The attenuation of the higher-order cross-section modes in a duct with a thin porous layer

A numerical method for sound propagation of higher-order cross-sectional modes in a duct of arbitrary cross-section and boundary conditions with nonzero, complex acoustic admittance has been considered. This method assumes that the cross-section of the duct is uniform and that the duct is of a considerable length so that the longitudinal modes can be neglected. The problem is reduced to a two-dimensional (2D) finite element (FE) solution, from which a set of cross-sectional eigen-values and eigen-functions are determined. This result is used to obtain the modal frequencies, velocities and the attenuation coefficients. The 2D FE solution is then extended to three-dimensional via the normal mode decomposition technique. The numerical solution is validated against experimental data for sound propagation in a pipe with inner walls partially covered by coarse sand or granulated rubber. The values of the eigen-frequencies calculated from the proposed numerical model are validated against those predicted by the standard analytical solution for both a circular and rectangular pipe with rigid walls. It is shown that the considered numerical method is useful for predicting the sound pressure distribution, attenuation, and eigen-frequencies in a duct with acoustically nonrigid boundary conditions. The purpose of this work is to pave the way for the development of an efficient inverse problem solution for the remote characterization of the acoustic boundary conditions in natural and artificial waveguides.     

Dynamics of energetic electrons in a solar flare loop and the flattening of its millimeter-wavelength radio emission spectrum

A comparison of predictions by the dynamical trap- plus- precipitation model of broadband microwave bursts and the empirical data on intensive solar flares at centimeter and millimeter wavelengths is presented. The main finding of the data analysis and model calculations is the spectral flattening throughout the entire burst duration. This effect occurs always in the decay phase of the flux peaks at frequencies well above the spectrum peak frequency and is independent of the burst duration. It was found that in some bursts the flux maxima at the higher frequencies are delayed compared to the maxima at the lower frequencies. These effects can be interpreted as being a consequence of the hardening of the electron energy spectrum in the decay phase of the burst.     

The second frequency spectrum of Timoshenko beams

A second spectrum of frequencies was reported in early analytical work on the vibrations of Timoshenko beams. However, in subsequent finite element modelling this phenomenon was either ignored or not definitively classified and recorded. In fact, from a recent finite element analysis with a high precision element it was even concluded that there is no separate second spectrum of frequencies except for the special case of hinged-hinged beams and it was asserted that previous investigators had misinterpreted some frequencies thus introducing the notion of second frequencies. In this paper, a simple linear beam element with independent displacement fields and reduced integration to eliminate shear locking is used and enables one to detect the second spectrum accurately. Guidelines are provided which help to identify and classify the frequencies into two separate spectra.     

Excitation of a rectangular waveguide by pulsed electric and magnetic currents

We consider the problem of excitation of electromagnetic fields by electric and magnetic currents that are present in a rectangular waveguide. The space-time Green’s functions are obtained for a waveguide with ideally conducting walls. The primary attention is given to analysis of the energy spectrum of radiation and the influence of the finite duration of the current pulse and the finite dimensions of the conductor with the current on the above spectrum. It is shown that the influence of the above properties on the spectrum is weak at low frequencies and increases with frequency. N. I. Lobachevsky State University, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 4, pp. 456–468, April, 1998.     

The proton recoil spectrum of thermal neutrons decaying in vacuum

The decay of neutrons in an evacuated tangential reactor beam tube is considered. Due to the thermal motion of the decaying neutrons, the proton recoil spectrum in the laboratory system (z-axis parallel to the beam tube) differs from that in the c.m. system. This difference is calculated for a point at the beam tube exit, and expressed in terms of the average¯q _z and of the variance of thez-component of neutron momentum. An activation method for measuring the neutron current densityJ _z (which is closely related to¯q _z) is suggested. The distortion of the spectrum by the finite resolution of an electrostatic spectrometer is also given. The results are to be used in connection with a planned measurement of the proton recoil spectrum in neutron decay.     

Gravitons from anomalous decay

The decay rate of the neutral pion into two gravitons is calculated from the gravitational anomaly in the axial current. Although this decay rate is negligible relative to the decay rate of the neutral pion into two photons, the rate of decay into gravitons is proportional to the seventh power of the mass of the decaying particle, and to the square of the gravitational constant. The possibility that a particle of very large mass, associated with an axial current anomaly, was present in the early universe is considered. Such a particle would decay at a significant rate into gravitons. As these gravitons would not be thermaiized, they would result in a (potentially observable) nonthermal spectrum of gravitational waves present today. The peak frequency of this gravitational wave spectrum would be indicative of the mass of the decaying particle. Alternatively, if the gravitational constant were large at early times, then the gravitational decay of the pion would be significant in the early universe, giving rise to a nonthermal gravitational wave spectrum.     

