988 research outputs found
Monopole Decay in a Variable External Field
The rate of monopole decay into a dyon and an electron in an inhomogeneous
external electric field is calculated by semiclassical methods. Comparison is
made to an earlier result where this quantity was calculated for a constant
field. Experimental and cosmological tests are suggested.Comment: 15 pages, 8 figures. v2: typos removed, list of references update
Fluctuations of temperature gradients in turbulent thermal convection
Broad theoretical arguments are proposed to show, formally, that the
magnitude G of the temperature gradients in turbulent thermal convection at
high Rayleigh numbers obeys the same advection-diffusion equation that governs
the temperature fluctuation T, except that the velocity field in the new
equation is substantially smoothed. This smoothed field leads to a -1 scaling
of the spectrum of G in the same range of scales for which the spectral
exponent of T lies between -7/5 and -5/3. This result is confirmed by
measurements in a confined container with cryogenic helium gas as the working
fluid for Rayleigh number Ra=1.5x10^{11}. Also confirmed is the logarithmic
form of the autocorrelation function of G. The anomalous scaling of
dissipation-like quantities of T and G are identical in the inertial range,
showing that the analogy between the two fields is quite deep
Destruction of a metastable string by particle collisions
We calculate the probability of destruction of a metastable string by
collisions of the Goldstone bosons, corresponding to the transverse waves on
the string. We find a general formula that allows to determine the probability
of the string breakup by a collision of arbitrary number of the bosons. We find
that the destruction of a metastable string takes place only in collisions of
even number of the bosons, and we explicitly calculate the energy dependence of
such process in a two-particle collision for an arbitrary relation between the
energy and the largest infrared scale in the problem, the length of the
critical gap in the string.Comment: 15 pages, 1 figur
Sign-symmetry of temperature structure functions
New scalar structure functions with different sign-symmetry properties are
defined. These structure functions possess different scaling exponents even
when their order is the same. Their scaling properties are investigated for
second and third orders, using data from high-Reynolds-number atmospheric
boundary layer. It is only when structure functions with disparate
sign-symmetry properties are compared can the extended self-similarity detect
two different scaling ranges that may exist, as in the example of convective
turbulence.Comment: 18 pages, 5 figures, accepted for publication in Physical Review
Mean- Field Approximation and Extended Self-Similarity in Turbulence
Recent experimental discovery of extended self-similarity (ESS) was one of
the most interesting developments, enabling precise determination of the
scaling exponents of fully developed turbulence. Here we show that the ESS is
consistent with the Navier-Stokes equations, provided the pressure -gradient
contributions are expressed in terms of velocity differences in the mean field
approximation (Yakhot, Phys.Rev. E{\bf 63}, 026307, (2001)). A sufficient
condition for extended self-similarity in a general dynamical systemComment: 8 pages, no figure
Comparative experimental study of local mixing of active and passive scalars in turbulent thermal convection
We investigate experimentally the statistical properties of active and
passive scalar fields in turbulent Rayleigh-B\'{e}nard convection in water, at
. Both the local concentration of fluorescence dye and the local
temperature are measured near the sidewall of a rectangular cell. It is found
that, although they are advected by the same turbulent flow, the two scalars
distribute differently. This difference is twofold, i.e. both the quantities
themselves and their small-scale increments have different distributions. Our
results show that there is a certain buoyant scale based on time domain, i.e.
the Bolgiano time scale , above which buoyancy effects are significant.
Above , temperature is active and is found to be more intermittent than
concentration, which is passive. This suggests that the active scalar possesses
a higher level of intermittency in turbulent thermal convection. It is further
found that the mixing of both scalar fields are isotropic for scales larger
than even though buoyancy acts on the fluid in the vertical direction.
Below , temperature is passive and is found to be more anisotropic than
concentration. But this higher degree of anisotropy is attributed to the higher
diffusivity of temperature over that of concentration. From the simultaneous
measurements of temperature and concentration, it is shown that two scalars
have similar autocorrelation functions and there is a strong and positive
correlation between them.Comment: 13 pages and 12 figure
On the Detection of Magnetic Helicity
Magnetic fields in various astrophysical settings may be helical and, in the
cosmological context, may provide a measure of primordial CP violation during
baryogenesis. Yet it is difficult, even in principle, to devise a scheme by
which magnetic helicity may be detected, except in some very special systems.
We propose that charged cosmic rays originating from known sources may be
useful for this purpose. We show that the correlator of the arrival momenta of
the cosmic rays is sensitive to the helicity of an intervening magnetic field.
If the sources themselves are not known, the method may still be useful
provided we have some knowledge of their spatial distribution.Comment: 5 pages, 1 figure, discussions and references added, submited to
Phys. Rev.
Inertial range scaling in numerical turbulence with hyperviscosity
Numerical turbulence with hyperviscosity is studied and compared with direct
simulations using ordinary viscosity and data from wind tunnel experiments. It
is shown that the inertial range scaling is similar in all three cases.
Furthermore, the bottleneck effect is approximately equally broad (about one
order of magnitude) in these cases and only its height is increased in the
hyperviscous case--presumably as a consequence of the steeper decent of the
spectrum in the hyperviscous subrange. The mean normalized dissipation rate is
found to be in agreement with both wind tunnel experiments and direct
simulations. The structure function exponents agree with the She-Leveque model.
Decaying turbulence with hyperviscosity still gives the usual t^{-1.25} decay
law for the kinetic energy, and also the bottleneck effect is still present and
about equally strong.Comment: Final version (7 pages
Semiclassical Calculation of Photon-Stimulated Schwinger Pair Creation
We consider the electron-positron pair creation by a photon in an external
constant electric field. The presented treatment is based on a purely
quasiclassical calculation of the imaginary part of the on-shell photon
polarization operator. By using this approach we find the pair production rate
for photons with polarization parallel as well as orthogonal to the external
electric field in the leading order in the parameter , which has
been recently found by other methods. For the orthogonal polarization we also
find a new contribution to the rate, which is leading in the ratio of the
photon energy to the electron mass . We also reproduce by a purely
geometrical calculation the exponential factor in the probability of the
stimulated pair creation at arbitrary energy of the photon.Comment: 16 pages, 4 figure
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