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NASA NTRS1987

Gravitational lensing by an ensemble of isothermal galaxies

Calculation of 28,000 models of gravitational lensing of a distant quasar by an ensemble of randomly placed galaxies, each having a singular isothermal mass distribuiton, is reported. The average surface mass density was 0.2 of the critical value in all models. It is found that the surface mass density averaged over the area of the smallest circle that encompasses the multiple images is 0.82, only slightly smaller than expected from a simple analytical model of Turner et al. (1984). The probability of getting multiple images is also as large as expected analytically. Gravitational lensing is dominated by the matter in the beam; i.e., by the beam convergence. The cases where the multiple imaging is due to asymmetry in mass distribution (i.e., due to shear) are very rare. Therefore, the observed gravitational-lens candidates for which no lensing object has been detected between the images cannot be a result of asymmetric mass distribution outside the images, at least in a model with randomly distributed galaxies. A surprisingly large number of large separations between the multiple images is found: up to 25 percent of multiple images have their angular separation 2 to 4 times larger than expected in a simple analytical model.

Katz, Neal↗

NASA NTRS1987

Global features of the semiannual oscillation in stratospheric temperatures and comparison between seasons and hemispheres

Four years of satellite-derived microwave and infrared radiances are analyzed for the three-dimensional and seasonal variation of semiannual oscillations (SAO) in stratospheric temperatures, with particular focus on high latitudes, to investigate the effect of stratospheric warmings on SAO. Separate analyses of individual seasons in each hemisphere reveal that the strongest SAO in temperature occur in the Northern Hemisphere (NH) winter polar upper stratosphere. These results, together with the latitudinal structure of the temperature SAO and the fact that the NH polar SAO is nearly out of phase with the lower latitude SAO, are consistent with the existence of a global-scale, meridional circulation on the SAO time scale. The results suggest that polar stratospheric warmings are an important source of SAO in both high and low latitude stratospheric temperature fields. Interannual variations, three-dimensional phase structure, and zonal asymmetry of SAO are also detailed. The SH stratospheric SAO is dominated by a localized feature in the high-latitude, eastern hemisphere which tilts westward with height.

Gao, Xin-Hai↗

NASA NTRS1987

Luminal surface fabrication for cardiovascular prostheses

Methodology used for the production of highly ordered surfaces on polytetrafluoroethylene (PTFE) Teflon, for use of this material as a surface layer in small-diameter vascular prostheses, is described. Directed argon-ion beam sputter etching is used to produce highly ordered specific surface morphologies, and photolithographic techniques are used for pattern definition. The methodology also includes surface polishing of the samples, sputter deposition of the intermediary layer of gold to enhance photoresist adhesion and improve pattern resolution, and rotating of the sample to minimize the effects of ion-beam nonuniformity and asymmetry. Use of these techniques resulted in the formation of regular arrays of sharply defined hexagonal pillars with smooth vertical walls and large relief.

Deininger, William D.↗

NASA NTRS1987

Influence of Reynolds number on forebody side forces for 3.5-diameter tangent-ogive bodies

Vortex asymmetries can arise on aircraft and missile forebodies at high angles of attack and have proven difficult to simulate in the wind tunnel because of a large array of parameters that can influence the test results. One of the most important of these parameters is Reynolds number. The current report compares, for tangent-ogive forebodies with fineness ratios of 3.5, flow patterns from oil-flow photographs to features in pressure distributions. The pressure data base is then used to illustrate the development of the sectional side force coefficient along both the tangent-ogive nose and an attached cylindrical afterbody for Reynolds numbers of 0.2, 0.8 and 3.8 million, based on maximum diameter, and angles of attack of 40, 50 and 60 deg. General commments concerning the effects of Reynolds number are presented, as well as an analysis of the effects of Reynolds number on the sectional side force coefficient near and at the apex of the pointed body. All data presented are for freestream Mach numbers on the order of 0.25.

