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At least 379 records · Page 21

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DOE OSTI2020

Dissolution of a cylindrical disk in Hele-Shaw flow: a conformal-mapping approach

Here we apply conformal mapping to find the evolving shapes of a dissolving cylinder in a potential flow. Similar equations can be used to describe melting in a flowing liquid phase. Results are compared with microfluidic experiments and numerical simulations. Shapes predicted by conformal mapping agree almost perfectly with experimental observations, after a modest (20 %) rescaling of the time. Finite-volume simulations show that the differences with experiment are connected to the underlying assumptions of the analytical model: potential flow and diffusion-limited dissolution. Approximate solutions of the equations describing the evolution of the shape of the undissolved solid can be derived from a Laurent expansion of the mapping function from the unit circle. Asymptotic expressions for the evolution of the area of the disk and the shift in its centre of mass have been derived at low and high Péclet number. Analytic approximations to the leading-order Laurent coefficients provide additional insight into the mechanisms underlying pore-scale dissolution.

58 GEOSCIENCES↗

NASA NTRS1975

Study of growth of single crystal ribbon in space

The technical feasibility is studied of growing single-crystal silicon ribbon in the space environment. Procedures are described for calculating the electromagnetic fields produced in a silicon ribbon by an rf shaping coil. The forces on the ribbon and the degree of shaping to be expected are determined. The expected steady-state temperature distribution in the ribbon is calculated in the one-dimensional approximation. Calculations on simplified models indicate, that lack of flatness of the shaped ribbon and excessive heating of the melt by the eddy currents induced by the shaping fields may pose problems. An analysis of the relative effects of various kinds of forces other than electromagnetic showed that in the space environment capillarity forces would dominate, and that the shape of the melt is thus principally determined by the shape of any solids with which it comes in contact. This suggests that ribbon may be produced simply by drawing between parallel wires. A concept is developed for a process of off-angle growth, in which the ribbon is pulled at an angle to the solidification front. Such a process promises to offer increased growth rate, better homogeneity, and thinner ribbon.

Wood, V. E.↗

NASA NTRS2003

Light Scattering by Gaussian Particles: A Solution with Finite-Difference Time Domain Technique

The understanding of single-scattering properties of complex ice crystals has significance in atmospheric radiative transfer and remote-sensing applications. In this work, light scattering by irregularly shaped Gaussian ice crystals is studied with the finite-difference time-domain (FDTD) technique. For given sample particle shapes and size parameters in the resonance region, the scattering phase matrices and asymmetry factors are calculated. It is found that the deformation of the particle surface can significantly smooth the scattering phase functions and slightly reduce the asymmetry factors. The polarization properties of irregular ice crystals are also significantly different from those of spherical cloud particles. These FDTD results could provide a reference for approximate light-scattering models developed for irregular particle shapes and can have potential applications in developing a much simpler practical light scattering model for ice clouds angular-distribution models and for remote sensing of ice clouds and aerosols using polarized light. (copyright) 2003 Elsevier Science Ltd. All rights reserved.

Sun, W.↗

NASA NTRS1971

Observations of low-energy /0.3- to 1.8-MeV/ differential spectrums of trapped protons.

Measurements of differential energy spectrums of trapped protons obtained from several passes during the period January to November 1969 using the polar orbiting, low-altitude Injun 5 satellite equipped with a special solid-state detector proton-electron telescope are presented. Results reveal the existence of a quasi-persistent peak in the differential energy spectrum in the McIlwain shell parameter (L) range 2 to 2.6 and in the energy range of approximately 0.32 to 0.72 MeV. The fact that the shape of the spectrum is stable for several days or can change with time scales as small as 4 hours suggests an impulsive acceleration mechanism deep in the radiation belts. Other features of the spectrum show that if the spectrum is represented by an exponential form in energy, the dependence of the spectral parameter is in general agreement with diffusion theory over the L range of approximately 2 to 4.

Venkatesan, D.↗

NASA NTRS1977

A generalized orthogonal coordinate system for describing families of axisymmetric and two-dimensional bodies

A generalized curvilinear orthogonal coordinate system is presented which can be used for approximating various axisymmetric and two-dimensional body shapes of interest to aerodynamicists. Such body shapes include spheres, ellipses, spherically capped cones, flat-faced cylinders with rounded corners, circular disks, and planetary probe vehicles. A set of transformation equations is also developed whereby a uniform velocity field approaching a body at any angle of attack can be resolved in the transformed coordinate system. The Navier-Stokes equations are written in terms of a generalized orthogonal coordinate system to show the resultant complexity of the governing equations.

Gnoffo, P. A.↗

NASA NTRS1976

Application of finite element approach to transonic flow problems

A variational finite element model for transonic small disturbance calculations is described. Different strategy is adopted in subsonic and supersonic regions. Blending elements are introduced between different regions. In the supersonic region, no upstream effect is allowed. If rectangular elements with linear shape functions are used, the model is similar to Murman's finite difference operators. Higher order shape functions, non-rectangular elements and discontinuous approximation of shock waves are also discussed.

