Numerical study of heat transfer in an optically thick semi-transparent spherical porous medium

In this paper, heat transfer by simultaneous convection, conduction and radiation in a semi-transparent spherical porous medium is investigated. The ROSSELAND approximation is adopted to take account of radiation in the heat transfer rate. The routine used here to solve the set of differential equations is taken from the IMSL MATH/LIBRARY. Various results are obtained for the dimensionless temperature profiles in the solid and fluid phases, the radiative, conductive, convective and total heat fluxes. The effects of some radiative properties of the medium on the heat transfer rate are examined.     

A spherical cavity in an Einstein Universe

Suitable metric forms for the regions inside and outside a spherical cavity in an Einstein universe are derived by means of perturbation. It is shown that for low proper pressure, the cavity behaves like negative Schwarzschild mass. Finally, the possibility of carrying over to the exact theory a proposed definition of the gravitational field in matter is examined.     

Computational approaches to aspect-ratio-dependent upper bounds and heat flux in porous medium convection

Direct numerical simulation (DNS) has shown that Rayleigh–Bénard convection in a fluid-saturated porous medium self-organizes into narrowly spaced plumes at (ostensibly) asymptotically high values of the Rayleigh number Ra. In this Letter a combination of DNS and upper bound theory is used to investigate the dependence of the Nusselt number Nu on the domain aspect ratio L at large Ra  . A novel algorithm is introduced to solve the optimization problems arising from the upper bound analysis, allowing for the best available bounds to be extended up to Ra≈2.65×10⁴$\mathit{Ra}\approx 2.65\times {10}^4$. The dependence of the bounds on L(Ra)$L(\mathit{Ra})$ is explored and a “minimal flow unit” is identified.     

Effect of the gravity on the photothermal waves in a semiconducting medium with an internal heat source and one relaxation time

The purpose of the present paper is to study the gravitational effect in a homogeneous isotropic semiconducting medium subjected to an internal heat source, based on Lord–Shulman theory. The problem is formulated in the dimensionless form and then solved analytically using the normal mode analysis method. The physical quantities are obtained and tested by a numerical study using the parameters of silicon as a target and presented graphically. The distribution of these quantities is represented graphically in the presence and absence of the internal heat source as well as gravity.     

Point source of polarized resonance radiation in an unbounded medium

The diffusion of polarized resonance radiation is considered. An explicit expression is given for the Green's function of the corresponding equation for an unbounded uniform space.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 53–58, April, 1974.     

Transient heat transfer in a spherical protective material, submitted to flux and mixed boundary conditions: an investigation on zirconia

An analytic solution is presented for describing combined radiation and conduction heat transfer in a spherical fiber thermal protection exposed to combined radiative and convective heating. The solution includes the equation of radiative transfer within the material, coupled to a transient energy equation that contains both radiative and convective terms. At elevated temperatures radiative transfer becomes important, and if several hot surfaces view each other, the radiation exchange process must be considered carefully. Some thermal protections are partially transparent to thermal radiation. Hence, an exchange process is complicated by radiation penetrating into and coming out of material. The radiation leaving an area depends on the temperature distribution inside that area and that is unknown and is affected by the exchange process to other areas. The analysis has allowed for unlimited spectral detail but assumes that the various material properties do not vary significantly with temperature. Transient temperature distributions are obtained for the boundary conditions of external radiation and convection. The present analysis includes the influence of reflectivity, surface radiative properties and spectral properties on the temperature distributions.     

Aeroelastic flutter of a disk rotating in an unbounded acoustic medium

The linear aeroelastic stability of an unbaffled flexible disk rotating in an unbounded fluid is investigated by modeling the disk-fluid system as a rotating Kirchhoff plate coupled to the irrotational motions of a compressible inviscid fluid. A perturbed eigenvalue formulation is used to compute systematically the coupled system eigenvalues. Both a semi-analytical and a numerical method are employed to solve the fluid boundary value problem. The semi-analytical approach involves a perturbation series solution of the dual integral equations arising from the fluid boundary value problem. The numerical approach is a boundary element method based on the Hadamard finite part. Unlike previous works, it is found that a disk with zero material damping destabilizes immediately beyond its lowest critical speed. Upon the inclusion of small disk material damping, the flutter speeds become supercritical and increase with decreasing fluid density. The competing effects of radiation damping into the surrounding fluid and disk material damping control the onset of flutter at supercritical speed. The results are expected to be relevant for the design of rotating disk systems in data storage, turbomachinery and manufacturing applications.     

