Page 25

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NASA NTRS1977

The ratio of deuterium to hydrogen in interstellar space. III - The lines of sight to Zeta Puppis and Gamma Cassiopeiae

An extensive set of measurements of the ratio of deuterium to hydrogen in the interstellar medium has been undertaken. The general observing program is described along with the data reduction techniques used to separate the complex deuterium and hydrogen profiles, and the results of the measurements for two stars, Zeta Pup and Gamma Cas. For Gamma Cas, log N(D)/N(H) is found to be approximately -4.8, in agreement with previous measurements. For Zeta Pup, a complicated structure of absorbing clouds in the interstellar medium is inferred. The best value of log N(D)/N(H) is about -4.6 for each of two components. One component is probably more complex than can be demonstrated directly at the resolution employed, and log N(D)N(H) could be -4.0 or larger in this component.

Vidal-Madjar, A.↗

NASA NTRS1978

Size-frequency distributions of primary and secondary lunar impact craters

The graphs of diameter vs frequency plotted in the present study for primary impact craters produced in four time intervals exhibit an appreciable change in form with age. A deficiency of small craters relative to an extrapolation from the large-diameter parts of the curves is greatest and extends to the largest sizes in the oldest population (pre-Nectarian), whose curve approximates a log-normal form. Successively younger populations (Nectarian, Imbrian, Copernican plus Eratosthenian) have successively more small craters relative to larger ones and more nearly log-log distributions. Many of the craters thought to be small old primaries are here identified as secondary craters of basins, on the basis of previously developed criteria.

Wilhelms, D. E.↗

NASA NTRS1982

CNO abundances in H II regions of the Magellanic clouds and the galaxy with implications regarding the nucleosynthesis of the CNO element group

Final abundance results of IUE observations of the UV spectra of three H II regions in the Small Magellanic Cloud and four H II regions in the Large Magellanic Cloud are presented. Calculated yields of carbon and oxygen derived are y(C)=.00063 and y(O)=.0016. The nucleosynthetic origin of nitrogen was evaluated as being predominantly a secondary element produced from carbon as its seed. Plotting log N/C versus log C/H yielded the rather unexpected result that log N/C decreases with lo C/H over the SMC-LMC-Orion range. The cause of this relationship is discussed.

Dufour, R. J.↗

NASA NTRS1979

Study of fatigue durability of advanced composite materials under conditions of accelerated loading

The effect of temperature on the tension-tension fatigue life of the T300/5208 graphite/epoxy angle-ply laminate system was investigated in an effort to develop an acceptable and reliable method of accelerated loading. Typical S log sub 10 N curves were determined experimentally at 25 C, 75 C, and 115 C. The time-temperature superposition principle was employed to find the shift factors of uniaxial fatigue strength, and a general linear equation of S log sub 10 N for shifting purpose was established. The combined techniques of cyclic creep measurements and optical microscopy upon fatigue failure allow some assessment of the possible physical basis of S log 10 N curve shifting. Before fatigue, the laminates at all test temperatures and stress levels undergo a unique damage mechanism during fatigue loading. It is concluded that an accelerated loading method is feasible.

Shih, H. M.↗

NASA NTRS1982

An experimental search for near-wall boundary conditions for large eddy simulation

Instantaneous wall shear stress and streamwise velocities have been measured simultaneously in a flat plate, turbulent boundary layer at moderate Reynolds number in an effort to provide experimental support for large eddy simulations. Data were obtained by using a buried-wire wall shear gage and a hot-wire rake positioned in the log region of the flow. All data processing was accomplished with digital data analysis techniques on a minicomputer. Fluctuations of the instantaneous U plus versus Y plus profiles about a mean law of the wall are shown to be significant and complex. Peak cross-correlation values between wall shear stress and the velocities are high and reflect the passage of a large structure inclined at a small angle to the wall. Estimates of this angle are consistent with those made by other investigators. Conditional sampling techniques were used to detect the passage of various sizes and types of flow disturbances (events) and to estimate their mean frequency of occurrence. Events characterized by large and sudden streamwise accelerations were found to be highly coherent throughout the log region and were strongly correlated with large fluctuations in wall shear-stress. Phase randomness between the near-wall quantities and the outer velocities was small. The results suggest that the flow events detected by conditional sampling applied to velocities in the log region may be related to the bursting process.

Robinson, S. K.↗

NASA NTRS1983

Minimizing inner product data dependencies in conjugate gradient iteration

The amount of concurrency available in conjugate gradient iteration is limited by the summations required in the inner product computations. The inner product of two vectors of length N requires time c log(N), if N or more processors are available. This paper describes an algebraic restructuring of the conjugate gradient algorithm which minimizes data dependencies due to inner product calculations. After an initial start up, the new algorithm can perform a conjugate gradient iteration in time c*log(log(N)).