Hafez, M. M.↗

NASA NTRS1978

Geometric model and analysis of rod-like large space structures

The application of geometrical schemes to large sphere antenna reflectors was investigated. The purpose of these studies is to determine the shape and size of flat segmented surfaces which approximate general shells of revolution and in particular spherical and paraboloidal reflective surfaces. The extensive mathematical and computational geometry analyses of the reflector resulted in the development of a general purpose computer program. This program is capable of generating the complete design parameters of the dish and can meet stringent accuracy requirements. The computer program also includes a graphical self contained subroutine which graphically displays the required design.

Nayfeh, A. H.↗

NASA NTRS1978

Venus - Detection of variations in stratospheric carbon monoxide

The reported observations of the J = 0-1 rotational transition (115271.2 MHz) of carbon monoxide in the Venusian atmosphere indicate that the distribution of stratospheric CO is variable. Observations were obtained with a 11-m antenna on several days in February 1977, when Venus was near maximum eastern elongation (k approximately 0.4), and in April 1977 during inferior conjunction (k approximately 0.01). Differences in the line intensities and shapes for the two data sets are described. The results are interpreted to suggest that on the nightside of Venus there is relatively more CO at higher altitudes and relatively less CO at lower altitudes than on the dayside of the planet.

Gulkis, S.↗

NASA NTRS1980

Calculations of the variability of ice cloud radiative properties at selected solar wavelengths

This study shows that there is surprising little difference in values of reflectance, absorptance, and transmittance for many of the intermediate-size particle spectra. Particle size distributions with mode radii ranging from approximately 50 to 300 microns, irrespective of particle shape and nearly independent of the choice of size distribution representation, give relatively similar flux values. The very small particle sizes, however, have significantly larger values of reflectance and transmittance with corresponding smaller values of absorptance than do the larger particle sizes. The very large particle modes produce very small values of reflectance and transmittance along with very large values of absorptance. Such variations are particularly noticeable when plotted as a function of wavelength.

Welch, R. M.↗

NASA NTRS1982

Environmental effects on galaxies in clusters

Influences from the cluster environment build up steadily over long times so that the internal dynamics of a galaxy in a cluster change appreciably over a cluster crossing time. The galaxy can no longer be considered as an isolated object. Slow rotation of elliptical galaxies and observations on galaxy alignment within clusters of galaxies provide two independent pieces of observational evidence for cluster influence. A first cut at studying cluster influences on the internal dynamics of galaxies is presented by means of numerical experiments started from a rotating barlike galaxy placed in an external force field like that which a galaxy sees in a cluster. Both pattern motion and observable rotation are tidally braked by the cluster force field. The experiments verify that the braking rate scales inversely as the square of the cluster crossing time. Systematic internal motions are reduced so that observational V/sigma is less than 0.2, in agreement with observations on elliptical galaxies but sharply reduced from V/sigma of approximately 1.0 for the original model. The final shapes of tidally braked galaxies need not be spherical.

Miller, R. H.↗

NASA NTRS1982

Wide range weight functions for the strip with a single edge crack

A closed form expression for the weight function for a strip with a single edge crack is presented. The expression is valid for relative crack lengths from zero to unity. It is based on the assumption that the shape of an opened edge crack can be approximated by a conic section. The results agree well with published values for weight functions, stress intensity factors, and crack mouth opening displacements.

Orange, T. W.↗

NASA NTRS1984

Stratospheric HF mixing ratio profiles in the northern and southern hemispheres

Spectra obtained by balloon-borne Michelson interferometers over South Australia in 1977 and New Mexico in 1978 have been analyzed to obtain HF mixing ratios using a recently developed, sensitive analysis method for Fourier transform spectroscopy. The stratospheric HF mixing ratio appears to be significantly larger in the northern hemisphere than in the southern hemisphere for similar respective latitudes with the profiles having a similar vertical shape: the HF mixing ratio increases from 20 to 25 km, stays approximately constant between 25 and 35 km, and increases upward from 35 km. A brief discussion is given of possible causes for HF profile shape.

Park, J. H.↗

NASA NTRS1985

Stratospheric ClO - In-situ detection with a new approach

Stratospheric chlorine monoxide concentrations were measured in situ using a new and improved observational method in which the detection package is lowered on a tether from a balloon-borne platform floating near 40 km altitude and then reeled back up. Two reel down and reel up sequences covering 38 to 31 km produced (ClO) profiles which agree with each other to + or - 12 percent. The measured (ClO) profile falls within the envelope of nine previous fast parachute drop results, but exhibits a different shape, peaking near 32 km as opposed to approximately 39 km for the average of the previous profiles. These results are lower than but consistent with the profiles derived from remote sensing methods, and indicate the need for simultaneous measurements of relevant species.

