22,623 research outputs found
Proceedings of the 21st annual Central Plains irrigation conference, Colby Kansas, February 24-25, 2009
Presented at the the 21st annual Central Plains irrigation conference on February 24-25, 2009 in Colby, Kansas
Recommended from our members
Modeling of Selective Area Laser Deposition Vapor Infiltration (SALDVI) of Silicon Carbide
Selective Area Laser Deposition Vapor Infiltration (SALDVI) is a developing
solid freeform fabrication (SFF) technique in which porous layers of powder are
densified by infiltrating the pore spaces with solid material deposited from a gas
precursor during laser heating. A 3D finite element model was developed that simulates
SALDVI of silicon carbide. The model predicts the laser input power and the distribution
of vapor deposited SiC within the powder bed as well as on the surface of the powder bed
(SALD). The model considers a moving Gaussian distribution laser beam, temperatureand porous-dependent thermal conductivity, specific heat and temperature-dependent
deposition rate. Furthermore, the model also includes closed-loop control of the laser
power to achieve a desired target processing temperature on the top surface of the power
bed. The simulation results agree fairly well with experimental data for simple
geometries and offer guidelines for further experimental studies of the SALDVI process.Mechanical Engineerin
Exclusion process for particles of arbitrary extension: Hydrodynamic limit and algebraic properties
The behaviour of extended particles with exclusion interaction on a
one-dimensional lattice is investigated. The basic model is called -ASEP
as a generalization of the asymmetric exclusion process (ASEP) to particles of
arbitrary length . Stationary and dynamical properties of the -ASEP
with periodic boundary conditions are derived in the hydrodynamic limit from
microscopic properties of the underlying stochastic many-body system. In
particular, the hydrodynamic equation for the local density evolution and the
time-dependent diffusion constant of a tracer particle are calculated. As a
fundamental algebraic property of the symmetric exclusion process (SEP) the
SU(2)-symmetry is generalized to the case of extended particles
Stationary states and phase diagram for a model of the Gunn effect under realistic boundary conditions
A general formulation of boundary conditions for semiconductor-metal contacts
follows from a phenomenological procedure sketched here. The resulting boundary
conditions, which incorporate only physically well-defined parameters, are used
to study the classical unipolar drift-diffusion model for the Gunn effect. The
analysis of its stationary solutions reveals the presence of bistability and
hysteresis for a certain range of contact parameters. Several types of Gunn
effect are predicted to occur in the model, when no stable stationary solution
exists, depending on the value of the parameters of the injecting contact
appearing in the boundary condition. In this way, the critical role played by
contacts in the Gunn effect is clearly stablished.Comment: 10 pages, 6 Post-Script figure
The Influence of Bars on Nuclear Activity
We test ideas on fueling of galactic nuclei by bar-driven inflow by comparing
the detection rate and intensity of nuclear H II regions and AGNs among barred
and unbarred galaxies in a sample of over 300 spirals selected from our recent
optical spectroscopic survey of nearby galaxies. Among late-type spirals
(Sc-Sm), but not early-type (S0/a-Sbc), we observe in the barred group a very
marginal increase in the detection rate of H II nuclei and a corresponding
decrease in the incidence of AGNs. The minor differences in the detection
rates, however, are statistically insignificant, most likely stemming from
selection effects and not from a genuine influence from the bar. The presence
of a bar seems to have no noticeable impact on the likelihood of a galaxy to
host either nuclear star formation or an AGN. The nuclei of early-type barred
spirals do exhibit measurably higher star-formation rates than their unbarred
counterparts, as indicated by either the luminosity or the equivalent width of
H-alpha emission. By contrast, late-type spirals do not show such an effect.
Bars have a negligible effect on the strength of the AGNs in our sample,
regardless of the Hubble type of the host galaxy. This result confirms similar
conclusions reached by other studies based on much smaller samples.Comment: To appear in the Astrophysical Journal. LaTex, 31 pages including 6
postscript figures and 3 tables. AAStex macros include
PVF2 Transducers for NDT
We have investigated the spatial dependence of the longitudinal piezoelectric stress constant e of PVF2. Experiments were performed on a series of no.minally identical brass-backed PVF2 longitudinal wave transducers in water using commercial PVF2 film. Computer programs were designed to predict the performance of the transducers as a function of ezz (Z) , the thickness mode piezoelectric stress constant as a function of position Z through the thickness of the film. Our experiments indicate that the coupling coefficient is uniform across the film thickness. These computer programs were also used to model the insertion loss and bandwidth performance of the transducers. High voltage pulses were applied to PVF2 transducers to determine the region of linearity and to find the maximum nondestructive voltage that can be used. Transducers of this type were also made using PVF films fabricated in the Stanford Center for Materials Research using commercial resin, and found to pertorm as well as transducers using commercial film. Transducers of the above kind have also been used as bulk wave sources in wedge transducer assemblies for the production of surface acoustic waves on ceramic plates, and initial observations of reflections from surface cracks have been made
Chaotic dynamics of electric-field domains in periodically driven superlattices
Self-sustained time-dependent current oscillations under dc voltage bias have
been observed in recent experiments on n-doped semiconductor superlattices with
sequential resonant tunneling. The current oscillations are caused by the
motion and recycling of the domain wall separating low- and high-electric-
field regions of the superlattice, as the analysis of a discrete drift model
shows and experimental evidence supports. Numerical simulation shows that
different nonlinear dynamical regimes of the domain wall appear when an
external microwave signal is superimposed on the dc bias and its driving
frequency and driving amplitude vary. On the frequency - amplitude parameter
plane, there are regions of entrainment and quasiperiodicity forming Arnol'd
tongues. Chaos is demonstrated to appear at the boundaries of the tongues and
in the regions where they overlap. Coexistence of up to four electric-field
domains randomly nucleated in space is detected under ac+dc driving.Comment: 9 pages, LaTex, RevTex. 12 uuencoded figures (1.8M) should be
requested by e-mail from the autho
Parallel algorithm with spectral convergence for nonlinear integro-differential equations
We discuss a numerical algorithm for solving nonlinear integro-differential
equations, and illustrate our findings for the particular case of Volterra type
equations. The algorithm combines a perturbation approach meant to render a
linearized version of the problem and a spectral method where unknown functions
are expanded in terms of Chebyshev polynomials (El-gendi's method). This
approach is shown to be suitable for the calculation of two-point Green
functions required in next to leading order studies of time-dependent quantum
field theory.Comment: 15 pages, 9 figure
- …