27,851 research outputs found
Continuation-conjugate gradient methods for the least squares solution of nonlinear boundary value problems
We discuss in this paper a new combination of methods for solving nonlinear boundary value problems containing a parameter. Methods of the continuation type are combined with least squares formulations, preconditioned conjugate gradient algorithms and finite element approximations.
We can compute branches of solutions with limit points, bifurcation points, etc.
Several numerical tests illustrate the possibilities of the methods discussed in the present paper; these include the Bratu problem in one and two dimensions, one-dimensional bifurcation and perturbed bifurcation problems, the driven cavity problem for the Navier–Stokes equations
Liquid Rocket Engine Turbopump Rotating-shaft Seals
A monograph is organized and presents, for effective use in design, the significant experience and knowledge accumulated in development and operational programs to date. It reviews and assesses current practices, and from them establishes firm guidance for achieving greater consistency in design, increased reliability in the end product, and greater efficiency in the design effort. The monograph is divided into two major sections: state of the art and design criteria
Double window viewing chamber assembly
A viewing chamber which permits observation of a sample retained therein includes a pair of double window assemblies mounted in opposed openings in the walls thereof so that a light beam can directly enter and exit from the chamber. A flexible mounting arrangement for the outer windows of the window assemblies enables the windows to be brought into proper alignment. An electrical heating arrangement prevents fogging of the outer windows whereas desiccated air in the volume between the outer and inner windows prevents fogging of the latter
Liquid rocket metal tanks and tank components
Significant guidelines are presented for the successful design of aerospace tanks and tank components, such as expulsion devices, standpipes, and baffles. The state of the art is reviewed, and the design criteria are presented along with recommended practices. Design monographs are listed
Solid rocket motor internal insulation
Internal insulation in a solid rocket motor is defined as a layer of heat barrier material placed between the internal surface of the case propellant. The primary purpose is to prevent the case from reaching temperatures that endanger its structural integrity. Secondary functions of the insulation are listed and guidelines for avoiding critical problems in the development of internal insulation for rocket motors are presented
Seismic properties of the crust and uppermost mantle of North America
Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies
Unboundedness of adjacency matrices of locally finite graphs
Given a locally finite simple graph so that its degree is not bounded, every
self-adjoint realization of the adjacency matrix is unbounded from above. In
this note we give an optimal condition to ensure it is also unbounded from
below. We also consider the case of weighted graphs. We discuss the question of
self-adjoint extensions and prove an optimal criterium.Comment: Typos corrected. Examples added. Cute drawings. Simplification of the
main condition. Case of the weight tending to zero more discussed
Effects of Large-Scale Convection on p-mode Frequencies
We describe an approach for finding the eigenfrequencies of solar acoustic
modes (p modes) in a convective envelope in the WKB limit. This approximation
restricts us to examining the effects of fluid motions which are large compared
to the mode wavelength, but allows us to treat the three-dimensional mode as a
localized ray. The method of adiabatic switching is then used to investigate
the frequency shifts resulting from simple perturbations to a polytropic model
of the convection zone as well as from two basic models of a convective cell.
We find that although solely depth-dependent perturbations can give frequency
shifts which are first order in the strength of the perturbation, models of
convective cells generate downward frequency shifts which are second order in
the perturbation strength. These results may have implications for resolving
the differences between eigenfrequencies derived from solar models and those
found from helioseismic observations.Comment: 27 pages + 6 figures; accepted for publication in Ap
Neutron spin-echo study of the critical dynamics of spin-5/2 antiferromagnets in two and three dimensions
We report a neutron spin-echo study of the critical dynamics in the
antiferromagnets MnF and RbMnF with three-dimensional (3D) and
two-dimensional (2D) spin systems, respectively, in zero external field. Both
compounds are Heisenberg antiferromagnets with a small uniaxial anisotropy
resulting from dipolar spin-spin interactions, which leads to a crossover in
the critical dynamics close to the N\'eel temperature, . By taking
advantage of the energy resolution of the spin-echo
spectrometer, we have determined the dynamical critical exponents for both
longitudinal and transverse fluctuations. In MnF, both the characteristic
temperature for crossover from 3D Heisenberg to 3D Ising behavior and the
exponents in both regimes are consistent with predictions from the
dynamical scaling theory. The amplitude ratio of longitudinal and transverse
fluctuations also agrees with predictions. In RbMnF, the critical
dynamics crosses over from the expected 2D Heisenberg behavior for
to a scaling regime with exponent , which has not been predicted
by theory and may indicate the influence of long-range dipolar interactions
The Effelsberg-Bonn HI Survey (EBHIS)
The new L-band 7-feed-array at the 100-m telescope in Effelsberg will be used
to perform an unbiased fully sampled HI survey of the entire northern
hemisphere observing the galactic and extragalactic sky using simultaneously
two different backends.
The survey will be extremely valuable for a broad range of research topics:
study of the low-mass end of the HI mass function (HIMF) in the local volume,
environmental and evolutionary effects (as seen in the HIMF), the search for
galaxies near low-redshift Lyman-alpha absorbers, and analysis of multiphase
and extraplanar gas, HI shells, and ultra-compact high-velocity-clouds.Comment: 2 pages, 1 figure, to appear in proceeding of "Galaxies in the Local
Volume" Sydney 8-13 July 200
- …