Crossing resonance of stochastically interacting wave fields

The dynamic susceptibilities (Green’s functions) of the system of two interacting wave fields of different physical natures with a stochastically inhomogeneous coupling parameter between them with zero mean value have been examined. The well-known self-consistent approximation taking into account all diagrams with noncrossing correlation/interaction lines has been generalized to the case of stochastically interacting wave fields. The analysis has been performed for spin and elastic waves. The results obtained taking into account the processes of multiple scattering of waves from inhomogeneities are significantly different from those obtained for this situation earlier in the Bourret approximation [R.C. Bourret, Nuovo Cimento 26, 1 (1962)]. Instead of frequencies degeneracy removal in the wave spectrum and the splitting of resonance peaks of dynamic susceptibilities, a wide single-mode resonance peak should be observed at the crossing point of the unperturbed dispersion curves. The fine structure appears at vertices of these wide peaks in the form of a narrow resonance on the Green’s-function curve of one field and a narrow antiresonance on the vertex of the Green’s-function curve of the other field.     

非均匀可激介质中的螺旋波

以Barkley模型为对象,研究了可激介质的非均匀性对螺旋波斑图形成的影响.该模型中各参数与可激介质的属性密切相关,通过参数涨落的正态分布来刻画非均匀性,数值研究了单参数以及多参数涨落的正态分布情形下螺旋波斑图的形成.研究表明,可激介质的非均匀性对于螺旋波波纹的粗细及疏密程度有较大影响.参数涨落分布的方差越大,形成的螺旋波波纹越粗糙.对于两参数均匀分布的极端情形,当参数分布大于某一范围,无法形成螺旋波.这些都与螺旋波旋转的角频率密切相关.螺旋波旋转的角频率越大,螺旋波波纹越粗,同时波纹越密集;反之,螺旋波 关键词： 螺旋波 非均匀介质 Barkley模型     

Interaction of a surface magnetostatic wave with an inhomogeneous granular HTSC medium

In the present paper the dispersion properties of surface magnetostatic waves (SMSWs) propagating in a stratified structure, ferrite film—high-temperature superconducting layer, have been investigated. The problem of SMSW propagation in an inhomogeneous stratified medium has been solved and the dispersion equation has been obtained. In the solution of this problem, the granular nature of the high-temperature superconducting (HTSC) medium and the exposure to a constant magnetization field have been taken into account. Upon exposure to the constant magnetic field the HTSC film becomes an inhomogeneous anisotropic medium with respect to a variable magnetic field. The nonhomogeneous wave equation describing the SMSW field in a granular HTSC medium has been solved by the method of sequential iterations. Corrections for the SMSW velocity and attenuation, calculated with the use of the exact dispersion equation, are significant in comparison with previously obtained approximate values and exceed 20 and 40%, respectively. The method of analyzing electromagnetic fields in inhomogeneous granular high-temperature media can be used to solve some other problems, where spatially inhomogeneous HTSC media are used. Tomsk State University of Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 45–48, April, 1999.     

Similarity solutions of isothermal flows behind a MGD blast wave in an inhomogeneous medium

Summary Self-similar solutions of isothermal flows behind a cylindrical magnetogasdynamic blast wave have been obtained. A strong cylindrical shock wave generated by a sudden line source explosion in an inhomogeneous medium of electrically conducting gas has been studied. Numerical and analytical treatments have been presented and a uniformly valid distribution of pressure, density and velocity profiles has been determined and the magnetic-field effects on the flow distributions have been investigated. The shock propagation law is determined by extending Whitham’s rule for a MGD flow with infinite electrical conductivity.

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Riassunto Si sono ottenute soluzioni autosimili di flussi isotermici dietro un’onda espansiva, magnetogasdinamica, cilindrica. è stata studiata un’onda d’urto forte cilindrica generata da un’improvvisa esplosione a sorgente lineare in un mezzo non omogeneo di un gas elettricamente conduttore. Si presentano trattamenti numerici e analitici e si è determinata una distribuzione uniformemente valida dei profili di pressione, densità e velocità e si sono studiati gli effetti del campo magnetico sulle distribuzioni del flusso. La legge di propagazione dell’urto si determina estendendo la regola di Whitham per un flusso MGD con conduttività elettrica infinita.

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Резюме Получются самоподобные решения для изотермических потоков за цилиндрической магнитогазодимамической ударной. Исследется сильная цилиндческая ударная волна, которая обрзуетсяв резуль↦ате внеапного взрыва линейного истоника в неоднородной среде электрически проводящего газа. Проводятся численное и аналитическое рассмотрение. Определяжтся распредение давления, профили плотности и скорости. йсследуется влияние магнитного поля на распредение потока. Определется закон распространения ударной волны, обобшая правило Витама для магнитогазодинамического потока с бесконечной электрической проводимостью.

