6,157 research outputs found
An abstract formulation of the concept of entropy
Entropy is presented as a concave function relating two sets of quantities called densities and field. It allows a simple classification of the standard relations of classical thermodynamics and yields a simple derivation of the conditions for concavity of the entropy function. It also allows a formal derivation of the equations of fluid motion. Dissipation, mixtures, and phase changes may also be included in the theory in a natural manner
Stationary and non-stationary fluid flow of a Bose-Einstein condensate through a penetrable barrier
We experimentally study the fluid flow induced by a broad, penetrable barrier
moving through an elongated dilute gaseous Bose-Einstein condensate. The
barrier is created by a laser beam swept through the condensate, and the
resulting dipole potential can be either attractive or repulsive. We examine
both cases and find regimes of stable and unstable fluid flow: At slow speeds
of the barrier, the fluid flow is stationary due to the superfluidity of the
condensate. At intermediate speeds, we observe a non-stationary regime in which
the condensate gets filled with dark solitons. At faster speeds, soliton
formation completely ceases and a remarkable absence of excitation in the
condensate is seen again.Comment: 4 pages, 4 figure
NIMBUS-7 SBUV (Solar Backscatter Ultraviolet) observations of solar UV spectral irradiance variations caused by solar rotation and active-region evolution for the period November 7, 1978 - November 1, 1980
Observations of temporal variations of the solar UV spectral irradiance over several days to a few weeks in the 160-400 nm wavelength range are presented. Larger 28-day variations and a second episode of 13-day variations occurred during the second year of measurements. The thirteen day periodicity is not a harmonic of the 28-day periodicity. The 13-day periodicity dominates certain episodes of solar activity while others are dominated by 28-day periods accompanied by a week 14-day harmonic. Techniques for removing noise and long-term trends are described. Time series analysis results are presented for the Si II lines near 182 nm, the Al I continuum in the 190 nm to 205 nm range, the Mg I continuum in the 210 nm to 250 nm range, the MgII H & K lines at 280 nm, the Mg I line at 285 nm, and the Ca II K & H lines at 393 and 397 nm
Some exact solutions in moving finite elements
It is shown that when the moving finite elements are used on a number of parabolic problems there are steady-state, stationary, similarity, or travelling-wave solutions that can be found numerically
Coherence vortices in one spatial dimension
Coherence vortices are screw-type topological defects in the phase of
Glauber's two-point degree of quantum coherence, associated with pairs of
spatial points at which an ensemble-averaged stochastic quantum field is
uncorrelated. Coherence vortices may be present in systems whose dimensionality
is too low to support spatial vortices. We exhibit lattices of such
quantum-coherence phase defects for a one-dimensional model quantum system. We
discuss the physical meaning of coherence vortices and propose how they may be
realized experimentally.Comment: 5 pages, 3 figure
Local Asymmetry and the Inner Radius of Nodal Domains
Let M be a closed Riemannian manifold of dimension n. Let f be an
eigenfunction of the Laplace-Beltrami operator corresponding to an eigenvalue
\lambda. We show that the volume of {f>0} inside any ball B whose center lies
on {f=0} is > C|B|/\lambda^n. We apply this result to prove that each nodal
domain contains a ball of radius > C/\lambda^n.Comment: 12 pages, 1 figure; minor corrections; to appear in Comm. PDE
Vortex density spectrum of quantum turbulence
The fluctuations of the vortex density in a turbulent quantum fluid are
deduced from local second-sound attenuation measurements. These measurements
are performed with a micromachined open-cavity resonator inserted across a flow
of turbulent He-II near 1.6 K. The power spectrum of the measured vortex line
density is compatible with a (-5/3) power law. The physical interpretation,
still open, is discussed.Comment: Submitted to Europhys. Let
Drag force on an oscillating object in quantum turbulence
This paper reports results of the computation of the drag force exerted on an
oscillating object in quantum turbulence in superfluid He. The drag force
is calculated on the basis of numerical simulations of quantum turbulent flow
about the object. The drag force is proportional to the square of the magnitude
of the oscillation velocity, which is similar to that in classical turbulence
at high Reynolds number. The drag coefficient is also calculated, and its value
is found to be of the same order as that observed in previous experiments. The
correspondence between quantum and classical turbulences is further clarified
by examining the turbulence created by oscillating objects.Comment: 7 pages, 5 figures, 1 tabl
Topologically non-trivial quantum layers
Given a complete non-compact surface embedded in R^3, we consider the
Dirichlet Laplacian in a layer of constant width about the surface. Using an
intrinsic approach to the layer geometry, we generalise the spectral results of
an original paper by Duclos et al. to the situation when the surface does not
possess poles. This enables us to consider topologically more complicated
layers and state new spectral results. In particular, we are interested in
layers built over surfaces with handles or several cylindrically symmetric
ends. We also discuss more general regions obtained by compact deformations of
certain layers.Comment: 15 pages, 6 figure
Direct measurement of quantum phase gradients in superfluid 4He flow
We report a new kind of experiment in which we generate a known superfluid
velocity in a straight tube and directly determine the phase difference across
the tube's ends using a superfluid matter wave interferometer. By so doing, we
quantitatively verify the relation between the superfluid velocity and the
phase gradient of the condensate macroscopic wave function. Within the
systematic error of the measurement (~10%) we find v_s=(hbar/m_4)*(grad phi)
- …