Numerical simulation of quasisurface magnetostatic waves in a ferrite film with two magnetic channels

Quasisurface magnetostatic waves propagating in a ferrite film along two magnetized channels are simulated numerically. It is shown that the interaction between the channels is manifested differently, depending on the wavelength. In the long-wavelength region the interaction between the channels has a distributed character; in the short-wavelength region the interaction between the channels appears as if it takes place at their boundary. The magnetized region of the ferrite film between the channels behaves both as a conductor of the alternating field and as a medium with eigenmodes, so that under certain conditions the waveguide can be transformed into a three-channel structure. The dispersion curves of the magnetostatic wave modes of a two-channel waveguide lie in zones bounded by the dispersion curves of the corresponding modes of a one-channel waveguide of double width. As the gap between the channels increases, the dispersion curves of the odd modes shift toward shorter wavelengths, and those of the even modes shift toward longer wavelengths. Zh. Tekh. Fiz. 68, 91–96 (February 1998)     

Numerical study of TE waves propagating in multibranch couplers

Abstract

The evolution of TE-polarized waves propagating in the linear and nonlinear multibranch coupler is studied. The propagation of the waves in both directions is calculated by the beam-propagation method (BPM). The numerical results of simulation show that the minimum insertion loss of the nonlinear multibranch coupler is less than 0.088 dB, whereas that of the linear multibranch coupler is more than 3 dB.     

Numerical study of TE waves propagating in multibranch couplers

The evolution of TE-polarized waves propagating in the linear and nonlinear multibranch coupler is studied. The propagation of the waves in both directions is calculated by the beam-propagation method (BPM). The numerical results of simulation show that the minimum insertion loss of the nonlinear multibranch coupler is less than 0.088 dB, whereas that of the linear multibranch coupler is more than 3 dB.     

Numerical study of converging shock waves in porous media

The dependences of the solutions to the hydrodynamic equations of compressed media that describe converging shock waves on the density of a substance ahead of a wave front are studied. The properties of Hugoniot adiabats that can explain the qualitatively different characters of these dependences for the equations of state of perfect gas and condensed matter are analyzed. The one-dimensional problems of converging shock waves in graphite and aluminum are considered, and the two-dimensional problem of the compression of graphite in a steel target with a conical cavity is solved. The latter problem is also investigated in terms of a simple model for a deformable solid that takes into account shear stresses.     

Retardation of electromagnetic waves by helices of large diameters

This paper is devoted to the study of the wave retardation by helices of large diameters, when the lengthL of one turn of the helix is larger than the vacuum wavelength, i.e.L/>1. The dispersion equation of the tape helix wound up on a dielectric cylinder was derived and the dependence of the phase velocity of slowed-down waves on the diameter of helix was calculated. The theory was verified experimentally at a frequencyf=2·385 GHz. Helices were wound up on polyfoam (=1·04) and plexiglass (=2·55) cylinders of various diameters. For the helix wound up on plexiglass cylinders it was found that the wave retardation depends on the form of conductor from which the helix is made. The retardation of waves is determined by an effective dielectric constant _ef which is equal either ( _i + _e)/2 for the tape helix or ( _{i e}) for the helix made from a wire of the circular cross-section; here _i is the dielectric constant of inner dielectric rod and _e is that of outer space.     

Droplet combustion in standing sound waves

Interaction between droplet combustion and acoustic oscillation is clarified. As the simplest model, an isolated fuel droplet is combusted in a standing sound wave. Apart from the conventional idea that oscillatory component of flow influences heat and mass transfer and promotes combustion, a new model that a secondary flow dominates combustion promotion is examined. The secondary flow, found by the authors in the previous work, is driven by acoustic radiation force due to Reynolds normal stress, and named as thermo-acoustic streaming. Since the force is described by the same equation as buoyancy, i.e., F = ΔρVg, the nature of the streaming is thought to be the same as natural convection. The flow patterns of the streaming are analyzed and its influence on burning rate of a droplet is predicted. Experimental investigation was mainly done with burning droplets located in the middle of node and anti-node of standing sound waves. This location realizes the strongest streaming. By varying sound pressure level, ambient pressure, and acoustic frequency, the strength of the streaming was controlled. Flame configuration including soot and burning rate were examined. Microgravity conditions were employed to clarify the influence of acoustic field through the streaming, since it is similar to and must be distinguished from natural convection. Experiments using microgravity conditions confirmed the new combustion promotion model and the way to quantify it. By introducing a new non-dimensional number Gr_a, that is the ratio of acoustic radiation force to viscosity, burning rate constants for various ambient and sound conditions are rearranged. As a result, it was found that the excess burning rate (k/k₀ − 1) is proportional to or , for weak sound and for strong sound, respectively.     

