109,893 research outputs found
Experimental study of the formation and collapse of an overhang in the lateral spread of smouldering peat fires
Smouldering combustion is the driving phenomenon of wildfires in peatlands, and is responsible for large amounts of carbon emissions and haze episodes world wide. Compared to flaming fires, smouldering is slow, low-temperature, flameless, and most persistent, yet it is poorly understood. Peat, as a typical organic soil, is a porous and charring natural fuel, thus prone to smouldering. The spread of smouldering peat fire is a multidimensional phenomenon, including two main components: in-depth vertical and surface lateral spread. In this study, we investigate the lateral spread of peat fire under various moisture and wind conditions. Visual and infrared cameras as well as a thermocouple array are used to measure the temperature profile and the spread rate. For the first time the overhang, where smouldering spreads fastest beneath the free surface, is observed in the laboratory, which helps understand the interaction between oxygen supply and heat losses. The periodic formation and collapse of overhangs is observed. The overhang thickness is found to increase with moisture and wind speed, while the spread rate decreases with moisture and increases with wind speed. A simple theoretical analysis is proposed and shows that the formation of overhang is caused by the spread rate difference between the top and lower peat layers as well as the competition between oxygen supply and heat losses
Agegraphic Chaplygin gas model of dark energy
We establish a connection between the agegraphic models of dark energy and
Chaplygin gas energy density in non-flat universe. We reconstruct the potential
of the agegraphic scalar field as well as the dynamics of the scalar field
according to the evolution of the agegraphic dark energy. We also extend our
study to the interacting agegraphic generalized Chaplygin gas dark energy
model.Comment: 8 page
The Electromagnetically Induced Transparency in Mechanical Effects of Light
We consider the dynamical behavior of a nanomechanical mirror in a
high-quality cavity under the action of a coupling laser and a probe laser. We
demonstrate the existence of the analog of electromagnetically induced
transparency (EIT) in the output field at the probe frequency. Our calculations
show explicitly the origin of EIT-like dips as well as the characteristic
changes in dispersion from anomalous to normal in the range where EIT dips
occur. Remarkably the pump-probe response for the opto mechanical system shares
all the features of the Lambda system as discovered by Harris and
collaborators.Comment: 4 pages, 5 figure
A Generic Dynamical Model of Gamma-ray Burst Remnants
The conventional generic model is deemed to explain the dynamics of
-ray burst remnants very well, no matter whether they are adiabatic or
highly radiative. However, we find that for adiabatic expansion, the model
could not reproduce the Sedov solution in the non-relativistic phase, thus the
model needs to be revised. In the present paper, a new differential equation is
derived. The generic model based on this equation has been shown to be correct
for both radiative and adiabatic fireballs, and in both ultra-relativistic and
non-relativistic phase.Comment: 10 pages, LaTeX, 4 postscript figures, accepted for publication in
MNRA
Optical Flashes and Very Early Afterglows in Wind Environments
The interaction of a relativistic fireball with its ambient medium is
described through two shocks: a reverse shock that propagates into the
fireball, and a forward shock that propagates into the medium. The observed
optical flash of GRB 990123 has been considered to be the emission from such a
reverse shock. The observational properties of afterglows suggest that the
progenitors of some GRBs may be massive stars and their surrounding media may
be stellar winds. We here study very early afterglows from the reverse and
forward shocks in winds. An optical flash mainly arises from the relativistic
reverse shock while a radio flare is produced by the forward shock. The peak
flux densities of optical flashes are larger than 1 Jy for typical parameters,
if we do not take into account some appropriate dust obscuration along the line
of sight. The radio flare always has a long lasting constant flux, which will
not be covered up by interstellar scintillation. The non-detections of optical
flashes brighter than about 9th magnitude may constrain the GRBs isotropic
energies to be no more than a few ergs and wind intensities to be
relatively weak.Comment: 21 pages, 6 figures, accepted by MNRAS on March 7, 200
Electromagnetically Induced Transparency from Two Phonon Processes in Quadratically Coupled Membranes
We describe how electromagnetically induced transparency can arise in
quadratically coupled optomechanical systems. Due to quadratic coupling the
underlying optical process involves a two phonon process in optomechanical
system and this two phonon process makes the mean amplitude, which plays the
role of atomic coherence in traditional EIT, zero. We show how the fluctuation
in displacement can play a role similar to atomic coherence and can lead to
EIT-like effects in quadratically coupled optomechanical systems. We show how
such effects can be studied using the existing optomechanical systems.Comment: 5 pages,4 figure
Can reactive coupling beat motional quantum limit of nano waveguides coupled to microdisk resonator
Dissipation is generally thought to affect the quantum nature of the system
in an adverse manner, however we show that dissipatively coupled nano systems
can be prepared in states which beat the standard quantum limit of the
mechanical motion. We show that the reactive coupling between the waveguide and
the microdisk resonator can generate the squeezing of the waveguide by
injecting a quantum field and laser into the resonator through the waveguide.
The waveguide can show about 70--75% of maximal squeezing for temperature about
1--10 mK. The maximum squeezing can be achieved with incident pump power of
only 12 W for a temperature of about 1 mK. Even for temperatures of 20 mK,
achievable by dilution refrigerators, the maximum squeezing is about 60%.Comment: 6 pages,2 figure
Transverse momentum dependence in the perturbative calculation of pion form factor
By reanalysing transverse momentum dependence in the perturbative calculation
of pion form factor an improved expression of pion form factor which takes into
account the transverse momentum dependenc in hard scattering amplitude and
intrinsic transverse momentum dependence associated with pion wave functions is
given to leading order, which is available for momentum transfers of the order
of a few GeV as well as for . Our scheme can be extended to
evaluate the contributions to the pion form factor beyond leading order.Comment: 13 pages in LaTeX, plus 3 Postscript figure
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