Vanrosendale, J.↗

NASA NTRS1982

High resolution EUV structure of the chromosphere-corona transition region above a sunspot

Rocket observations are reported for a sunspot at high spatial and spectral resolution in the extreme ultraviolet region (EUV) between 1170 and 1715 A. The instrument and data reduction are described, and the appearance of the sunspot features in EUV emission lines is presented together with a correlation of the nonthermal velocity field with the Doppler velocity field. For sunspots the shape of the differential emission measure curve shows that the minimum is shifted to lower temperatures by over a factor of two, while the slope on a log-log scale near log of effective temperature equal to 5.3 is increased by a factor of two. This means that the energy balance is dominated by radiative losses from the large amount of transition region plasma, and an upper limit of a million K exists on the extrapolated temperature above the umbra. Static energy balance models of the umbra show that the observed enthalpy flux can balance radiative losses above 30,000 K for a fill factor greater than 0.1. An umbral model is compared with the chromospheric sunspot model of Kneer and Mattig (1978).

Nicolas, K. R.↗

NASA NTRS1986

Instantaneous velocity profile measurements in a turbulent boundary layer

Instantaneous wall shear stress and streamwise velocities have been measured simultaneously in a flat-plate, turbulent boundary layer at moderate Reynolds number in an effort to provide experimental support for large eddy simulations. Data were obtained using a buried-wire, wall shear gage and a hot-wire rake positioned in the log region of the flow. Fluctuations of the instantaneous U(+) versus Y(+) profiles about a mean law of the wall are shown to be significant and complex. Peak cross-correlation values between wall shear stress and the velocities are high, and reflect the passage of a large structure inclined at a small angle to the wall. Estimates of this angle are consistent with those made by other investigators. Conditional sampling techniques were used to detect the passage of various sizes and types of flow disturbances (events), and to estimate their mean frequency of occurrence. Events characterized by large aand sudden streamwise accelerations were found to be highly coherent throughout the log region and were strongly correlated with large fluctuations in wall shear stress. Phase randomness between the near-wall quantities and the outer velocities was small. The results suggest that the flow events detected by conditional sampling applied to velocities in the log region may be related to the bursting process.

Robinson, S. K.↗

NASA NTRS1987

Evaluation of AIS-2 (1986) data over hydrothermally altered granitoid rocks of the Singatse Range (Yerington) Nevada and comparison with 1985 AIS-1 data

The Airborne Imaging Spectrometer-2 (AIS-2) flights along 2 subparallel lines (bearing 013) were designed to traverse 3 major rock assemblages - the Triassic sedimentary sequence; the granitoid rocks of the Yerington batholith and the Tertiary ignimbritic ash flow and ash fall tuffs. The first 2 sites are hydrothermally altered to a quartz-sericite-tourmaline mineralogy. The first AIS-2 data set showed numerous line dropouts and a considerable number of randomly distributed dark pixels. A second decommutation reduced the dropout essentially to near zero and the dark pixels by about 75 percent. Vertical striping was removed by histogram matching, column by column. A log residual spectrum was calculated which showed the departure of a 2 x 2 pixel area from the spatially and spectrally averaged scene. A 1:1 correlation was found with the log residual AIS-2 data and a large open pit area of gypsum. An area with known sericite agreed with the overflight data, and an area known to be free of any significant amount of O-H bearing materials showed no evidence of any in the AIS-2 log residuals.

Lyon, R. J. P.↗

NASA NTRS1988

Parallel matrix multiplication on the Connection Machine

Matrix multiplication is a computation and communication intensive problem. Six parallel algorithms for matrix multiplication on the Connection Machine are presented and compared with respect to their performance and processor usage. For n by n matrices, the algorithms have theoretical running times of O(n to the 2nd power log n), O(n log n), O(n), and O(log n), and require n, n to the 2nd power, n to the 2nd power, and n to the 3rd power processors, respectively. With careful attention to communication patterns, the theoretically predicted runtimes can indeed be achieved in practice. The parallel algorithms illustrate the tradeoffs between performance, communication cost, and processor usage.

Tichy, Walter F.↗

NASA NTRS1988

The development of an Ada programming support environment database: SEAD (Software Engineering and Ada Database), user's manual

This is a manual for users of the Software Engineering and Ada Database (SEAD). SEAD was developed to provide an information resource to NASA and NASA contractors with respect to Ada-based resources and activities that are available or underway either in NASA or elsewhere in the worldwide Ada community. The sharing of such information will reduce the duplication of effort while improving quality in the development of future software systems. The manual describes the organization of the data in SEAD, the user interface from logging in to logging out, and concludes with a ten chapter tutorial on how to use the information in SEAD. Two appendices provide quick reference for logging into SEAD and using the keyboard of an IBM 3270 or VT100 computer terminal.