Numerical study of influence of molecular diffusion in the Mild combustion regime

In this paper, the importance of molecular diffusion versus turbulent transport in the moderate or intense low-oxygen dilution (Mild) combustion mode has been numerically studied. The experimental conditions of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154] were used for modelling. The EDC model was used to describe the turbulence–chemistry interaction. The DRM-22 reduced mechanism and the GRI 2.11 full mechanism were used to represent the chemical reactions of an H₂/methane jet flame. The importance of molecular diffusion for various O₂ levels, jet Reynolds numbers and H₂ fuel contents was investigated. Results show that the molecular diffusion in Mild combustion cannot be ignored in comparison with the turbulent transport. Also, the method of inclusion of molecular diffusion in combustion modelling has a considerable effect on the accuracy of numerical modelling of Mild combustion. By decreasing the jet Reynolds number, decreasing the oxygen concentration in the airflow or increasing H₂ in the fuel mixture, the influence of molecular diffusion on Mild combustion increases.     

Magnetic interactions in ferromagnetic manganite nanotubes of different diameters

In this work we present a magnetic study of La_0.67Sr_0.33MnO₃ (LSMO) and La_0.67Ca_0.33MnO₃ (LCMO) nanotubes with nominal external diameters (?) of 100, 200, 600 and 800 nm. The 800 nm diameter nanotubes have walls of around 50 nm thickness in all the cases. The walls are constituted by an assembly of nanoparticles with a non-Gaussian size distribution presenting a maximum at 24 ± 6 nm (LSMO) and 25 ± 8 nm (LCMO). We carried out isothermal remanent magnetization (IRM) and dc demagnetization (DCD) experiments. We determined that the crystallites are single magnetic domains with a magnetic dead layer on the surface which avoids exchange interactions among grains. We conclude that the dominating interactions are of dipolar type of the same magnitude for all the samples.     

不同沙粒底面下气泡脉动特性实验研究

通过高速摄影系统对电火花气泡与不同沙粒底面间的相互作用进行了实验研究,并改变气泡与沙粒底面之间的距离.实验结果表明:气泡在与沙粒底面的相互作用中会产生两种明显不同的现象,即形成与近刚性壁面类似的气泡射流以及“蘑菇状”气泡,“蘑菇状”气泡撕裂形成两个气泡,随后产生两个反方向的沿轴线方向的射流.沙粒底面边界具有刚性与弹性两种特征.另外,随着气泡与沙粒底面之间的距离d的增大,气泡脉动周期先增大然后减小,存在气泡脉动周期峰值.对于不同的沙粒底面边界,出现气泡脉动周期峰值的距离d随着沙底粒径的增大而越小.     

交替行为对螺旋波影响的数值模拟研究

在离散可激发介质Greenberg-Hasting模型中引入交替(alternans)行为,研究了交替行为对螺旋波的影响.数值结果表明:在适当选择参数下,交替对螺旋波有很大影响,例如交替导致螺旋波的形状振荡,形成呼吸螺旋波,交替使螺旋波漫游、漂移,甚至使螺旋波漫游出系统的边界,交替使螺旋波破碎形成小螺旋波、反靶波和时空混沌等,首次在均匀介质中观察到交替导致传导障碍,使螺旋波破碎和消失,并对发生这些现象的机理进行了分析. 关键词： 离散可激发介质 螺旋波 靶波 漫游     

Numerical study of Fermi-Pasta-Ulam recurrence for water waves over finite depth

Highly accurate direct numerical simulations have been performed for two-dimensional free-surface potential flows of an ideal incompressible fluid over a constant depth h, in the gravity field g. In each numerical experiment, at t = 0 the free surface profile was in the form y = A ₀cos(2πx/L), and the velocity field v = 0. The computations demonstrate the phenomenon of Fermi-Pasta-Ulam (FPU) recurrence takes place in such systems for moderate initial wave amplitudes A ₀ ≲ 0.12h and spatial periods at least L ≲ 120h. The time of recurrence T _FPU is well fitted by the formula T _FPU(g/h)^1/2 ≈ 0.16(L/h)²(h/A ₀)^1/2.     

