2,081 research outputs found
Laudatores Temporis Acti, or Why Cosmology is Alive and Well - A Reply to Disney
A recent criticism of cosmological methodology and achievements by Disney
(2000) is assessed. Some historical and epistemological fallacies in the said
article have been highlighted. It is shown that---both empirically and
epistemologically---modern cosmology lies on sounder foundations than it is
portrayed. A brief historical account demonstrates that this form of
unsatisfaction with cosmology has had a long tradition, and rather meagre
results in the course of the XX century.Comment: 11 pages, no figures; a criticism of astro-ph/0009020; Gen. Rel.
Grav., accepted for publicatio
Dynamical diffraction in sinusoidal potentials: uniform approximations for Mathieu functions
Eigenvalues and eigenfunctions of Mathieu's equation are found in the short
wavelength limit using a uniform approximation (method of comparison with a
`known' equation having the same classical turning point structure) applied in
Fourier space. The uniform approximation used here relies upon the fact that by
passing into Fourier space the Mathieu equation can be mapped onto the simpler
problem of a double well potential. The resulting eigenfunctions (Bloch waves),
which are uniformly valid for all angles, are then used to describe the
semiclassical scattering of waves by potentials varying sinusoidally in one
direction. In such situations, for instance in the diffraction of atoms by
gratings made of light, it is common to make the Raman-Nath approximation which
ignores the motion of the atoms inside the grating. When using the
eigenfunctions no such approximation is made so that the dynamical diffraction
regime (long interaction time) can be explored.Comment: 36 pages, 16 figures. This updated version includes important
references to existing work on uniform approximations, such as Olver's method
applied to the modified Mathieu equation. It is emphasised that the paper
presented here pertains to Fourier space uniform approximation
Inhibition of Tendon Cell Proliferation and Matrix Glycosaminoglycan Synthesis by Non-Steroidal Anti-Inflammatory Drugs in vitro
The purpose of this study was to investigate the effects of some commonly used non-steroidal anti-inflammatory drugs (NSAIDs) on human tendon. Explants of human digital flexor and patella tendons were cultured in medium containing pharmacological concentrations of NSAIDs. Cell proliferation was measured by incorporation of 3H-thymidine and glycosaminoglycan synthesis was measured by incorporation of 35S-Sulphate. Diclofenac and aceclofenac had no significant effect either on tendon cell proliferation or glycosaminoglycan synthesis. Indomethacin and naproxen inhibited cell proliferation in patella tendons and inhibited glycosaminoglycan synthesis in both digital flexor and patella tendons. If applicable to the in vivo situation, these NSAIDs should be used with caution in the treatment of pain after tendon injury and surgery
Tool wear prediction on sheet metal forming die of automotive part based on numerical simulation method
Tool wear is a main concern in the sheet metal forming of automobile parts due to the development and application of new materials and the high requirement of parts quality in automobile production. However, it is difficult and time-consuming to predict tool wear using a traditional method. This paper provides a rapid numerical simulation approach for predicting the tool wear on sheet metal forming die. The simulation was carried out using the finite element software AutoForm™. A tool wear model was presented as a foundation of the simulation. The recommended protection method for the die surface and the prediction of tool worn areas was obtained from the simulation. The predicted results were in accordance with the results obtained from the on-site production. The influences of the contact pressure and drawing depth on the tool worn area distribution were also investigated based on the simulation outcome
Coagulation by Random Velocity Fields as a Kramers Problem
We analyse the motion of a system of particles suspended in a fluid which has
a random velocity field. There are coagulating and non-coagulating phases. We
show that the phase transition is related to a Kramers problem, and use this to
determine the phase diagram, as a function of the dimensionless inertia of the
particles, epsilon, and a measure of the relative intensities of potential and
solenoidal components of the velocity field, Gamma. We find that the phase line
is described by a function which is non-analytic at epsilon=0, and which is
related to escape over a barrier in the Kramers problem. We discuss the
physical realisations of this phase transition.Comment: 4 pages, 3 figure
Temporal variability in detrital 10Be concentrations in large Himalayan catchments
