Granular flows in a rotating drum: the scaling law between velocity and thickness of the flow

The flow of dry granular material in a half-filled rotating drum is studied. The thickness of the flowing zone is measured for several rotation speeds, drum sizes and beads sizes (size ratio between drum and beads ranging from 47 to 7400). Varying the rotation speed, a scaling law linking mean velocity vs. thickness of the flow, v∼h^m, is deduced for each couple (beads, drum). The obtained exponent m is not always equal to 1, the value previously reported for a drum in litterature, but varies with the geometry of the system. For small size ratios, exponents higher than 1 are obtained due to a saturation of the flowing zone thickness. The exponent of the power law decreases with the size ratio, leading to exponents lower than 1 for high size ratios. These exponents imply that the velocity gradient of a dry granular flow in a rotating drum is not constant. More fundamentally, these results show that the flow of a granular material in a rotating drum is very sensible to the geometry, and that the deduction of the “rheology” of a granular medium flowing in such a geometry is not obvious.     

On reflection of radiation from moving objects

An analytical expression for the field reflected from a nonuniformity of the velocity of a moving medium is obtained. Specific features of the reflected radiation depending on the velocity and sizes of the non-uniformity domain and on the angle of incidence of radiation relative to the direction of motion are studied. The feasibility of experimental detection of the reflected radiation is demonstrated.     

Application of integral transforms to a description of the Brownian motion by a non-Markovian random process

The one-dimensional Brownian motion and the Brownian motion of a spherical particle in an infinite medium are described by the conventional methods and integral transforms considering the entrainment of surrounding particles of the medium by the Brownian particle. It is demonstrated that fluctuations of the Brownian particle velocity represent a non-Markovian random process. A harmonic oscillator in a viscous medium is also considered within the framework of the examined model. It is demonstrated that for rheological models, random dynamic processes are also non-Markovian in character. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 66–74, February, 2009.     

Acoustic Monitoring of the Sea Medium Variability: Experimental Testing of New Methods

Two new methods of acoustic monitoring of the sea medium are tested in a shelf region. One of the methods is the difference-pulse technique in which the variability of the medium is characterized by the difference in the energies of the sequences of arrivals of two multiray signals. The other method is the self-sustained oscillator technique in which the information on the changes that occur in the water temperature, the velocity of currents, and other parameters of the medium is obtained from the frequency deviation of a self-sustained oscillator that has a feedback loop closed through the underwater sound channel.     

Direct numerical simulation of passive scalar in decaying compressible turbulence

1IntroductionDirectnumericalsimulation(DNS)becomesanimportanttoolinrecentresearchofturbulence[1].DNSofcompressibleturbulenceismoredifficultthanthatoftheincompressibleturbulence.WhentheturbulentMachnumberisgreaterthan0.3theshockletsmayappearinthecompressibleturbulentflowfields.Thereasonandmechanismofshockletsexistencearenotclearyet.TheturbulentMachnumberinDNScannotbeveryhighwiththepresentexistingnumericalmethodsandcomputerresource.Fortheproblemofcompressibleisotropicturbulencewiththeinitia…     

室温条件下掺铒光纤中光脉冲群速可控特性的研究

利用相干布居振荡技术在介质吸收光谱上产生烧孔，孔宽大约为基态粒子数恢复时间的倒数. 由增益理论分析得到不同抽运光功率对介质吸收状态的影响. 在介质的吸收区域，振荡导致光脉冲经历饱和吸收，脉冲传输延迟；在介质的增益区域，振荡又导致光脉冲经历增益饱和，脉冲传输超前. 应用此技术在掺铒光纤中实现了光速人为可控. 在掺铒光纤晶体中观测到了最慢为2.857×10³m/s的光速减慢传输，相应感生群折射率为10.5×10⁴. 根据布居振荡效应及增益理论，由速率方程出发，得到了 关键词： 光谱烧孔 相干布居振荡 饱和吸收 慢光     

Evaluation of ultrasonic bubble detectors in vitro using calibrated microbubbles at selected velocities.

Two standard Doppler ultrasonic devices, currently used for detecting bubbles in vivo, have been evaluated and compared in vitro using carefully calibrated uniform micro-bubbles rising at terminal velocity through a static aqueous medium. Two unexpected findings were observed: (a) the focal length of the transducer apparently decreases for smaller bubble sizes, and (b) a significant horizontal convection current was produced by one of the instruments. When the medium was in motion, it was found that the sensitivity varied markedly with bubble velocity, varying from a minimum detectable diameter of 40 mum at 55 cm/sec to 170 mum at 20 cm/sec. These findings are discussed with regard to the limitations of the Doppler technique for monitoring gas emboli in vivo and as an early warning for decompression sickness in divers.     

Computation of effective propagation parameters in the optical domain with a finite difference time domain method

The study of optical scattering by heterogeneous media is a complex topic where homogenization is very helpful and rigorous methods are useful. Finite difference time domain (FDTD) method coupled with Monte Carlo process is used to compute the effective parameters of heterogeneous media. Effective parameters based on the coherent field propagation of a beam in the medium are determined in bidimensional geometry and for both polarizations. It is applied to media composed of small particles embedded in an host medium, for relevant ranges of particle sizes and optical constants for both binder and particles. The results are compared to the Maxwell–Garnett and Bruggeman mixing laws and the Foldy–Twersky and Keller perturbative approximations, leading to the assessment of their validity domain.     

