Engineering topics
Publications and source records attributed to Brown, R. A..
The sensitivity to silicone contamination of a wide variety of adhesive bond systems is discussed. Generalizations regarding factors that make some bond systems more sensitive to contamination than others are inferred and discussed. The effect of silane adhesion promoting primer on the contamination sensitivity of two epoxy/steel bond systems is also discussed.
A method of determining surface pressures in oceanic storm systems using Earth Remote-Sensing Satellite (ERS-1) scatterometer data is employed to determine the lowest pressure in 25 storms. This method uses the surface winds as a lower boundary condition on a planetary boundary layer model to determine gradient winds and, thereby, pressure gradients. An optimization scheme referenced to a pressure outside the storm provides a pressure field and an estimate of the low pressure. The values are compared to European Center for Medium-Range Weather Forecasts (ECMWF) analyses in each case; there is good agreement, with some expected differences.
Goals for planetary exploration during the next decade include: (1) determine how our solar system formed, and understand whether planetary systems are a common phenomenon through out the cosmos; (2) explore the diverse changes that planets have undergone throughout their history and that take place at present, including those that distinguish Earth as a planet; (3) understand how life might have formed on Earth, whether life began anywhere else in the solar system, and whether life (including intelligent beings) might be a common cosmic phenomenon; (4) discover and investigate natural phenomena that occur under conditions not realizable in laboratories; (5) discover and inventory resources in the solar system that could be used by human civilizations in the future; and (6) make the solar system a part of the human experience in the same way that Earth is, and hence lay the groundwork for human expansion into the solar system in the coming century. The plan for solar system exploration is motivated by these goals as well as the following principle: The solar system exploration program will conduct flight programs and supporting data analysis and scientific research commensurate with United States leadership in space exploration. These programs and research must be of the highest scientific merit, they must be responsive to public excitement regarding planetary exploration, and they must contribute to larger national goals in technology and education. The result will be new information, which is accessible to the public, creates new knowledge, and stimulates programs of education to increase the base of scientific knowledge in the general public.
A new method is presented for the solution of free-boundary problems using Lagrangian finite element approximations defined on locally refined grids. The formulation allows for direct transition from coarse to fine grids without introducing non-conforming basis functions. The calculation of elemental stiffness matrices and residual vectors are unaffected by changes in the refinement level, which are accounted for in the loading of elemental data to the global stiffness matrix and residual vector. This technique for local mesh refinement is combined with recently developed mapping methods and Newton's method to form an efficient algorithm for the solution of free-boundary problems, as demonstrated here by sample calculations of cellular interfacial microstructure during directional solidification of a binary alloy.
Thin-film solidification experiments with a succinonitrile-acetone alloy are used to observe the long time-scale dynamics of cellular crystal growth at growth rates only slightly above the critical value VC = Vc(lambda sub c) for the onset of morphological instability. Under these conditions only very small amplitude cells are observed with wavelengths near the value predicted by linear stability theory lambda = lambda sub c. At long times, microstructures with wavelengths significantly finer than lambda suc c form by nucleation at defects across the interface. These interfaces do not have a unique microstructure, but seem to exhibit spatiotemporal chaos on a long time scale caused by the continual birth and death of cells by tip splitting and cell annihilation in grooves.
A new boundary-conforming mapping is developed for the calculation of highly deformed cellular solidification interfaces in a model of directional solidification of a binary alloy. The mapping is derived through a variational formulation that is designed so that the grid penetrates the grooves between cells along the interface without causing a loss of ellipticity of the mapping equations. A finite element/Newton method is presented for simultaneous solution of the free boundary problem described by the solutal model of directional solidification and the mapping equations. Results are compared to previous calculations and demonstrate the importance of accurate representation of the interface shape for understanding the solution structure.
The 'LPL echelle' is a CCD spectrograph with a resolving power of 160,000 and enough sensitivity to detect the brighter airglow emissions in a few minutes. It uses an R3.2 grating in a novel out-of-plane mounting that permits any wavelength to be placed on the blaze. A single order is selected by a medium-band interference filter. Lower dispersions and broader wavelength coverage are obtained by replacing the echelle by a 600 line/mm grating or a Littrow prism.
The panel met to identify and assess strategies for recovering the Hubble Space Telescope (HST) capabilities degraded by a spherical aberration. The panels findings and recommendations to correct the problem with HST are given. The optical solution is a pair of mirrors for each science instrument field of view. The Corrective Optics Space Telescope Axial Replacement (COSTAR) is the proposed device to carry and deploy the corrective optics. A 1993 servicing mission is planned.
The ability to determine surface pressure fields from satellite scatterometer data was shown by Brown and Levy (1986). The surface winds are used to calculate the gradient winds above the planetary boundary layer, and these are directly related to the pressure gradients. There are corrections for variable stratification, variable surface roughness, horizontal inhomogeneity, humidity and baroclinity. The Seasat-A Satellite Scatterometer (SASS) data have been used in a systematic study of 50 synoptic weather events (regions of approximately 1000 X 1000 km). The preliminary statistics of agreement with national weather service surface pressure maps are calculated. The resulting surface pressure maps can be used together with SASS winds and Scanning Multichannel Microwave Radiometer (SMMR) water vapor and liquid water analyses to provide good front and storm system analyses.
