75,631 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
Universal local pair correlations of Lieb-Liniger bosons at quantum criticality
The one-dimensional Lieb-Liniger Bose gas is a prototypical many-body system
featuring universal Tomonaga-Luttinger liquid (TLL) physics and free fermion
quantum criticality. We analytically calculate finite temperature local pair
correlations for the strong coupling Bose gas at quantum criticality using the
polylog function in the framework of the Yang-Yang thermodynamic equations. We
show that the local pair correlation has the universal value in the quantum critical regime, the TLL phase and the
quasi-classical region, where is the pressure per unit length rescaled by
the interaction energy with interaction
strength and linear density . This suggests the possibility to test
finite temperature local pair correlations for the TLL in the relativistic
dispersion regime and to probe quantum criticality with the local correlations
beyond the TLL phase. Furthermore, thermodynamic properties at high
temperatures are obtained by both high temperature and virial expansion of the
Yang-Yang thermodynamic equation.Comment: 8 pages, 6 figures, additional text and reference
Climate Response to Negative Greenhouse Gas Radiative Forcing in Polar Winter
Greenhouse gas (GHG) additions to Earthâs atmosphere initially reduce global outgoing longwave radiation, thereby warming the planet. In select environments with temperature inversions, however, increased GHG concentrations can actually increase local outgoing longwave radiation. Negative top of atmosphere and effective radiative forcing (ERF) from this situation give the impression that local surface temperatures could cool in response to GHG increases. Here we consider an extreme scenario in which GHG concentrations are increased only within the warmest layers of winter nearâsurface inversions of the Arctic and Antarctic. We find, using a fully coupled Earth system model, that the underlying surface warms despite the GHG addition exerting negative ERF and cooling the troposphere in the vicinity of the GHG increase. This unique radiative forcing and thermal response is facilitated by the high stability of the polar winter atmosphere, which inhibit thermal mixing and amplify the impact of surface radiative forcing on surface temperature. These findings also suggest that strategies to exploit negative ERF via injections of shortâlived GHGs into inversion layers would likely be unsuccessful in cooling the planetary surface.Key PointsIncreased GHG concentrations in polar inversion layers cause negative top of atmosphere instantaneous and effective radiative forcingPolar and global surface temperatures warm despite this negative radiative forcingSurface warming and tropospheric cooling result from high stability and increased surface downwelling longwave fluxPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142965/1/grl56994_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142965/2/grl56994.pd
Information on the Pion Distribution Amplitude from the Pion-Photon Transition Form Factor with the Belle and BaBar Data
The pion-photon transition form factor (TFF) provides strong constraints on
the pion distribution amplitude (DA). We perform an analysis of all existing
data (CELLO, CLEO, BaBar, Belle) on the pion-photon TFF by means of light-cone
pQCD approach in which we include the next-to-leading order correction to the
valence-quark contribution and estimate the non-valence-quark contribution by a
phenomenological model based on the TFF's limiting behavior at both
and . At present, the pion DA is not definitely determined, it is
helpful to have a pion DA model that can mimic all the suggested behaviors,
especially to agree with the constraints from the pion-photon TFF in whole
measured region within a consistent way. For the purpose, we adopt the
conventional model for pion wavefunction/DA that has been constructed in our
previous paper \cite{hw1}, whose broadness is controlled by a parameter . We
fix the DA parameters by using the CELLO, CLEO, BABAR and Belle data within the
smaller region ( GeV), where all the data are consistent
with each other. And then the pion-photon TFF is extrapolated into larger
region. We observe that the BABAR favors which has the behavior close
to the Chernyak-Zhitnitsky DA, whereas the recent Belle favors which
is close to the asymptotic DA. We need more accurate data at large region
to determine the precise value of , and the definite behavior of pion DA can
be concluded finally by the consistent data in the coming future.Comment: 6 pages, 5 figures. Slightly changed and references update
The trispectrum in ghost inflation
We calculate the trispectrum in ghost inflation where both the contact
diagram and scale-exchange diagram are taken into account. The shape of
trispectrum is discussed carefully and we find that the local form is absent in
ghost inflation. In general, for the non-local shape trispectrum there are not
analogous parameters to and which can
completely characterize the size of local form trispectrum.Comment: 19 pages, 8 figures; clarifications and corrections added, version
accepted for publication in JCA
Second-order equation of state with the full Skyrme interaction: toward new effective interactions for beyond mean-field models
In a quantum Fermi system the energy per particle calculated at the second
order beyond the mean-field approximation diverges if a zero-range interaction
is employed. We have previously analyzed this problem in symmetric nuclear
matter by using a simplified nuclear Skyrme interaction, and proposed a
strategy to treat such a divergence. In the present work, we extend the same
strategy to the case of the full nuclear Skyrme interaction. Moreover we show
that, in spite of the strong divergence ( , where is
the momentum cutoff) related to the velocity-dependent terms of the
interaction, the adopted cutoff regularization can be always simultaneously
performed for both symmetric and nuclear matter with different
neutron-to-proton ratio. This paves the way to applications to finite nuclei.Comment: 15 figure
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