Hall, Robert M.↗

NASA NTRS1987

Model-following output feedback controller for oblique-wing aircraft

A variable-skew oblique-wing offers a substantial aerodynamic performance advantage for aircraft missions that require both high efficiency in subsonic flight and supersonic dash or cruise. The most obvious characteristics of the oblique-wing concept is the asymmetry of the wing-skew angle that causes a significant aerodynamic and inertial cross coupling between the aircraft longitudinal and lateral-directional axes. This paper presents a technique for synthesizing a decoupling controller while providing the desired stability augmentation. The use of output feedback in control law synthesis without the requirement of state estimation is presented. An explicit model-following control system with output feedback is developed. The effectiveness of the control laws developed in achieving the desired decoupling is illustrated for a given flight condition for an oblique-wing aircraft.

Alag, Gurbux Singh↗

NASA NTRS1987

Determination of the scaled optical thickness of clouds from reflected solar radiation measurements

A method for determination of scaled optical thickness from reflected solar radiation measurements is described in which measurements of the reflected function are compared with asymptotic expressions for the reflection function of optically thick layers. Analytic formulas are derived which show the dependence of the reflection and transmission functions of nonabsorbing atmospheres on cloud optical thickness, ground albedo, and the asymmetry factor. An expression is derived which shows that the ground albedo produces a constant bias in the derived optical thickness, regardless of the value of the measured reflection function. Results from an analysis of high-resolution images, obtained in Oklahoma, of the reflection function of clouds shows the impact of details of the single scattering phase function on the derived optical thickness.

King, Michael D.↗

NASA NTRS1987

Scattering parameters for aspherical hydrometeors at microwave frequencies

Scattering parameters for ensembles of aspherical ice and liquid hydrometeors were calculated at Psi = 50 deg for frequencies of 37.0, 50.3, and 85.6 GHz. Hydrometeors were assumed to be oblate spheroids whose semiminor axes were aligned at Psi = 0 deg. The angle of 50 deg is determined by the viewing angle of recent satellite-borne radiometers. Backscattering phase functions and extinction coefficients at Psi = 0 deg were also computed at 18.0, 37.0, and 85.6 GHz to provide information for future nadir scanning radars. The hydrometeors were assumed to be characterized by Marshall-Palmer (1948) size distributions. The extended boundary condition method was used to calculate the extinction and backscattering coefficients as well as the albedo for single scattering and the asymmetry factor of the phase function. Results are fitted to simple functions of the rainfall rate. Some comparisons intended to illustrate the differences and similarities of these results with those obtained from equal volume spheres are also presented.

Kummerow, C.↗

NASA NTRS1988

Stabilizing Silicon-Ribbon Growth At Early Stages

Device mechanically stabilizes buttons from which silicon ribbons grown by dendritic-web process. Eliminates tendency for button to tip and interrupt pull because of asymmetry. Silicon seed hangs from holder through stabilizing tube. While button growing, tube elevated above melt surface. When ready for pulling of ribbon, tube comes to rest on button and prevents tipping. Large button used. Even if button assumes highly-asymmetrical weight distrubtion in early stages of growth, little danger of tipping and breakage of nascent ribbon.

Henry, Paul K.↗

NASA NTRS1985

Constitutive modeling for single crystal superalloys

A crystallographic approach to constitutive modeling of single crystal superalloys is discussed. The approach is based on identifying the active slip planes and slip directions. The shear stresses are computed on each of the slip planes from applied stress components. The slip rate is then computed on each slip system and the microscopic inelastic strain rates are the sum of the slip in the individual slip systems. The constitutive model was implemented in a finite element code using twenty noted isoparametric solid elements. Constants were determined for octahedral and cube slip systems. These constants were then used to predict tension-compression asymmetry and fatigue loops. Other data was used to model the tensile and creep response.

