Low-Frequency Radiation from an Axisymmetric Current in an Ionospheric Plasma

We consider the radiation of low-frequency electromagnetic waves from an axisymmetric current and propagation of these waves in a homogeneous ionospheric plasma with Hall and Pedersen conductivities. We obtain an analytical expressions for the fields excited by pulsed radial and ring currents. It is shown that electromagnetic radiation propagates as a wave inside a cone along the magnetic field. The field propagation outside the cone is described by a diffusion law. The spatio-temporal characteristics of the signal are determined by the ionospheric conductivities.     

Upwind monotonic interpolation methods for the solution of the time dependent radiative transfer equation

We describe the use of upwind monotonic interpolation methods for the solution of the time-dependent radiative transfer equation in both optically thin and thick media. These methods, originally developed to solve Eulerian advection problems in hydrodynamics, have the ability to propagate sharp features in the flow with very little numerical diffusion. We consider the implementation of both explicit and implicit versions of the method. The explicit version is able to keep radiation fronts resolved to only a few zones wide when higher order interpolation methods are used. Although traditional implementations of the implicit version suffer from large numerical diffusion, we describe an implicit method which considerably reduces this diffusion.     

The non-equilibrium Marshak wave problem

An analytic solution to a particular Marshak wave problem is given. The radiative transfer model used is the gray, non-equilibrium diffusion approximation which allows the radiation and material fields to be out of equilibrium. This solution should be useful as a reference problem for validating time-dependent radiative transfer computer codes, as well as investigating the convergence, as a function of space and time step size, for such codes. The coupling of the radiation field to the material field in a multigroup code, a difficult numerical problem, can also be tested against this solution. Typical numerical results are given for surface quantities, integral quantities, and the distribution of radiative energy and material temperature as a function of space and time.     

Characteristic parameters of diffusive supersonic radiation transport in low density materials

Diffusive heat waves play an important role in radiation hydrodynamics. In low density material, it may be possible that the radiative energy flux dominates the material energy flux and thus energy flow can be determined. In this paper by means of a simple algebraic method, the expressions characterizing the condition of diffusion approximation and supersonic transport of heat wave are found. In this case, the ratio of the radiative energy flux to the material energy flux is directly proportional to the product of Mach number M multiplied by optical depth \tau. And it may also be expressed by radiation temperature heating material. The material density and length may be determined in order to achieve above-mentioned conditions when the driven temperature and duration are given.     

地下强爆炸辐射流体动力学过程的二维数值模拟

为评估强爆炸的毁伤效应,利用流固耦合界面处理方法,考虑辐射输运引起的能量沉积过程,建立同时求解辐射流体力学方程和流体弹塑性方程的二维耦合计算方法,给出地下强爆炸时,爆室内辐射波传播、辐射能量沉积到岩土表层、岩土表层膨胀、冲击波在岩土介质中传播、爆室内高压压缩爆室壁、爆室壁振荡增大的完整物理过程.     

含双圆柱亚表面缺陷板条材料的表面温度分布

基于非傅里叶热传导方程,采用复变函数法和镜像法,研究了含双圆柱亚表面缺陷板条材料热波散射的温度场,并给出了热波散射温度场的解析解。分析了入射波波数、热扩散长度、缺陷的埋藏深度以及板条材料的厚度等对板条表面温度分布的影响。温度波由调制光束在材料表面激发,缺陷表面的边界条件为绝热。该分析方法和数值结果可为工程材料结构的传热分析、热波成像和材料内部缺陷评估,以及热物理反问题研究提供参考。     

Floquet-bloch waves in bound one-dimensional photonic crystals

Floquet-Bloch waves in a bound one-dimensional photonic crystal are considered. It is shown that a single Floquet-Bloch wave can be excited in a one-dimensional photonic crystal located between two homogeneous media. An exact solution of the wave equation corresponding to this case is represented in the form of a set of heterogeneous waves. For the case of incidence of the plane wave from a homogeneous medium on a bounded one-dimensional photonic crystal, the functions for the reflection and transmission coefficients are obtained based on the exact solution of the wave equation. It is shown that the transmission function for the one-dimensional photonic crystal has a form similar to that for the traditional Fabri-Perot interferometer and is determined by the interference of the Floquet-Bloch waves excited in the crystal. The evolution of the amplitude profile of the decaying Floquet-Bloch waves and the reflection spectrum of the bounded one-dimensional photonic crystal are considered in the first-order forbidden band.     