Accurately quantifying sediment fluxes in large rivers draining tectonically active landscapes is complicated by the stochastic nature of sediment inputs. Cosmogenic 10Be concentrations measured in modern river sands have been used to estimate 102- to 104-year sediment fluxes in these types of catchments, where upstream drainage areas are often in excess of 10 000 km2. It is commonly assumed that within large catchments, the effects of stochastic sediment inputs are buffered such that 10Be concentrations at the catchment outlet are relatively stable in time. We present 18 new 10Be concentrations of modern river and dated Holocene terrace and floodplain deposits from the Ganga River near to the Himalayan mountain front (or outlet). We demonstrate that 10Be concentrations measured in modern Ganga River sediments display a notable degree of variability, with concentrations ranging between ∼9000 and 19 000 atoms g−1. We propose that this observed variability is driven by two factors. Firstly, by the nature of stochastic inputs of sediment (e.g. the dominant erosional process, surface production rates, depth of landsliding, degree of mixing) and, secondly, by the evacuation timescale of individual sediment deposits which buffer their impact on catchment-averaged concentrations. Despite intensification of the Indian Summer Monsoon and subsequent doubling of sediment delivery to the Bay of Bengal between ∼11 and 7 ka, we also find that Holocene sediment 10Be concentrations documented at the Ganga outlet have remained within the variability of modern river concentrations. We demonstrate that, in certain systems, sediment flux cannot be simply approximated by converting detrital concentration into mean erosion rates and multiplying by catchment area as it is possible to generate larger volumetric sediment fluxes whilst maintaining comparable average 10Be concentrations
Unmixing in Random Flows
We consider particles suspended in a randomly stirred or turbulent fluid.
When effects of the inertia of the particles are significant, an initially
uniform scatter of particles can cluster together. We analyse this 'unmixing'
effect by calculating the Lyapunov exponents for dense particles suspended in
such a random three-dimensional flow, concentrating on the limit where the
viscous damping rate is small compared to the inverse correlation time of the
random flow (that is, the regime of large Stokes number). In this limit
Lyapunov exponents are obtained as a power series in a parameter which is a
dimensionless measure of the inertia. We report results for the first seven
orders. The perturbation series is divergent, but we obtain accurate results
from a Pade-Borel summation. We deduce that particles can cluster onto a
fractal set and show that its dimension is in satisfactory agreement with
previously reported in simulations of turbulent Navier-Stokes flows. We also
investigate the rate of formation of caustics in the particle flow.Comment: 39 pages, 8 figure
Reflectionless Potentials and PT Symmetry
Large families of Hamiltonians that are non-Hermitian in the conventional
sense have been found to have all eigenvalues real, a fact attributed to an
unbroken PT symmetry. The corresponding quantum theories possess an
unconventional scalar product. The eigenvalues are determined by differential
equations with boundary conditions imposed in wedges in the complex plane. For
a special class of such systems, it is possible to impose the PT-symmetric
boundary conditions on the real axis, which lies on the edges of the wedges.
The PT-symmetric spectrum can then be obtained by imposing the more transparent
requirement that the potential be reflectionless.Comment: 4 Page
On the Aggregation of Inertial Particles in Random Flows
We describe a criterion for particles suspended in a randomly moving fluid to
aggregate. Aggregation occurs when the expectation value of a random variable
is negative. This random variable evolves under a stochastic differential
equation. We analyse this equation in detail in the limit where the correlation
time of the velocity field of the fluid is very short, such that the stochastic
differential equation is a Langevin equation.Comment: 16 pages, 2 figure
Resonances and the thermonuclear reaction rate
We present an approximate analytic expression for thermonuclear reaction rate
of charged particles when the cross section contains a single narrow or wide
resonance described by a Breit-Wigner shape. The resulting expression is
uniformly valid as the effective energy and resonance energy coalesce. We use
our expressions to calculate the reaction rate for
C(p,)N.Comment: 4 pages, 1 figure, presented at the VIII International Conference on
Nucleus-Nucleus in Moscow (Russia) on June 17-21, 200
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