On the use of the Rosseland and Planck mean absorption coefficients in the non-equilibrium radiative transfer equation

The use of the Rosseland and Planck mean absorption coefficients in the non-equilibrium (time-dependent) radiative transfer equation is investigated in the gray approximation for a prescribed temperature distribution. The corresponding gray radiative flux at the surface of a halfspace is compared to an exact (multi-frequency) solution obtained from multiple collision theory. An improved mean absorption coefficient is then obtained using the exact angle integrated intensity at the surface, and the corresponding gray approximation is shown to be markedly accurate.     

Modelling radiative mean absorption coefficients

We define and compute mean absorption coefficients for the macroscopic models of radiative transfer. These coefficients take into account the anisotropic form of the photon emission and lead to a better computation of a photonic flow far from the radiative equilibrium. They are deduced by averaging a specific radiative intensity on the space of frequency and are generalized versions of the Planck means. This intensity is obtained by minimizing the mathematical entropy with the constraint of the reconstruction of radiative moments and constitutes the closure of the M ₁ radiative model. We discuss the influence of these coefficients, extend them to the case of multi-frequency problems and perform a numerical comparison with the former Planck mean.     

Drying processes in the presence of temperature gradients --Pore-scale modelling

The European Physical Journal E - The influence of temperature gradients on the drying of water-saturated porous networks has been studied. We have focussed on the influence of the temperature on...     

On the use temperature parameterized rate coefficients in the estimation of non-equilibrium reaction rates

The present paper considers a detailed analysis of the nonequilibrium effects for a model reactive system with the Chapman–Eskog (CE) solution of the Boltzmann equation as well as an explicit time dependent solution. The elastic cross sections employed are a hard sphere cross section and the Maxwell molecule cross section. Reactive cross sections which model reactions with and without activation energy are used. A detailed comparison is carried out with these solutions of the Boltzmann equation and the approximation introduced by Cukrowski and coworkers [J. Chem. Phys. 97 (1992) 9086; Chem. Phys. 89 (1992) 159; Physica A 188 (1992) 344; Chem. Phys. Lett. A 297 (1998) 402; Physica A 275 (2000) 134; Chem. Phys. Lett. 341 (2001) 585; Acta Phys. Polonica B 334 (2003) 3607.] based on the temperature of the reactive particles. We show that the Cukrowski approximation has limited applicability for the large class of reactive systems studied in this paper. The explicit time dependent solutions of the Boltzmann equation demonstrate that the CE approach is valid only for very slow reactions for which the corrections to the equilibrium rate coefficient are very small.     

Anomalous absorption of thermal neutrons in copper in the presence of strong mechanical stresses

A study has been made of the interaction between an ultraweak flux of thermal neutrons (I _n=20 n/s·cm²) and a copper cathode during electrolysis in a 1M solution of KOH in H₂O. An effect involving the generation of appreciable elastic and plastic deformations, revealed as transverse bending of the copper plate and damage to its surface, was observed in the copper. It was shown that the average cross section for absorption of thermal neutrons in the copper is increased approximately twenty times compared with its equilibrium value. Fiz. Tverd. Tela (St. Petersburg) 40, 254–259 (February 1998)     

Recoil-free resonant gamma-ray absorption in57Fe nuclei in the presence of a strong microwavefield

A calculation of the cross-section of resonant gamma-ray absorption in the presence of strong microwave fields has been performed. This calculation is based on papers published earlier [1-3], in which the possibility of nuclear recoil compensation at the absorption of gamma-rays, by means of optical photons, was considered. Preliminary experimental results concerning there coil-free gamma absorption in the presence of a 3 GHz strong microwave field are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

On collision duration in the presence of strong overlapping resonance levels

The time delay of wave packets in the case of strong overlapping resonances is discussed. It is shown that the duration of collision at very large level density is much larger than the lifetime of a separate level. It has a value of the order of 2π?/nD, where n is a number of open channels and D is the average level spacing. As a consequence, if the width Γ ? nD, the correlation length of cross section fluctuations must be of the order of nD but not Γ.     

Interference absorption coefficient of X-rays in crystals in the presence of temperature gradient

We investigate experimentally and theoretically the behavior of the interference coefficient of absorption of X-ray radiation of a quartz single crystal in Laue geometry in the presence of temperature gradient. The total intensity of transmitted X-ray radiation and that reflected from different families of reflecting atomic planes of the quartz single crystal has been recorded. It was shown that with the increase in temperature gradient the coefficient of absorption of X-ray radiation decreases abruptly reaching its minimal value at a certain value of the temperature gradient.     

Optical absorption in a narrow-band InSb cylindrical layered nanowire in the presence of strong electrostatic field

On the example of InSb we consider the influence of a strong homogeneous electrostatic field on the state of carriers in a narrow-band quantized cylindrical layer in the case of Kaine dispersion law. Explicit expressions for the energy spectrum and wave function envelopes of charge carriers in the heterolayer are obtained. It is shown that under action of the strong external field the rotational motion, with respect to angular variables, of charge carriers becomes oscillatory in a narrow cone of the azimuth angle. We have obtained corresponding selection rules and calculated the threshold frequencies of absorption for interband and intraband electro-optical transitions in the layer. The threshold frequencies are determined by the geometric sizes of the sample and the magnitude of the external field.     