Excitation of an Additional Region of Short-Wavelength Plasma Oscillations in Ionospheric Heating Experiments

We discuss transfer of plasma waves, excited by a powerful radio wave due to its scattering on artificial ionospheric irregularities, into an additional region of very short plasma oscillations polarized almost perpendicular to the magnetic field. Such a region can exist in the magnetized ionospheric plasma due to the strong spatial dispersion. We take into account the plasma-wave diffusion over the spectrum caused by multiple scattering on irregularities, as well as the nonlinear process of plasma-wave interaction due to induced scattering by ions. The latter process leads to the transfer of primary plasma waves into the additional region. The induced scattering is considered in the differential approximation valid for sufficiently smooth plasma-wave spectra. The numerical calculations are performed for a Maxwellian plasma in which suprathermal electrons are absent. It is shown that in this case, the additional region of plasma waves is excited if the pump frequency is close to but slightly less than the fourth electron gyroharmonic, so that the absorption of primarily excited plasma waves becomes sufficiently strong. Application of our calculations to the results of ionospheric experiments is discussed.     

辐射传输的格子Boltzmann模拟

辐射动力学理论是描述辐射传输是重要手段,基于此,本文建立了辐射能和辐射动量守恒方程,并基于Chapman-Enskog多尺度展开方法实现了从辐射传输Boltzmann方程到宏观方程的推导,进而建立了适于一维辐射传输的2分量格子Boltzmann模型。数值结果与精确解吻合较好,表明本文提出的LBM方法具有很好的准确性和稳...     

The influence of numerical diffusion on the solution of the radiative transfer equations

We investigate the explicit numerical solution strategies of multi-dimensional radiative transfer equations which are commonly used, e.g., to determine the radiation emerging from astrophysical objects surrounded by absorbing and scattering matter. For explicit grid solvers, we identify numerical diffusion as a severe source of error in first-order discretization schemes, underestimated in former work about radiative transfer. Using the simple example of a beam propagating through vacuum, we illustrate the influence of the diffusion on the solution and discuss various techniques to reduce it. In view of the large required storage for implicit solvers, we propose to use second-order explicit grid techniques to solve 3D radiative transfer problems.     

Travelling waves in a radiation-combustion free-boundary model for flame propagation

We study a combustion-radiation model which models premixed flames propagating in a gaseous mixture with inert dust. This model combines diffusion of mass and temperature with reaction at the flame front. We choose a free boundary model to describe the propagating flames and take a linearized approximation to model the radiation, but we keep a nonlinear reaction term which is temperature dependent. The radiative transfer of thermal energy emitted and absorbed by dust is modelled using the Eddington equation. We analyse the bifurcation diagram of the travelling wave solution curve. In a specific parameter plane, travelling waves are given by a single smooth curve which is parameterized by the flame temperature.     

Lagrange solution for three wavelength solitary wave clusters in nematic liquid crystals

We investigate the interaction of three optical solitary waves propagating with angular momentum in bulk nematic liquid crystals. The resulting cluster of solitary waves, or nematicons, is shown to orbit about its common centre of “mass”. An elongated isosceles triangle configuration is derived, this solution being the equivalent of the Lagrange solution of Newtonian gravitation. This triangle solution is found to be stable owing to diffractive radiation. A modulation theory explains the existence of the triangle solution as due to the non-monotonicity of an effective potential for the interaction of the solitary waves. This modulation theory also gives good agreement with numerical solutions for the trajectories of the nematicons in the three colours. Finally, it is shown that a cut-off in the shed diffractive radiation prevents the break-up of the triangle due to radiative losses.     

A high-order discontinuous Galerkin method for wave propagation through coupled elastic–acoustic media

We introduce a high-order discontinuous Galerkin (dG) scheme for the numerical solution of three-dimensional (3D) wave propagation problems in coupled elastic–acoustic media. A velocity–strain formulation is used, which allows for the solution of the acoustic and elastic wave equations within the same unified framework. Careful attention is directed at the derivation of a numerical flux that preserves high-order accuracy in the presence of material discontinuities, including elastic–acoustic interfaces. Explicit expressions for the 3D upwind numerical flux, derived as an exact solution for the relevant Riemann problem, are provided. The method supports h-non-conforming meshes, which are particularly effective at allowing local adaptation of the mesh size to resolve strong contrasts in the local wavelength, as well as dynamic adaptivity to track solution features. The use of high-order elements controls numerical dispersion, enabling propagation over many wave periods. We prove consistency and stability of the proposed dG scheme. To study the numerical accuracy and convergence of the proposed method, we compare against analytical solutions for wave propagation problems with interfaces, including Rayleigh, Lamb, Scholte, and Stoneley waves as well as plane waves impinging on an elastic–acoustic interface. Spectral rates of convergence are demonstrated for these problems, which include a non-conforming mesh case. Finally, we present scalability results for a parallel implementation of the proposed high-order dG scheme for large-scale seismic wave propagation in a simplified earth model, demonstrating high parallel efficiency for strong scaling to the full size of the Jaguar Cray XT5 supercomputer.     