     

Energy characteristics of coupling channels between points of an inhomogeneous medium

The translational characteristics of points of a continuous medium are studied for a source and receiver with fixed locations and given properties. Within the framework of geometrical optics, the energycoupling coefficient (AC) and anisotropy coefficient (AC) are introduced, which determine the size and angle distribution of the coupling channels between the source and observation region through a point of the inhomogeneous medium. The spatial structures of the AC and AC are investigated for some typical waveguides encountered in practice. The possibilities of using the AC and AC in systems for viewing and tomographic monitoring of inhomogeneous media are analyzed.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 5, pp. 443–455, May. 1993.     

Nuclear magnetic resonance in inhomogeneous magnetic fields

The response of the spin system has been investigated by numerical simulations in the case of a nuclear magnetic resonance (NMR) experiment performed in inhomogeneous static and radiofrequency fields. The particular case of the NMR-MOUSE was considered. The static field and the component of the radiofrequency field perpendicular to the static field were evaluated as well as the spatial distribution of the maximum NMR signal detected by the surface coil. The NMR response to various pulse sequences was evaluated numerically for the case of an ensemble of isolated spins (1/2). The behavior of the echo train in Carr-Purcell-like pulse sequences used for measurements of transverse relaxation and self-diffusion was simulated and compared with the experiment. The echo train is shown to behave qualitatively differently depending on the particular phase schemes used in these pulse sequences. Different echo trains are obtained, because of the different superposition of Hahn and stimulated echoes forming mixed echoes as a result of the spatial distribution of pulse flip angles. The superposition of Hahn and stimulated echoes originating from different spatial regions leads to distortions of the mixed echoes in intensity, shape, and phase. The volume selection produced by Carr-Purcell-like pulse sequences is also investigated for the NMR-MOUSE. The developed numerical simulation procedure is useful for understanding a variety of experiments performed with the NMR-MOUSE and for improving its performance. Copyright 2000 Academic Press.     

The role of surface tension in elastic wave scattering in an inhomogeneous poroelastic medium

It is well known that many porous media such as rocks have heterogeneities at nearly all scales. We applied Biot's poroelastic theory to study the propagation of elastic waves in isotropic porous matrix with spherical inclusions. It is assumed that the heterogeneity dimension exceeds significantly the pore size. Modified boundary conditions on poroelastic interface are used to take into account the surface tension effects. The effective wavenumber is calculated using the Waterman and Truell multiple scattering theory, which relates the effective wave number to the amplitude of the wave field scattered by a single inclusion. The calculations were performed for a medium containing fluid-filled cavities or porous inclusions contrasting in saturating fluid elastic properties. The results obtained show that when we consider elastic wave propagation in poroelastic medium containing soft inclusions, it is necessary to take into account the capillary pressure. The influence of the surface tension depends on the diffraction parameter and it is a maximum in the low frequency range.     

The role of surface tension in elastic wave scattering in an inhomogeneous poroelastic medium

It is well known that many porous media such as rocks have heterogeneities at nearly all scales. We applied Biot's poroelastic theory to study the propagation of elastic waves in isotropic porous matrix with spherical inclusions. It is assumed that the heterogeneity dimension exceeds significantly the pore size. Modified boundary conditions on poroelastic interface are used to take into account the surface tension effects. The effective wavenumber is calculated using the Waterman and Truell multiple scattering theory, which relates the effective wave number to the amplitude of the wave field scattered by a single inclusion. The calculations were performed for a medium containing fluid-filled cavities or porous inclusions contrasting in saturating fluid elastic properties. The results obtained show that when we consider elastic wave propagation in poroelastic medium containing soft inclusions, it is necessary to take into account the capillary pressure. The influence of the surface tension depends on the diffraction parameter and it is a maximum in the low frequency range.     

Spatial statistics of a partially coherent wave in a randomly inhomogeneous cubic medium

The spatial statistics of a partially coherent light wave in a randomly inhomogeneous medium is considered on the basis of perturbation theory. The equations for the spatial coherence function of the light wave in a nonlinear irregular medium were solved. The variation in the fluctuation field dispersion, in its amplitude and phase, as well as the behavior of the radius of spatial correlation for various perturbations of the incident radiation, were investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 27, No. 1, pp. 56–64, January, 1984.     

Suppressing amplitude fluctuations of the wave propagating in a randomly inhomogeneous medium

We suggest using spatial processing of the wave field with the aid of the double (in source and observer coordinates) weighted Fourier transform (DWFT) to suppress amplitude fluctuations (amplitude scintillations) during wave propagation in a randomly inhomogeneous medium. We examine the influence of sizes of receiving and transmitting antenna systems on the efficiency of the spatial processing in suppressing scintillations for both weak and strong intensity fluctuations. We demonstrate that the efficiency of the suppression of amplitude fluctuations by the inverse DWFT depends on the excess of sizes of fan beam projections of receiving and transmitting antenna systems over the Fresnel radius in a region with irregularities.