On the temperature dependence of the mean number of clusters

The random variable number of cluster defined on the configurations of a ferromagnetic Ising model at zero field and inverse temperature on a graphG is considered. The Gibbs average at=0 is proved to be greater than the one at>0 if the degree ofG is not greater than 3.     

On the acoustic performance of rectangular splitter silencers in the presence of mean flow

Dissipative splitter silencers are often used to reduce the noise emitted in ventilation and gas turbine systems. It is well known that the acoustic performance of a splitter silencer changes under the influence of the convective effects of a mean gas flow and so in this article a theoretical model is developed to include the effects of mean flow. The theoretical model is based on a hybrid finite element method which enables the inclusion of bull nose fairings and a perforated screen separating the mean gas flow from a bulk reacting porous material. Predictions are compared against experimental measurements obtained both with and without mean flow. Good agreement between prediction and measurement is generally observed in the absence of mean flow, although it is seen that for silencers with a low percentage open area the silencer insertion loss is over predicted at higher frequencies. When mean flow is present, problems with the experimental methodology are observed at relatively modest mean flow velocities, and so comparison between prediction and experiment is limited to relatively low face velocities. However, experiment and theory both show that the insertion loss reduces at low frequencies when mean flow is in the direction of sound propagation, and at high frequencies the influence of mean flow is generally much smaller. Following additional theoretical investigations it is concluded that the influence of mean flow on splitter silencer performance should be accounted for at low frequencies when silencer airway velocities are greater than about 20 m/s; however, at higher frequencies one may generally neglect the effect of mean flow, even at higher velocities. Predictions obtained using the hybrid method are also compared to a simplified point collocation approach and it is demonstrated that the computationally efficient point collocation method may be used to investigate the effects of mean flow in a splitter silencer without loss of accuracy.     

Analysis op mean statistical spectral dependences of coefficients of aerosol absorption in the range 0.59–10 μm

A comparison of the mean statistical coefficients of aerosol absorption in the transparency windows of the atmosphere, obtained by the authors, with the results of the corresponding measurements known from published sources indicates the feasibility of obtaining data for optical weather prognosis at different geographical locations.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 10, pp. 103–106, October, 1972.     

Transmission coefficients in the presence of multistep-direct reactions

Formal expressions for transmission coefficients governing the multistep-compound process and introduced in an earlier paper, are explicitly worked out, using the idea of a multistep-direct process populating the compound nucleus. A perturbative treatment of inelastic scattering, combined with a “never come back” and the sudden approximation, and a statistical average over exit channels lead to simple, intuitively appealing expressions which contain shell-model type matrix elements and partial level densities and are therefore amenable to numerical evaluation.     

Electron states and light absorption in a thin ellipsoidal quantum lens in the presence of a strong magnetic field

Within the framework of adiabatic approximation the energy levels and direct interband light absorption in a thin ellipsoidal quantum lens in the presence of a magnetic field are studied. Analytical expressions for the particle energy spectrum and for the absorption threshold frequencies in the regime of strong size quantization for the cases of ellipsoidal and spherical cross-section segments are obtained. Selection rules for quantum transitions are revealed.     

Temperature scaling of absorption coefficients

A recently published conversion of room temperature absorption coefficients to absorption cross-sections at standard temperature, may lead to large errors due to inadequate temperature scaling. Specific examples are given and the general relation for temperature scaling of absorption coefficients is derived. Application is presented to temperature profiling of the atmospheres using differential absorption lidar measurements.     

Two-photon interference in the presence of absorption

Experiments on two-photon interference are discussed in the case when there is absorption of all the modes participating in the process of spontaneous parametric down-conversion (SPDC) of light. The objects of investigation are 10-to 80-Å-thick ultrathin gold films deposited on fused-silica substrates. Conditions are determined under which the effect of absorption of the signal and pump waves on the interference pattern is small. It is shown that, under these conditions, the visibility of the interference pattern and the shape of the frequency-angular spectrum at the signal frequency are determined by the optical parameters of the medium at the idler frequency, which belongs to the near-infrared region.     

On the temperature variations of the integrated absorption intensity in the oxygen fundamental

The goal of this note is to comment on the unusual temperature variations of the integrated absorption intensity measured recently in the region of the oxygen fundamental. More accurate than previous ones and covering wider temperature interval, the observations by Baranov and Lafferty showed that the integrated intensity reaches its minimum at roughly 280 K and then increases steadily at elevated temperature. We suggest that this behaviour can be understood in terms of the flatness of intermolecular potential accompanied by the dominance of the absorption inducing collisions at extremely short distances corresponding to repulsive interaction.     

Spontaneous symmetry breaking in the presence of strong fields

We analyse the properties of spontaneous symmetry breaking in the presence of external sources, concentrating on the question whether these sources can lead to a change of the vacuum expectation value of the Higgs field. We discuss in particular how the meaning of the Higgs-Kibble mechanisms is modified by such a vacuum change. The consequences of the presence of a strong electric charge are investigated in the framework of the Glashow-Salam-Weinberg model. Finally we discuss the possibility of actually observing this process.