Rotating spiral waves in excitable media: The analytical results

In this paper we calculate the rotation frequency of spiral waves as a function of the parameters of the excitable medium. We give the complete analytical solution in the special case of the Rinzel-Keller model and suggest an analytical procedure for general two-component relaxational reaction-diffusion models. This procedure is based on the Greenberg equation and stability analysis; it is applicable when the core of a spiral wave is large as compared with the characteristic diffusion length. Construction of spiral wave solutions for the waves inside ring channels, circular regions, or around holes is discussed. Multi-armed spiral waves are investigated.     

A numerical model for multigroup radiation hydrodynamics

We present in this paper a multigroup model for radiation hydrodynamics to account for variations of the gas opacity as a function of frequency. The entropy closure model (M₁) is applied to multigroup radiation transfer in a radiation hydrodynamics code. In difference from the previous grey model, we are able to reproduce the crucial effects of frequency-variable gas opacities, a situation omnipresent in physics and astrophysics. We also account for the energy exchange between neighbouring groups which is important in flows with strong velocity divergence. These terms were computed using a finite volume method in the frequency domain. The radiative transfer aspect of the method was first tested separately for global consistency (reversion to grey model) and against a well-established kinetic model through Marshak wave tests with frequency-dependent opacities. Very good agreement between the multigroup M₁ and kinetic models was observed in all tests. The successful coupling of the multigroup radiative transfer to the hydrodynamics was then confirmed through a second series of tests. Finally, the model was linked to a database of opacities for a Xe gas in order to simulate realistic multigroup radiative shocks in Xe. The differences with the previous grey models are discussed.     

On accounting for the effect of particles of a condensed dispersed phase on radiant energy transfer in gaseous media

Questions concerning the formation of the optical properties of dense gaseous and plasma media in relation to the specific features of radiant energy transfer are considered. The integral equations describing the radiation trapping are investigated as a new class of generalized wave equations of Schrödinger type. Starting from the methods of quantum mechanics, original analytical and numerical approaches are suggested for solving problems of the radiative kinetics of both spatially homogeneous and inhomogeneous absorbing media containing dispersed particles. In terms of the quasi-classical approximation, two classes of reference problems for determination of phase factors are formulated. Solutions for a number of model problems are presented that demonstrate the efficiency of the methods developed.     

Nichtgleichgewichtsdichten inhomogener Plasmen mit Photoabsorption

The electron and population densities in nonequilibrium plasmas are computed. For these computations rate equations dependent on geometrical dimensions were used. Such rate equations result if the unknown radiation intensity is eliminated. This is achived by using the radiative transport equation. Our density computations are valid for a quasi neutral plasma. The total pressure or the density of the heavy particles and the distribution of the electron energy have to be assumed. For homogeneous plasmas the inelastic collision processes and the radiation processes are considered. For inhomogeneous plasmas the diffusion processes of the excited atoms are considered in addition to the collision and radiation processes. Using this method of calculation the electron and population densities of a cesium plasma are computed. We assumed maxwellian distributions and a total pressure of 1 Torr.     

辐射扩散计算方法若干研究进展

辐射流体力学研究辐射的传输对流体运动的影响,并在此条件下研究流体的运动规律.实际应用问题中辐射流体力学所描述的是非常复杂的物理过程,数值模拟是主要的研究手段之一.模拟通常采用流体计算和辐射计算分裂求解的方法.讨论求解辐射扩散方程时迫切需要解决的一些计算方法问题,包括大变形网格上扩散计算格式与非线性迭代方法,并简要介绍部分研究进展.     

介质初始密度对波阵面辐射特性的影响

 通过一维辐射流体动力学数值模拟,仔细研究了工作介质的初始密度对在氖中传播的强冲击波的波阵面辐射特性、波阵面亮度温度谱分布的作用。证明对于不同强度的冲击波,工作介质选用不同的初始密度,对其中传播的强冲击波波阵面辐射能力的提高是比较有效的。     

Exact Non-Singular Waves in the Anti-de Sitter Universe

A class of radiative solutions of Einstein's field equations with a negative cosmological constant and a pure radiation is investigated. The space-times, which generalize the Defrise solution, represent exact gravitational waves which interact with null matter and propagate in the anti–de Sitter universe. Interestingly, these solutions have homogeneous and non-singular wave-fronts for all freely moving observers. We also study properties of sandwich and impulsive waves which can be constructed in this class of space-times.