4,045 research outputs found
Experiments to investigate particulate materials in reduced gravity fields
Study investigates agglomeration and macroscopic behavior in reduced gravity fields of particles of known properties by measuring and correlating thermal and acoustical properties of particulate materials. Experiment evaluations provide a basis for a particle behavior theory and measure bulk properties of particulate materials in reduced gravity
Vibrational Stability of NLC Linac accelerating structure
The vibration of components of the NLC linac, such as accelerating structures
and girders, is being studied both experimentally and analytically. Various
effects are being considered including structural resonances and vibration
caused by cooling water in the accelerating structure. This paper reports the
status of ongoing work.Comment: 3 pages 8 figures Presented at EPAC 2002 Paris Franc
Vibrational Stability of NLC Linac and Final Focus Components
Vertical vibration of linac components (accelerating structures, girders and
quadrupoles) in the NLC has been studied experimentally and analytically.
Effects such as structural resonances and vibration caused by cooling water
both in accelerating structures and quadrupoles have been considered.
Experimental data has been compared with analytical predictions and simulations
using ANSYS. A design, incorporating the proper decoupling of structure
vibrations from the linac quadrupoles, is being pursued.Comment: 3 pages, 8 figures presented at the LINAC 2002 conference, Gyeongju
Kore
Effect of Cooling Water on Stability of NLC Linac Components
Vertical vibration of linac components (accelerating structures, girders and
quadrupoles) in the NLC has been studied experimentally and analytically.
Effects such as structural resonances and vibration caused by cooling water
both in accelerating structures and quadrupoles have been considered.
Experimental data has been compared with analytical predictions and simulations
using ANSYS. A design, incorporating the proper decoupling of structure
vibrations from the linac quadrupoles, is being pursued.Comment: 6 Pages 13 Figures Presented at The Nanobeam 2002 Workshop (Lausanne
Switzerland
Breakdown of disordered media by surface loads
We model an interface layer connecting two parts of a solid body by N
parallel elastic springs connecting two rigid blocks. We load the system by a
shear force acting on the top side. The springs have equal stiffness but are
ruptured randomly when the load reaches a critical value. For the considered
system, we calculate the shear modulus, G, as a function of the order
parameter, \phi, describing the state of damage, and also the ``spalled''
material (burst) size distribution. In particular, we evaluate the relation
between the damage parameter and the applied force and explore the behaviour in
the vicinity of material breakdown. Using this simple model for material
breakdown, we show that damage, caused by applied shear forces, is analogous to
a first-order phase transition. The scaling behaviour of G with \phi is
explored analytically and numerically, close to \phi=0 and \phi=1 and in the
vicinity of \phi_c, when the shear load is close but below the threshold force
that causes material breakdown. Our model calculation represents a first
approximation of a system subject to wear induced loads.Comment: 15 pages, 7 figure
Towards a modeling of the time dependence of contact area between solid bodies
I present a simple model of the time dependence of the contact area between
solid bodies, assuming either a totally uncorrelated surface topography, or a
self affine surface roughness. The existence of relaxation effects (that I
incorporate using a recently proposed model) produces the time increase of the
contact area towards an asymptotic value that can be much smaller than
the nominal contact area. For an uncorrelated surface topography, the time
evolution of is numerically found to be well fitted by expressions of
the form [, where the exponent depends on
the normal load as , with close to 0.5. In
particular, when the contact area is much lower than the nominal area I obtain
, i.e., a logarithmic time increase of the
contact area, in accordance with experimental observations. The logarithmic
increase for low loads is also obtained analytically in this case. For the more
realistic case of self affine surfaces, the results are qualitatively similar.Comment: 18 pages, 9 figure
Reactor monitoring and safeguards using antineutrino detectors
Nuclear reactors have served as the antineutrino source for many fundamental
physics experiments. The techniques developed by these experiments make it
possible to use these very weakly interacting particles for a practical
purpose. The large flux of antineutrinos that leaves a reactor carries
information about two quantities of interest for safeguards: the reactor power
and fissile inventory. Measurements made with antineutrino detectors could
therefore offer an alternative means for verifying the power history and
fissile inventory of a reactors, as part of International Atomic Energy Agency
(IAEA) and other reactor safeguards regimes. Several efforts to develop this
monitoring technique are underway across the globe.Comment: 6 pages, 4 figures, Proceedings of XXIII International Conference on
Neutrino Physics and Astrophysics (Neutrino 2008); v2: minor additions to
reference
Frictional sliding without geometrical reflection symmetry
The dynamics of frictional interfaces play an important role in many physical
systems spanning a broad range of scales. It is well-known that frictional
interfaces separating two dissimilar materials couple interfacial slip and
normal stress variations, a coupling that has major implications on their
stability, failure mechanism and rupture directionality. In contrast,
interfaces separating identical materials are traditionally assumed not to
feature such a coupling due to symmetry considerations. We show, combining
theory and experiments, that interfaces which separate bodies made of
macroscopically identical materials, but lack geometrical reflection symmetry,
generically feature such a coupling. We discuss two applications of this novel
feature. First, we show that it accounts for a distinct, and previously
unexplained, experimentally observed weakening effect in frictional cracks.
Second, we demonstrate that it can destabilize frictional sliding which is
otherwise stable. The emerging framework is expected to find applications in a
broad range of systems.Comment: 14 pages, 5 figures + Supplementary Material. Minor change in the
title, extended analysis in the second par
Static Versus Dynamic Friction: The Role of Coherence
A simple model for solid friction is analyzed. It is based on tangential
springs representing interlocked asperities of the surfaces in contact. Each
spring is given a maximal strain according to a probability distribution. At
their maximal strain the springs break irreversibly. Initially all springs are
assumed to have zero strain, because at static contact local elastic stresses
are expected to relax. Relative tangential motion of the two solids leads to a
loss of coherence of the initial state: The springs get out of phase due to
differences in their sizes. This mechanism alone is shown to lead to a
difference between static and dynamic friction forces already. We find that in
this case the ratio of the static and dynamic coefficients decreases with
increasing relative width of the probability distribution, and has a lower
bound of 1 and an upper bound of 2.Comment: 10 pages, 2 figures, revtex
Magnetic friction in Ising spin systems
A new contribution to friction is predicted to occur in systems with magnetic
correlations: Tangential relative motion of two Ising spin systems pumps energy
into the magnetic degrees of freedom. This leads to a friction force
proportional to the area of contact. The velocity and temperature dependence of
this force are investigated. Magnetic friction is strongest near the critical
temperature, below which the spin systems order spontaneously.
Antiferromagnetic coupling leads to stronger friction than ferromagnetic
coupling with the same exchange constant. The basic dissipation mechanism is
explained. If the coupling of the spin system to the heat bath is weak, a
surprising effect is observed in the ordered phase: The relative motion acts
like a heat pump cooling the spins in the vicinity of the friction surface.Comment: 4 pages, 4 figure
- …