The use of strong magnetic fields for the control of particle settling in metallic systems is investigated by altering the fluid mechanics in the melt. The fluid mechanism of particle settling is analyzed for the motion around single, axisymmetric particles in the limit of creeping flow for a fluid with a large electrical conductivity. The drag is found to increase proportionately to the intensity of the magnetic field or the Hartmann number Ha. The flowfield forms boundary layers, which thin out with increasing Ha, along the surfaces parallel to the flow. For axisymmetric bodies, the boundary layer separates as the poles of the surface are approached and encloses regions of almost stagnant fluid. These regions spread upstream and downstream along the body with increasing Ha, thereby trapping the particle in a column of stagnant fluid.
The techniques used to extract oceanographic and meteorological parameters from satellite radar scatterometer data are described and demonstrated, primarily on the basis of Seasat data. The principles of backscatter measurement and the general relationships between wind and wave fields are summarized, and the construction of geophysical data records from scatterometer swaths is explained. Particular attention is given to surface-layer dynamics and stresses; planetary-boundary-layer dynamics; averaging methods; the estimation of heat, moisture, and CO2 flux coefficients; the determination of pressure fields; storm monitoring; synoptic-scale analysis; climate analysis; and ocean dynamics. The capabilities and potential applications of the planned ERS-1, NSCAT, and SCANSCAT satellite scatterometers are briefly indicated.
The bifurcation diagram corresponding to the Eckhaus stability curve has been constructed for the one-dimensional Swift-Hohenberg equation in a finite domain. Finite-amplitude solutions with particular spatial wavelength recover linear stability, as predicted by the Eckhaus curve, after a sequence of secondary bifurcations from the branch of solutions with this wavelength. No connectivity between the primary-solution branches is admissible if the stability predicted by this bifurcation diagram is to correspond to the prediction of the Eckhaus analysis. The Eckhaus curve does not exist if nonlinear couplings destroy this pattern. This is demonstrated by analysis of a coupled pair of Swift-Hohenberg equations.
Gas flow and energy and species transport in laser-induced chemical vapor deposition (LICVD) of amorphous silicon films by silane pyrolysis are analyzed by finite element analysis of a two-dimensional model for the process. Spatial nonuniformity of the deposited film is shown to result from diffusion controlled transport of products between the beam and substrate. Deposition profiles are affected by buoyancy-driven convection only at increased gas pressures. Horizontal orientation of the reactor with respect to gravity is optimal because the stagnation-like flow, that results adjacent to the substrate, enhances mixing, and smoothes the film profile.
The nonlinear dynamics of nonaxisymmetric inviscid charged conducting drops near the Rayleigh charge limit (R = 4) is investigated analytically. It is shown that only axisymmetric spheroid drops bifurcate from the sphere family when the charge is increased, that oblate spheroids at R greater than 4 are unstable to nonaxisymmetric disturbances governing drop breakup, and that prolate spheroids at R less than 4 are unstable only to axisymmetric disturbances tending to increase the length of the drop along its symmetry axis. The effects of external electric fields and rigid-body rotation are also analyzed, and the solutions for the amplitude equations at R just less than 4 (equivalent to the dynamical equations of the Henon-Heiles Hamiltonian) are explored.
This research program focuses on analysis of the transport mechanisms in solidification processes, especially one of interest to the Microgravity Sciences and Applications Program of NASA. Research during the last year has focused on analysis of the dynamics of the floating zone process for growth of small-scale crystals, on studies of the effect of applied magnetic fields on convection and solute segregation in directional solidification, and on the dynamics of microscopic cell formation in two-dimensional solidification of binary alloys. Significant findings are given.
Telescopic observations were made of gases and plasmas in the Jupiter system, and related phenomena such as the recently-discovered sodium atmosphere of Mercury. Sodium absorption in Io's atmosphere was observed to distances as large as 5 Io radii during the mutual-eclipse season of the Jovian satellites. Analysis of Jovian disk spectra has proceeded, as planned, from an emphasis on the H2 ortho-para ratio to analysis of methane absorptions and determination of cloud structure. Several 2-channel photometers were constructed in 1978 for the Uranus event at which the rings were discovered and a smaller, single-channel unit was built more recently. Although the photometers themselves are still fully usable, the data-acquisition systems have major deficiencies.
The moderate-amplitude, three-dimensional oscillations of an inviscid drop are described in terms of spherical harmonics. Specific oscillation modes are resonantly coupled by quadratic nonlinearities caused by inertia, capillarity, and drop deformation. The equations describing the interactions of these modes are derived from the variational principle for the appropriate Lagrangian by expressing the modal amplitudes to be functions of a slow time scale and by preaveraging the Lagrangian over the time scale of the primary oscillations. Stochastic motions are predicted for nonaxisymmetric deformations starting from most initial conditions, even those arbitrarily close to the axisymmetric shapes. The stochasticity is characterized by a redistribution of the energy contained in the initial deformation over all the degrees of freedom of the interacting modes.
Capillarity, acting to set the shape of the melt/gas interfaces, and heat transfer can interact to cause limits to steady-state growth of thin silicon sheets by the Edge-Defined Film-Fed Growth (EFG) method. A finite-element/Newton solution method for a two-dimensional thermal-capillary model of EFG is used to show that limiting values of pull rate exist beyond which steady-state growth is impossible. The pull rate limit is also predicted by a one-dimensional heat transfer model valid when the die sides and menisci are almost parallel and when the thermal conductivities of melt, crystal, and die are all equal. Both the one- and two-dimensional heat transfer models show that heat loss from the melt is dominated by conduction into the crystal and slow heat release to the ambient along the length of the ribbon. The limiting pull rate results from the reduced efficiency of conduction through the melt caused by the curvature of the meniscus which increases height of the die top above the level of the melt. Thermal-capillary limits are predicted for both positive and negative pressure differences across the meniscus.