Brune, Wm. H.↗

NASA NTRS1985

Wide-range weight functions for the strip with a single edge crack

Orange, T. W.↗

NASA NTRS1989

A preliminary design of interior structure and foundation of an inflatable lunar habitat

A preliminary structural design and analysis of an inflatable habitat for installation on the moon was completed. The concept takes the shape of a sphere with a diameter of approximately 16 meters. The interior framing provides five floor levels and is enclosed by a spherical air-tight membrane holding an interior pressure of 14.7 psi (101.4kpa). The spherical habitat is to be erected on the lunar surface with the lower one third below grade and the upper two thirds covered with a layer of lunar regolith for thermal insulation and shielding against radiation and meteoroids. The total dead weight (earth weight) of the structural aluminum, which is of vital interest for the costly space transportation, is presented. This structural dead weight represents a preliminary estimate without including structural details. The design results in two versions: one supports the weight of the radiation shielding in case of deflation of the fabric enclosure and the other assumes that the radiation shielding is self supporting. To gain some indication of the amount of structural materials needed if the identical habitat were installed on Mars and Earth, three additional design versions were generated where the only difference is in gravity. These additional design versions are highly academic since the difference will be much more than in gravity alone. The lateral loading due to dust storms on Mars and wind loads on Earth are some examples. The designs under the lunar gravity are realistic. They may not be adequate for final material procurement and fabrication, however, as the connection details, among other reasons, may effect the sizes of the structural members.

Yin, Paul K.↗

NASA NTRS1990

Optical neural network system for pose determination of spinning satellites

An optical neural network architecture and algorithm based on a Hopfield optimization network are presented for multitarget tracking. This tracker utilizes a neuron for every possible target track, and a quadratic energy function of neural activities which is minimized using gradient descent neural evolution. The neural net tracker is demonstrated as part of a system for determining position and orientation (pose) of spinning satellites with respect to a robotic spacecraft. The input to the system is time sequence video from a single camera. Novelty detection and filtering are utilized to locate and segment novel regions from the input images. The neural net multitarget tracker determines the correspondences (or tracks) of the novel regions as a function of time, and hence the paths of object (satellite) parts. The path traced out by a given part or region is approximately elliptical in image space, and the position, shape and orientation of the ellipse are functions of the satellite geometry and its pose. Having a geometric model of the satellite, and the elliptical path of a part in image space, the three-dimensional pose of the satellite is determined. Digital simulation results using this algorithm are presented for various satellite poses and lighting conditions.

Lee, Andrew↗

NASA NTRS1994

Enhanced cosmic-ray acceleration rates in highly inclined astrophysical shocks

The theory of cosmic-ray acceleration by subluminal magnetohydrodynamics (MHD) astrophysical shocks according to the test particle approximation is extended to highly oblique shocks where flow speeds which appear nonrelativistic in the shock rest frame appear on transformation to the de Hoffmann-Teller or E identically equal to 0 frame to have upstream flow speeds approaching c. Monte Carlo simulation shows that relative to the predictions of diffusion theory, as the upstream E identically equal to 0 frame velocity approaches c, flatter spectra and faster acceleration rates occur. These spectral and acceleration time changes are similar to those found for relativistic, parallel MHD shocks and may affect all nonthermal active galactic nuclei emission from relativistic electrons. Also there is an additional means of increasing the upper limit of the cosmic-ray spectrum expected from active galactic nuclei, although the approximation used may not accurately reproduce the spectral shape.

Lieu, R.↗

NASA NTRS1996

The Multigrid-Mask Numerical Method for Solution of Incompressible Navier-Stokes Equations

A multigrid-mask method for solution of incompressible Navier-Stokes equations in primitive variable form has been developed. The main objective is to apply this method in conjunction with the pseudospectral element method solving flow past multiple objects. There are two key steps involved in calculating flow past multiple objects. The first step utilizes only Cartesian grid points. This homogeneous or mask method step permits flow into the interior rectangular elements contained in objects, but with the restriction that the velocity for those Cartesian elements within and on the surface of an object should be small or zero. This step easily produces an approximate flow field on Cartesian grid points covering the entire flow field. The second or heterogeneous step corrects the approximate flow field to account for the actual shape of the objects by solving the flow field based on the local coordinates surrounding each object and adapted to it. The noise occurring in data communication between the global (low frequency) coordinates and the local (high frequency) coordinates is eliminated by the multigrid method when the Schwarz Alternating Procedure (SAP) is implemented. Two dimensional flow past circular and elliptic cylinders will be presented to demonstrate the versatility of the proposed method. An interesting phenomenon is found that when the second elliptic cylinder is placed in the wake of the first elliptic cylinder a traction force results in a negative drag coefficient.

Ku, Hwar-Ching↗