Stouffer, Donald C.↗

NASA NTRS1987

Phase purity of NiCo2O4, a catalyst candidate for electrolysis of water

NiCo2O4 is shown to be difficult to obtain as a pure phase, and may never have been so obtained. High resolution x-ray diffractometry is required for its precise characterization. Film XRD is not likely to show the asymmetry in the spinel diffraction lines, caused by poorly crystallized NiO, as seen in diffractometer traces. The Co3O4 which is expected to accompany NiO as an impurity in NiCo2O4 syntheses has the same diffraction pattern as the binary oxide. Firings of the co-precipitated hydroxides at 300, 350, and 400 C, including one in pure O2, failed to produce single phase cobaltate. Scanning electron microscopy showed all the sintered products to range over several orders of magnitude in agglomerate/particle size. Surface areas by BET were all in the range 40 to 110 m sq/g, equivalent to particles of 200 to 100 Angstrom diameter. The spinel diffraction line breadths were compatible with those approximate dimensions.

Singer, J.↗

NASA NTRS1987

Solar Dynamics and G-modes

Various properties of r-modes that would occupy the solar convection zone were computed. Values were calculated for the energy loss from the coefficient of eddy viscosity using the solar convective zone model. Viscous damping is strong and removes one-third or more of the mode energy each oscillation period. A fast Fourier transform (FFT) of daily Zurich sunspot number was made for eight sets of data, taken near peaks of solar cycles 14 to 21. The FFTs were evaluated with the Wolff and Hickey model. The horizontal component of F sub h of the force exerted by a planetary torque was calculated to see is it works on the photospheric layers at a rate comparable to 0.000001 solar luminosity. The time span of torque in the external sector of the Sun was calculated. The duration of the effect of two planets in conjunction on sunspot numbers was examined. The north-south asymmetry of solar activity was investigated.

Blizard, Jane B.↗

NASA NTRS1988

A modeling analysis program for the JPL table mountain Io sodium cloud data

Research in the third and final year of this project is divided into three main areas: (1) completion of data processing and calibration for 34 of the 1981 Region B/C images, selected from the massive JPL sodium cloud data set; (2) identification and examination of the basic features and observed changes in the morphological characteristics of the sodium cloud images; and (3) successful physical interpretation of these basic features and observed changes using the highly developed numerical sodium cloud model at AER. The modeling analysis has led to a number of definite conclusions regarding the local structure of Io's atmosphere, the gas escape mechanism at Io, and the presence of an east-west electric field and a System III longitudinal asymmetry in the plasma torus. Large scale stability, as well as some smaller scale time variability for both the sodium cloud and the structure of the plasma torus over a several year time period are also discussed.

Smyth, William H.↗

NASA NTRS1986

An experimental investigation of the flap-lag-torsion aeroelastic stability of a small-scale hingeless helicopter rotor in hover

A small scale, 1.92 m diam, torsionally soft, hingeless helicopter rotor was investigated in hover to determine isolated rotor stability characteristics. The two-bladed, untwisted rotor was tested on a rigid test stand at tip speeds up to 101 m/sec. The rotor mode of interest is the lightly damped lead-lag mode. The dimensionless lead-lag frequency of the mode is approximately 1.5 at the highest tip speed. The hub was designed to allow variation in precone, blade droop, pitch control stiffness, and blade pitch angle. Measurements of modal frequency and damping were obtained for several combinations of these hub parameters at several values of rotor speed. Steady blade bending moments were also measured. The lead-lag damping measurements were found to agree well with theoretical predictions for low values of blade pitch angle. The test data confirmed the predicted effects of precone, droop, and pitch control stiffness parameters on lead-lag damping. The correlation between theory and experiment was found to be poor for the mid-to-high range of pitch angles where the theory substantially overpredicted the experimental lead-lag damping. The poor correlation in the mid-to-high blade pitch angle range is attributed to low Reynolds number nonlinear aerodynamics effects not included in the theory. The experimental results also revealed an asymmetry in lead-lag damping between positive and negative thrust conditions.