Non-dispersive traveling waves in inclined shallow water channels

Existence of traveling waves propagating without internal reflection in inclined water channels of arbitrary slope is demonstrated. It is shown that traveling non-monochromatic waves exist in both linear and nonlinear shallow water theories in the case of a uniformly inclined channel with a parabolic cross-section. The properties of these waves are studied. It is shown that linear traveling waves should have a sign-variable shape. The amplitude of linear traveling waves in a channel satisfies the same Green's law, which is usually derived from the energy flux conservation for smoothly inhomogeneous media. Amplitudes of nonlinear traveling waves deviate from the linear Green's law, and the behavior of positive and negative amplitudes are different. Negative amplitude grows faster than positive amplitude in shallow water. The phase of nonlinear waves (travel time) is described well by the linear WKB approach. It is shown that nonlinear traveling waves of any amplitude always break near the shoreline if the boundary condition of the full absorption is applied.     

Propagation of solitary waves in channels of decreasing depth

The changes which occur in a right-going solitary wave as it travels a channel of decreasing depth are discussed. In addition to the changes in the solitary wave, we have found through a judicious use of the conservation laws two secondary structures (a shelf and a reflection). Each of these structures is small with respect to the solitary wave, though the mass flux associated with each is of the same order as that of the solitary wave. Of interest is that the amplitude of the reflected wave does not satisfy Green's law. But rather, the amplitude of the reflected wave is constant along left-going characteristics. This finding allows us to satisfy the mass flux conservation laws to leading order and establishes the perturbed Korteweg-deVries equation as a consistent approximation for the right-going profile.     

Formation of detonation waves in channels and interaction of the waves with penetrable screens

Two-dimensional channel flows with shock waves resulting from the detonation of a combustible gas mixture are considered. Conditions for detonation and the parameters of the shock waves are determined. The feasibility of reducing the shock wave intensity and loads on the structure by mounting a set of mesh screens in the channel is investigated. The numerical computation of detonation initiation in an air-hydrogen mixture and subsequent passage of shock waves through the mesh screens is carried out. Basic quantitative characteristics of shock wave reduction depending on the mesh screen penetrability and mutual arrangement of variously penetrable screens are obtained.     

Numerical time reversal of waves

A numerical time reversal of waves is proposed instead of the conventional time reversal of wave procedure used in underwater acoustics. In the numerical method, the test sound source and the receiving arrays are used, as in the conventional method, but the transmission of the received signals after their time reversal into the same medium, as well as the measurement of the field obtained in this way at the point of the test source, is replaced by computations. To use the proposed technique for obtaining the same results as those provided by the conventional time reversal of waves, the teset source should be placed at different depths. A simplified numerical algorithm with the test source operating at a single depth is proposed and justified. This version of the time reversal of waves is successfully applied to the experiment in the Barents Sea. In contrast to the conventional method, the proposed technique allows one to study the stability of the sea medium with currents.     

Nonlinear acoustic waves in channels with variable cross sections

The point symmetry group is studied for the generalized Webster-type equation describing nonlinear acoustic waves in lossy channels with variable cross sections. It is shown that, for certain types of cross section profiles, the allowed symmetry group is extended and the invariant solutions corresponding to these profiles are obtained. Approximate analytic solutions to the generalized Webster equation are derived for channels with smoothly varying cross sections and arbitrary initial conditions.     

Shock waves in narrow channels filled with superfluid helium

Formation of fourth-sound shock waves in narrow channels filled with superfluid helium is studied. Physical and mathematical conditions at the surface of discontinuity are established. These conditions differ somewhat from those for first- and second-sound waves. The velocity of discontinuity coincides with the speed of fourth sound. The jumps of temperature and the superfluid velocity are shown to be of the first order of the pressure jumps.