Coherent spin-current oscillations in transverse magnetic fields

We address the coherence of the dynamics of spin-currents with components transverse to an external magnetic field for the spin-1/2 Heisenberg chain. We study current autocorrelations at finite temperatures and the real-time dynamics of currents at zero temperature. Besides a coherent Larmor oscillation, we find an additional collective oscillation at higher frequencies, emerging as a coherent many-magnon effect at low temperatures. Using numerical and analytical methods, we analyze the oscillation frequency and decay time of this coherent current-mode versus temperature and magnetic field.     

Free vibrations of circular cylindrical shells

The finite element method is used to predict the dynamic behaviour of circular cylindrical shells in free vibrations. A suitable shape function for the circumferential displacement distribution has been proposed. This reduces the three-dimensional character of the problem to a two-dimensional one. The simultaneous iteration method to determine the eigen-frequencies and eigenvectors is utilised for solving the eigenvalue problem. The accuracy of the method has been checked by verifying the results of known cases. Finally an experimental shell structure containing elastic rings welded at the ends has also been analysed and the experimental results compared with the theoretical ones.     

Quasiadiabatic decay of capillary turbulence on the charged surface of liquid hydrogen

We study the free decay of capillary turbulence on the charged surface of liquid hydrogen. We find that decay begins from the high frequency end of the spectral range, while most of the energy remains localized at low frequencies. The apparent discrepancy with the self-similar theory of nonstationary wave turbulent processes is accounted for in terms of a quasiadiabatic decay wherein fast nonlinear wave interactions redistribute energy between frequency scales in the presence of finite damping at all frequencies. Numerical calculations based on this idea agree well with experimental data.     

中华和钟之双音鎛钟的声频特征和振动方式研究

本文对中华和种之中央Bo钟的声频特性和振动方式进行了研究。利用谱对其辐射声信号进行了谱分析;利用时频分析,研究了其谱结构随时间变化情况;为了了解其频率变化及音高情况,计算了其不同时刻的瞬时平均频率。同时,通过不同位置的振动信号谱分析,重建了其在不同频率下的振动方式。     

Nonstationary wave turbulence in an elastic plate

We report experimental results on the decay of wave turbulence in an elastic plate obtained by stopping the forcing from a stationary turbulent state. In the stationary case, the forcing is seen to induce some anisotropy and a spectrum in disagreement with the weak turbulence theory. After stopping the forcing, almost perfect isotropy is restored. The decay of energy is self-similar and the observed decaying spectrum is in better agreement with the prediction of the weak turbulence theory. The dissipative part of the spectrum is partially consistent with the theoretical prediction based on previous work by Kolmakov. This suggests that the nonagreement with the weak turbulence theory is mostly due to a spurious effect of the forcing related to the finite size of the system.     

Simulation of a Kolmogorov phase screen

Two new methods for modelling Kolmogorov phase fluctuations over a finite aperture are described. The first method relies on the incorporation of subharmonics in order to model accurately the low frequencies of the Kolmogorov spectrum. The second method provides a less accurate, but much faster method for simulating the Kolmogorov spectrum by using a midpoint displacement algorithm used in computer graphics.     

Non-linear behaviour and stability of trapped micron-sized cylindrical gas bubbles in an ultrasonic field

Stable arrays of cylindrical bubbles with diameters 3 and 4 μm were produced by trapping air in the pores of hydrophobic Nuclepore filters. These bubble arrays were irradiated by beams of cw and pulsed ultrasound at carrier frequencies near the resonance frequencies of the trapped bubbles. By examining the frequency spectrum of the field scattered by the arrays it was found that the bubbles may oscillate as non-linear stable cavities of long duration. Using short pulses, the bubbles remain stable up to excitation pressure amplitudes of several bars. Mechanisms for the eventual growth and decay of the bubbles are discussed in light of the observations.     

Radiative decay of mesons and anomaly in 2+1 dimensions

In 2+1 space-time, we study the radiative decay of mesons with Yukawa couplings to two and four component fermions. In a four component theory, the electromagnetic current is free of parity odd radiative corrections; however an axialU(1) current is shown to develop a finite vacuum expectation value. Unlike even dimensions this anomaly is temperature dependent and does not play any role in the radiative decay. We calculate the effective action for the mesons and show that a finite kinetic term for these fields is induced at the one-loop level and vanishes as the temperature goes to infinity.     