Doppler-Rabi oscillations of a moving atom due to the roentgen interaction in a cavity

The dynamics of energy exchange between an atom moving in a high-Q cavity and the cavity field is analyzed by taking into account the Roentgen interaction. A two-level atom coupled to a Fock or coherent state of an optical or microwave cavity is considered. The mean cavity photon number required for high-frequency Doppler-Rabi oscillations to occur is relatively high for both Fock-and coherent-state cavity fields and increases with the atomic transition frequency. Conditions are found when the Roentgen interaction plays a key role in the Doppler-Rabi oscillations and must be taken into account, in addition to the conventional electric field-dipole interaction.     

Nonlinear dynamics of a pulse in an anisotropic medium with birefringence

The dynamics of a wave packet propagating in an anisotropic nonlinear optical fiber with strong linear coupling of the orthogonal components of the pulse is studied. The regimes of modulation instability and formation of soliton-like pulses are analyzed for different polarization angles of the incident light with allowance made for the first-and second-order dispersion effects and the nonlinear effects of self-and cross-modulation.     

Configuration of normal vibrations in an unbounded medium containing dislocations with allowance for energy dissipation

On the basis of the calibration model of plastically incompressible medium with dissipation we consider the structure of vibrations in an unbounded medium with dislocations. Inclusion of dissipation of weakly damped low-frequency vibrations of volume contraction and high-frequency longitudinal sound vibrations is shown to cause a considerable change in the structure of normal vibrations. For a certain dissipation weakly damped low-frequency vibrations of volume contraction and high-frequency longitudinal sound vibrations are described by one branch of the dispersion relation. The structure of the vibrations changes from liquid-like vibrations to longitudinal sound vibrations in the neighborhood of , which together with the boundary frequency ₀ of optical vibrations is characteristic. The calibration model including energy dissipation does not describe the Cosser liquid.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 40–44, March, 1995.     

Increasing the cold atom density in an integrating spherical cavity

A novel method of angled incidence for diffuse laser cooling of ⁸⁷Rb is presented to improve the distribution of cold atom density in an integrating sphere. The angled injection scheme could cool more atoms in the middle of the sphere compared to the previous normal injection scheme. The loading time of the cold atoms for the angled injection scheme is twice as that of the normal injection scheme. The SNR and the contrast of the detected signal would be improved in the angled injection scheme.     

Ultrashort light pulse generation under coherent interaction of a field with a medium

We consider the dynamics of propagation and compression of ultrashort electromagnetic-wave pulses in a multicomponent two-level field, one component of which emits or absorbs light in a regime of coherent interaction of the radiation with the substance. We consider also the question of the dynamics of the field in planar superconducting structures (such as, e.g., layered high-temperature superconductors of the YBa₂Cu₃O_7=x type). With Josephson tunneling of the electrons between the layers it becomes possible here to describe the field-induced current in a model using Bloch equations for particles with near-zero frequency of the transition between levels and with infinite relaxation times, i.e., to reduce the problem to that of the dynamics of a field coherently interacting with a two-level medium.Lebedev Physics Institute, Academy of Sciences of the USSR. University Center for Laser Research, Oklahoma State University. translated from Preprint No. 69, Lebedev Physics Institute, Moscow, 1991.     

High-repetition-rate coherent femtosecond pulse amplification with an external passive optical cavity

We demonstrate a general technique for enhancement of femtosecond pulses from a pulse train through their coherent buildup inside a high-finesse cavity. Periodic extraction of the intracavity pulse by means of a fast switch provides a net energy gain of 42 to >70 times for 38-58-fs pulse durations. Starting with an actively stabilized but otherwise standard mode-locked laser system, we demonstrate pulses of >200-nJ energy.     

Dyadic Green‘s function for an unbounded gyroelectric chiral medium in cylindrical coordinates

The dyadic Green‘s function for an unbounded gyroelectric chiral medium is formulated in the Fourier domain in cylindrical coordinates.After a triple Fourier integral of the dyadic Green‘s function has been obtained,the integral is reduced by performing the integration over the transverse Fourier variable.     

On calculation of the heat flux from a spherical particle in a molecular gas

The problem of calculation of the heat flux from a spherical particle in a molecular gas is considered. The results of numerical calculations for the collision integral similar to that in the BGK model are presented under the condition of purely diffuse reflection of gas molecules from the surface of a particle.     

Anomalous heat flux inhibition under high frequency radiation pulse absorption in electron-ion collisions

Anomalous heat flux inhibition under high frequency radiation pulse absorption in electron-ion collisions is revealed for plasma with multiply charged ions. The inhibition takes place in time moments greater than momentum relaxation time, but less than energy relaxation time of the electrons, which give main contribution to the heat flux value.