Liaw, Morris↗

NASA NTRS1989

Non-LTE analysis of extremely helium-rich stars. The hot sdO stars LSE 153, 259 and 263

Results of a non-LTE fine analysis based mainly on high-resolution CASPEC spectra for three extremely helium-rich sdO stars are discussed in order to explain hydrogen deficiency in single stars. High temperature (Teff = 70,000 to 75,000 K) and a position in the log Teff - log g diagram were found close to the Eddington limit. Various abundance estimates are derived for hydrogen (upper limits only), carbon, nitrogen, and magnesium. Hydrogen is reduced to less than 10 percent by number in LSE 153 and LSE 263, and to less than 5 percent in LSE 259. The hydrogen deficiency is accompanied by nitrogen- and carbon-enrichment in LSE 153 and LSE 259 only. In LSE 263, carbon is depleted by about 1 dex. Stellar masses obtained by assuming that a core mass-luminosity relation holds for these stars, were found to be in the range 0.6-0.9 solar mass, yielding luminosities log L/L:solar = 3.7-4.5. Two of the program stars (LSE 153 and 259) appear to be possible successors of the R CrB and helium B stars, whereas the third star (LSE 263) displays a much lower carbon content in its photosphere making it an exceptional case among the known hydrogen deficient stars.

Husfeld, D.↗

NASA NTRS1990

Polar exponential sensor arrays unify iconic and Hough space representation

The log-polar coordinate system, inherent in both polar exponential sensor arrays and log-polar remapped video imagery, is identical to the coordinate system of its corresponding Hough transform parameter space. The resulting unification of iconic and Hough domains simplifies computation for line recognition and eliminates the slope quantization problems inherent in the classical Cartesian Hough transform. The geometric organization of the algorithm is more amenable to massively parallel architectures than that of the Cartesian version. The neural architecture of the human visual cortex meets the geometric requirements to execute 'in-place' log-Hough algorithms of the kind described here.

Weiman, Carl F. R.↗

NASA NTRS1991

Massively parallel algorithms for trace-driven cache simulations

Trace driven cache simulation is central to computer design. A trace is a very long sequence of reference lines from main memory. At the t(exp th) instant, reference x sub t is hashed into a set of cache locations, the contents of which are then compared with x sub t. If at the t sup th instant x sub t is not present in the cache, then it is said to be a miss, and is loaded into the cache set, possibly forcing the replacement of some other memory line, and making x sub t present for the (t+1) sup st instant. The problem of parallel simulation of a subtrace of N references directed to a C line cache set is considered, with the aim of determining which references are misses and related statistics. A simulation method is presented for the Least Recently Used (LRU) policy, which regradless of the set size C runs in time O(log N) using N processors on the exclusive read, exclusive write (EREW) parallel model. A simpler LRU simulation algorithm is given that runs in O(C log N) time using N/log N processors. Timings are presented of the second algorithm's implementation on the MasPar MP-1, a machine with 16384 processors. A broad class of reference based line replacement policies are considered, which includes LRU as well as the Least Frequently Used and Random replacement policies. A simulation method is presented for any such policy that on any trace of length N directed to a C line set runs in the O(C log N) time with high probability using N processors on the EREW model. The algorithms are simple, have very little space overhead, and are well suited for SIMD implementation.

Nicol, David M.↗

NASA NTRS1992

Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. A direct theoretical connection is made between homogeneous shear flow in equilibrium and the log-layer of fully-developed turbulent channel flow. It is shown that if a second-order closure model is calibrated to yield good equilibrium values for homogeneous shear flow it will also yield good results for the log-layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second-order closure models introduce an ad hoc wall reflection term in order to mask deficient predictions for the log-layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling.

Abid, R.↗

NASA NTRS1992

A comparison of techniques for extracting emissivity information from thermal infrared data for geologic studies

This article evaluates three techniques developed to extract emissivity information from multispectral thermal infrared data. The techniques are the assumed Channel 6 emittance model, thermal log residuals, and alpha residuals. These techniques were applied to calibrated, atmospherically corrected thermal infrared multispectral scanner (TIMS) data acquired over Cuprite, Nevada in September 1990. Results indicate that the two new techniques (thermal log residuals and alpha residuals) provide two distinct advantages over the assumed Channel 6 emittance model. First, they permit emissivity information to be derived from all six TIMS channels. The assumed Channel 6 emittance model only permits emissivity values to be derived from five of the six TIMS channels. Second, both techniques are less susceptible to noise than the assumed Channel 6 emittance model. The disadvantage of both techniques is that laboratory data must be converted to thermal log residuals or alpha residuals to facilitate comparison with similarly processed image data. An additional advantage of the alpha residual technique is that the processed data are scene-independent unlike those obtained with the other techniques.

Hook, Simon J.↗

NASA NTRS1993

Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

Abid, R.↗

NASA NTRS1993

The solubility of gold in silicate melts: First results

The effects of oxygen fugacity and temperature on the solubility of Au in silicate melts were determined. Pd-Au alloys were equilibrated with silicate of anorthite-diopside eutectic composition at different T-fO2 conditions. The behavior of Au was found to be similar to that of Pd reported recently. Au solubilities for alloys with 30 to 40 at. percent Au decrease at 1400 C from 12 ppm in air to 160 ppb at a log fO2 = -8.7. The slope of the log(Me-solubility) vs. log(fO2) curve is close to 1/4 for Au and the simultaneously determined Pd suggesting a formal valence of Au and Pd of 1+. Near the IW buffer Pd and Au solubilities become even less dependent on fO2 perhaps reflecting the presence of some metallic Au and Pd.

Borisov, A.↗