54 research outputs found
Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations
The approach of the elastic continuum limit in small amorphous bodies formed
by weakly polydisperse Lennard-Jones beads is investigated in a systematic
finite-size study. We show that classical continuum elasticity breaks down when
the wavelength of the sollicitation is smaller than a characteristic length of
approximately 30 molecular sizes. Due to this surprisingly large effect
ensembles containing up to N=40,000 particles have been required in two
dimensions to yield a convincing match with the classical continuum predictions
for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk
systems. The existence of an effective length scale \xi is confirmed by the
analysis of the (non-gaussian) noisy part of the low frequency vibrational
eigenmodes. Moreover, we relate it to the {\em non-affine} part of the
displacement fields under imposed elongation and shear. Similar correlations
(vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table
Three-dimensional lattice-Boltzmann simulations of critical spinodal decomposition in binary immiscible fluids
We use a modified Shan-Chen, noiseless lattice-BGK model for binary
immiscible, incompressible, athermal fluids in three dimensions to simulate the
coarsening of domains following a deep quench below the spinodal point from a
symmetric and homogeneous mixture into a two-phase configuration. We find the
average domain size growing with time as , where increases
in the range , consistent with a crossover between
diffusive and hydrodynamic viscous, , behaviour. We find
good collapse onto a single scaling function, yet the domain growth exponents
differ from others' works' for similar values of the unique characteristic
length and time that can be constructed out of the fluid's parameters. This
rebuts claims of universality for the dynamical scaling hypothesis. At early
times, we also find a crossover from to in the scaled structure
function, which disappears when the dynamical scaling reasonably improves at
later times. This excludes noise as the cause for a behaviour, as
proposed by others. We also observe exponential temporal growth of the
structure function during the initial stages of the dynamics and for
wavenumbers less than a threshold value.Comment: 45 pages, 18 figures. Accepted for publication in Physical Review
Phase separating binary fluids under oscillatory shear
We apply lattice Boltzmann methods to study the segregation of binary fluid
mixtures under oscillatory shear flow in two dimensions. The algorithm allows
to simulate systems whose dynamics is described by the Navier-Stokes and the
convection-diffusion equations. The interplay between several time scales
produces a rich and complex phenomenology. We investigate the effects of
different oscillation frequencies and viscosities on the morphology of the
phase separating domains. We find that at high frequencies the evolution is
almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low
viscosity) and diffusive (high viscosity) regimes, respectively. When the
period of the applied shear flow becomes of the same order of the relaxation
time of the shear velocity profile, anisotropic effects are clearly
observable. In correspondence with non-linear patterns for the velocity
profiles, we find configurations where lamellar order close to the walls
coexists with isotropic domains in the middle of the system. For particular
values of frequency and viscosity it can also happen that the convective
effects induced by the oscillations cause an interruption or a slowing of the
segregation process, as found in some experiments. Finally, at very low
frequencies, the morphology of domains is characterized by lamellar order
everywhere in the system resembling what happens in the case with steady shear.Comment: 1 table and 12 figures in .gif forma
Physics Opportunities with the 12 GeV Upgrade at Jefferson Lab
This white paper summarizes the scientific opportunities for utilization of
the upgraded 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) and
associated experimental equipment at Jefferson Lab. It is based on the 52
proposals recommended for approval by the Jefferson Lab Program Advisory
Committee.The upgraded facility will enable a new experimental program with
substantial discovery potential to address important topics in nuclear,
hadronic, and electroweak physics.Comment: 64 page
Recommended from our members
Remote temperature-measurement instrumentation for a heated rotating turbine disk
Thermographic-phosphor (TP) remote temperature sensors were installed on a turbine disk and subjected to thermal and centrifugal stresses in a spin-pit test. The sensors were placed at three different radii on the disk, which was run at 6600, 9330, 11400, and 13200 rpm at nominal temperatures of ambient, 300) degree)F, 600)degree)F, 900)degree)F, and 1250)degree)F (149)degree)C, 316)degree)C, 482)degree)C, and 677)degree)C, respectively). The paper gives details of the TP temperature-measurement method, phosphor bonding to the disk, calibration, optical-system design, and electronics instrumentation. The temperatures measured by the TP sensors were compared with those measured by thermocouples mounted on the disk. A number of the thermocouples behaved erratically after we operated the disk at 677)degree)C for an extended period. Nevertheless, for those cases where they could be compared with confidence, the agreement between the TP sensors and the thermocouples was good. 6 refs., 7 figs., 4 tabs
High sensitivity transuranic waste assay by simultaneous photon and thermal-neutron interrogation using an electron linear accelerator
Recommended from our members
Application of linear accelerator technology to the detection of trace amounts of transuranics in waste barrels
Electron linear accelerators (linacs), as sources of photons and neutrons, can produce a significant number of fissions in transuranic isotopes contained in large barrels of waste material. Both photons and thermal neutrons have been used to detect about 1 mg of plutonium in 105-kg matrices. A sequential interrogation with neutrons and photons, easily possible with linacs, can show both fertile and fissile constituents among the heavy-mass isotopes. The advantages of linacs in solving existing assay problems include: (1) high available beam current; (2) variable beam current, beam energy, pulse width, and pulse repetition frequency; and (3) beam-scanning ability. They also are compatible with passive assay instruments. Their versatility makes it likely that they will remain useful as assay technology advances
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