23,695 research outputs found
Second Law Violations in Lovelock Gravity for Black Hole Mergers
We study the classical second law of black hole thermodynamics, for Lovelock
theories (other than General Relativity), in arbitrary dimensions. Using the
standard formula for black hole entropy, we construct scenarios involving the
merger of two black holes in which the entropy instantaneously decreases. Our
construction involves a Kaluza-Klein compactification down to a dimension in
which one of the Lovelock terms is topological. We discuss some open issues in
the definition of the second law which might be used to compensate this entropy
decrease.Comment: 15 pages, 1 figure, v2 Title change & minor revisions to match
published version, v3 fixed accidental deletion of author name
Reconstruction subgrid models for nonpremixed combustion
Large-eddy simulation of combustion problems involves highly nonlinear terms that, when filtered, result in a contribution from subgrid fluctuations of scalars, Z, to the dynamics of the filtered value. This subgrid contribution requires modeling. Reconstruction models try to recover as much information as possible from the resolved field Z, based on a deconvolution procedure to obtain an intermediate field ZM. The approximate reconstruction using moments (ARM) method combines approximate reconstruction, a purely mathematical procedure, with additional physics-based information required to match specific scalar moments, in the simplest case, the Reynolds-averaged value of the subgrid variance. Here, results from the analysis of the ARM model in the case of a spatially evolving turbulent plane jet are presented. A priori and a posteriori evaluations using data from direct numerical simulation are carried out. The nonlinearities considered are representative of reacting flows: power functions, the dependence of the density on the mixture fraction (relevant for conserved scalar approaches) and the Arrhenius nonlinearity (very localized in Z space). Comparisons are made against the more popular beta probability density function (PDF) approach in the a priori analysis, trying to define ranges of validity for each approach. The results show that the ARM model is able to capture the subgrid part of the variance accurately over a wide range of filter sizes and performs well for the different nonlinearities, giving uniformly better predictions than the beta PDF for the polynomial case. In the case of the density and Arrhenius nonlinearities, the relative performance of the ARM and traditional PDF approaches depends on the size of the subgrid variance with respect to a characteristic scale of each function. Furthermore, the sources of error associated with the ARM method are considered and analytical bounds on that error are obtained
Summary of the NOW'98 Phenomenology Working Group
Summary of the Phenomenology Working Group at the Europhysics Neutrino
Oscillation Workshop (NOW'98), 7-9 September 1998, Amsterdam.Comment: 66 page
Recommended from our members
Do arsenic levels in rice pose a health risk to the UK population?
Consumption of rice and rice products can be a significant exposure pathway to inorganic arsenic (iAs), which is a group 1 carcinogen to humans. The UK follows the current European Commission regulations so that iAs concentrations must be 0.1 mg kg-1 were selected for As speciation using HPLC-ICP-MS. Based on the average concentration of iAs of our samples, we calculated values for the Lifetime Cancer Risk (LCR), Target Hazard Quotient (THQ) and Margin of Exposure (MoE). We found a statistically significant difference between organically and non-organically grown rice. We also found that brown rice contained a significantly higher concentration of iAs compared to white or wild rice. Notably, 28 rice samples exceeded the iAs maximum limit stipulated by the EU (0.1 mg kg-1) with an average iAs concentration of 0.13 mg kg-1; therefore consumption of these rice types could be riskier for infants than adults. Based on the MoE, it was found that infants up to 1 year must be restricted to a maximum of 20 g per day for the 28 rice types to avoid carcinogenic risks. We believe that consumers could be better informed whether the marketed product is fit for infants and young children, via appropriate product labelling containing information about iAs concentration
Hawking radiation as tunneling from squashed Kaluza-Klein black hole
We discuss Hawking radiation from a five-dimensional squashed Kaluza-Klein
black hole on the basis of the tunneling mechanism. A simple manner, which was
recently suggested by Umetsu, is possible to extend the original derivation by
Parikh and Wilczek to various black holes. That is, we use the two-dimensional
effective metric, which is obtained by the dimensional reduction near the
horizon, as the background metric. By using same manner, we derive both the
desired result of the Hawking temperature and the effect of the back reaction
associated with the radiation in the squashed Kaluza-Klein black hole
background.Comment: 16 page
Critical points in the Bragg glass phase of a weakly pinned crystal of CaRhSn
New experimental data are presented on the scan rate dependence of the
magnetization hysteresis width ( critical current
density ) in isothermal scans in a weakly pinned single crystal
of CaRhSn, which displays second magnetization peak (SMP)
anomaly as distinct from the peak effect (PE). We observe an interesting
modulation in the field dependence of a parameter which purports to measure the
dynamical annealing of the disordered bundles of vortices injected through the
sample edges towards the destined equilibrium vortex state at a given .
These data, in conjunction with the earlier observations made while studying
the thermomagnetic history dependence in in the tracing of the minor
hysteresis loops, imply that the partially disordered state heals towards the
more ordered state between the peak field of the SMP anomaly and the onset
field of the PE. The vortex phase diagram in the given crystal of
CaRhSn has been updated in the context of the notion of the
phase coexistence of the ordered and disordered regions between the onset field
of the SMP anomaly and the spinodal line located just prior to the
irreversibility line. A multi-critical point and a critical point in the
() region of the Bragg glass phase have been marked in this phase diagram
and the observed behaviour is discussed in the light of recent data on
multi-critical point in the vortex phase diagram in a single crystal of Nb.Comment: To appear in Current trends in Vortex State Studies - Pramana J.
Physic
Temperature dependence of the band gap shrinkage due to electron-phonon interaction in undoped n-type GaN
The photoluminescence spectra of band-edge transitions in GaN is studied as a
function of temperature. The parameters that describe the temperature
dependence red-shift of the band-edge transition energy and the broadening of
emission line are evaluated using different models. We find that the
semi-empirical relation based on phonon-dispersion related spectral function
leads to excellent fit to the experimental data. The exciton-phonon coupling
constants are determined from the analysis of linewidth broadening
New Exactly Solvable Model of Strongly Correlated Electrons Motivated by High T_c Superconductivity
We present a new model describing strongly correlated electrons on a general
-dimensional lattice. It differs from the Hubbard model by interactions of
nearest neighbours, and it contains the - model as a special case. The
model naturally describes local electron pairs, which can move coherently at
arbitrary momentum. By using an -pairing mechanism we can construct
eigenstates of the hamiltonian with off-diagonal-long-range-order (ODLRO).
These might help to relate the model to high- superconductivity. On a
one-dimensional lattice, the model is exactly solvable by Bethe Ansatz.Comment: 10 pages, using latex, Phys.Rev.Lett. 68 (1992) 296
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