Zerfall negativer Myonen in derK-Schale

The decay rate of bound muons from theK-shell and the spectrum of the decay electrons are calculated for seven elements up to lead. The muon is represented by a nonrelativistic wave function in the potential of a realistic nuclear charge distribution. The electron wave function is expanded in partial waves which allows to take into account exactly the relativistic and the finite nuclear size effects. The decay rate decreases monotonically but slowly with increasing atomic number. This agrees with earlier theoretical predictions but contradicts most of the experimental results for medium as well as high atomic number.     

A probe-based method for measuring the transport coefficient in the tokamak edge region

A new method for measuring the diffusion coefficient in the edge plasma of fusion devices is presented. The method is based on studying the decay of the plasma fluctuation spectrum inside a small ceramic tube having its mouth flush with a magnetic surface and its axis aligned along the radial direction. The plasma fluctuations are detected by an electrode, radially movable inside the tube. In the experiment described herein, which was performed in the edge region of the CASTOR tokamak, the electrode measured the floating potential. The experimental arrangement is the same used for the direct plasma potential measurements according to the “Ball-pen probe” [1], the design of which is based on the Katsumata probe principle. When the electrode protrudes from the tube, the measured signal shows the floating potential fluctuations of the plasma. Retracting the electrode into the tube, the signal power spectrum displays a decay. This decay is different for different frequencies, and is exponential. Assuming a mainly diffusive behaviour of the plasma inside the tube, the spectrum decay length can be used to derive a value of the diffusion coefficient. The measurement were performed at different radial positions in the CASTOR edge region, so that a radial profile of the diffusion coefficient was obtained. Typical values ofD are of (2–3) m²/s, consistent with expectations from the global particle balance. The radial profile shows a tendency of the diffusion coefficient to increase going deeper into the plasma.     

Spontaneous emission from a Λ three-level atom in an anisotropic photonic crystal

The spontaneous emission properties of a Λ type atom embedded in a three-dimensional anisotropic photonic crystal are investigated. Only one of the two atomic transition frequencies is considered near the photonic band edge. The atomic decay properties such as the time-evolution of the excited-state population and the instant and effective decay rates are studied in detail. It is found that there exists a wide region for the difference of the transition frequency from the band edge, in which only diffusion fields with frequencies being near or far away from the band edge appear in the emitted field. The spontaneous emitted field and its spectrum depend not only on the detuning of the transition frequency from the band edge, but also on the distance from the atom. Therefore, during the propagating process, the propagating field is partially transferred into the diffusion field.     

直线电机用高温超导块材磁体作用力—磁场特性分析

本文以探究高温超导块材磁体在同步直线电机实际应用特性为目标,搭建了对磁场、三维电磁力信号同时采集的多物理场测试平台.实验探究了作为直线电机次级的高温超导块材磁体所受电磁力、俘获磁通随充磁电流、电机初级三相交流电电流幅值、频率等参数的变化规律.理论分析了高温超导块材俘获磁通在行波磁场中的衰减规律.基于所得结论,提出可有效抑制磁场衰减的方法—引入铁磁材料,并重点讨论该方法对初次级间电磁力的影响.     

Simulation of a Kolmogorov phase screen

Abstract

Two new methods for modelling Kolmogorov phase fluctuations over a finite aperture are described. The first method relies on the incorporation of subharmonics in order to model accurately the low frequencies of the Kolmogorov spectrum. The second method provides a less accurate, but much faster method for simulating the Kolmogorov spectrum by using a midpoint displacement algorithm used in computer graphics.     

Effect of four-photon interactions on coherent population trapping in Λ-systems

The resonance fluorescence spectrum of a Λ-system excited by two resonant light fields is calculated using a Markov analysis. Analytical formulas are derived in the strong-field limit within and beyond the rotating wave approximation. It is shown that the resonance fluorescence of the system does not vanish during coherent population trapping. Its spectrum consists of two multiplets which are similar to a triplet in the resonance fluorescence spectrum of a two-level atom and lie at the electronic transition frequencies, together with two triplets located at the frequencies of four-photon processes involving the optical excitation fields. The latter are fundamental in character and impose limits on the lower bound of the dephasing rate for the Raman resonance owing to the effect of radiative decay of the dipole transitions on the dynamics of the ground state. The effect of four-photon dephasing on the absorption spectrum of a Λ-system is analyzed and found to lead to a substantial reduction in the depth of a dip in the absorption spectrum which vanishes as the laser field strength is increased. Zh. éksp. Teor. Fiz. 113, 144–167 (January 1998)