Sharpe, David L.↗

NASA NTRS1988

A simplified orthotropic formulation of the viscoplasticity theory based on overstress

An orthotropic, small strain viscoplasticity theory based on overstress is presented. In each preferred direction the stress is composed of time (rate) independent (or plastic) and viscous (or rate dependent) contributions. Tension-compression asymmetry can depend on direction and is included in the model. Upon a proper choice of a material constant one preferred direction can exhibit linear elastic response while the other two deform in a viscoplastic manner.

Sutcu, M.↗

NASA NTRS1987

Conditionally-averaged structures in wall-bounded turbulent flows

The quadrant-splitting and the wall-shear detection techniques were used to obtain ensemble-averaged wall layer structures. The two techniques give similar results for Q4 events, but the wall-shear method leads to smearing of Q2 events. Events were found to maintain their identity for very long times. The ensemble-averaged structures scale with outer variables. Turbulence producing events were associated with one dominant vortical structure rather than a pair of counter-rotating structures. An asymmetry-preserving averaging scheme was devised that allowed a picture of the average structure which more closely resembles the instantaneous one, to be obtained.

Guezennec, Yann G.↗

NASA NTRS1987

Solid rocket motor aft field joint flow field analysis

An efficient Navier-Stokes analysis was successfully applied to simulate the complex flow field in the vicinity of a slot in a solid rocket motor with segment joints. The capability of the computer code to resolve the flow near solid surfaces without using a wall function assumption was demonstrated. In view of the complex nature of the flow field in the vicinity of the slot, this approach is considered essential. The results obtained from these calculations provide valuable design information, which would otherwise be extremely difficult to obtain. The results of the axisymmetric calculations indicate the presence of a region of reversed axial flow at the aft-edge of the slot and show the over-pressure in the slot to be only about 10 psi. The results of the asymmetric calculations indicate that a pressure asymmetry more than two diameters downstream of the slot has no noticeable effect on the flow field in the slot. They also indicate that the circumferential pressure differential caused in the slot due to failure of a 15 deg section of the castable inhibitor will be approximately 1 psi.

Sabnis, Jayant S.↗

NASA NTRS1986

The Io sulfur torus in 1981

A Fabry-Perot spectrometer was used to obtain images of the Io torus in emission lines of S II (wavelength 6716 and 6731) and S III (wavelength 9531) in February and March 1981, on the 2.1 meter telescope at KPNO. The S II and S III images showed a large variation in brightness and radial extent. There is an indication the S II and S III emissions in the warm torus are correlated. The S II and S III emissions in the warm torus also have similar scale heights along the magnetic field lines of approximately 0.6 to 0.72 R sub J. The east-west asymmetry in the S II images taken at similar magnetic longitudes, but 2.5 Jovian rotations apart, supports the theory of convective motions suggested by others. In addition to the images, simultaneous measurements of the S II (6731 wavelength) line profile were also made on one night using a Fabry-Perot scanning spectrometer on the 4 meter at KPNO. The S II spectral scans implied ion temperatures of 52 (+ or - 10) x 10 to the 3rd at 5.2 to 5.6 R sub J from Jupiter and a minimum temperature of at least 3 x 10 to the 5th K at 6 R sub J from Jupiter.

Oliversen, Ronald J.↗

NASA NTRS1988

Anisotropic constitutive modeling for nickel-base single crystal superalloys

An anisotropic constitutive model was developed based on crystallographic slip theory for nickel base single crystal superalloys. The constitutive equations developed utilizes drag stress and back stress state variables to model the local inelastic flow. Specially designed experiments were conducted to evaluate the existence of back stress in single crystal superalloy Rene N4 at 982 C. The results suggest that: (1) the back stress is orientation dependent; and (2) the back stress state variable is required for the current model to predict material anelastic recovery behavior. The model was evaluated for its predictive capability on single crystal material behavior including orientation dependent stress-strain response, tension/compression asymmetry, strain rate sensitivity, anelastic recovery behavior, cyclic hardening and softening, stress relaxation, creep and associated crystal lattice rotation. Limitation and future development needs are discussed.

Sheh, Michael Y.↗