Fluctuations in the diffusion of particles in a non-uniform medium

We study the thermal fluctuations of density and flow velocity of particles diffusing in a non-uniform medium with memory character. The fluctuation spectra are derived from the macroscopic equations with the aid of the generalized Nyquist theorem. A comparison is made with other methods.     

The velocity component effect to the mass resolution within de Heer and Milani criteria

The velocity component effect to the mass resolution was considered for two different positions of beam source in the time-of-flight mass spectrometer based on de Heer and Milani criteria. The results estimated for various potassium cluster sizes showed that resolution is reduced at considerable level and the cluster range is limited to the small mass sizes for the position along with the cluster beam axis. The resolution is high and the effect appears close to K₁₀₀₀ cluster sizes depending upon perpendicular axis used.     

Transmission of quasi-monochromatic light wave through dispersive medium and measurement of refractive index

Relations used in the measurements of the refractive index in a non-absorbing dispersive medium are discussed, taking into account the quasi-monochromaticity of the light wave. It is shown that when interference methods using compensation are employed, it is always necessary to allow for corrections for dispersion. Relations describing the diffraction of a ray at the boundary of vacuum and a non-absorbing dispersive medium can be obtained only from the continuity of the Poynting vector at the interface of these media. The stationary phase method may lead to erroneous conclusions. The group velocity vector, which for diffraction describes correctly not only the absolute magnitude but also the direction of this velocity in a dispersive non-absorbing medium, is of the form $$v_g = \frac{{k(\omega _0 )}}{{|k|}}|\upsilon _g |$$ whereω ₀ is the frequency belonging to the maximum amplitude of a given quasi-monochromatic wave.     

The Role of a Specific Response of Media in the Vavilov-Cherenkov Effect

It is shown that the condition υ > υ_p (ω), which is necessary in order to trigger the Vavilov-Cherenkov effect, determines the interval of radiated frequencies and it alone cannot establish a strong restriction to the velocity of the particle. It is exhibited that it is possible to define a general lower bound for the velocity of the particle, which does not depend on the frequency when both, a specific response of the medium is taken into account and the mentioned condition is considered. The minimum value of the phase velocity of light in the medium determines the existence of such general lower bound.     

Anisotropy of the velocity space of electromagnetic radiation in a moving medium

Anisotropy arising in moving media is considered. In these media, the phase velocity of light nonlinearly depends on the velocity vector field of the medium due to anisotropic binding forces between lattice atoms. Observations of the optical anisotropy of light in a rotating optically transparent medium are discussed. Laser radiation with wavelength ?? = 0.632991 ± 1 × 10^?7 ??m propagating in an interferometer was passed through a rotating optical disk D = 62 mm in diameter. The projection of the beam??s path length in the medium onto the flat surface of the disk is l = 41 mm; the refractive index of the glass and its thickness are, respectively, n = 1.71250 for ?? = 632.8 nm and 10 mm; and the angle of incidence of the beam on the flat surface of the disk is ?₀ = 60°. The optical disk is rotated in two directions, and its rotation frequency may reach 250 Hz. Experimental data confirm the linear dependence of the fringe shift on the velocity of the medium up to 29.6 m/s. The measurement accuracy is sufficient to detect angular variations ??? = 3 × 10^?5 in the position of fringes at a fixed rotation velocity of the optical disk.     

Transition radiation of sound under uniform motion of momentum and mass sources in inhomogeneous media

Theoretical investigations of transition radiation of sound under the uniform motion of small momentum and mass (heat) sources in a gas with chaotic inhomogeneities of density and temperature are briefly discussed. We analyze two basic methods for calculating the source losses by the radiation of acoustic waves. The first method is based on the calculation of the energy flux in the Born approximation over small perturbations of the parameters of the medium, and allows us, in particular, to clarify the problem of the angular distribution of the radiation. The second method is based on the use of the effective permittivity tensor, which links together the spatiotemporal Fourier transforms of the average momentum of a unit volume of the medium and the average (over the ensemble) velocity of the wave perturbations. The dependence of the radiation reaction force on the velocity of the motion of source is analyzed for the case of small-scale and large-scale inhomogeneities of the medium.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 1–2, pp. 44–55, January–February, 1995.     

Application of the Model of Spheroidal Scatterers for Determination of Particles Sizes of Aqueous Suspensions by the Laser-Polarimetry and Dynamic-Light-Scattering Methods

Results of measurements of particle sizes of a dispersed medium by the methods of laser polarimetry and dynamic light scattering are presented for water suspensions of ZnO, CuO, TiO₂, and BaTiO₃ with particles of irregular shape. When using the laser-polarimetry method, the particle-size distributions within the framework of the spheroidal-scatterer model are reconstructed. The same model was used to correct data obtained by the method of dynamic light scattering. The data obtained are compared with results obtained by electron microscopy. It is shown that, at values of the size parameter of 2–5, correction of the results of measurements obtained by the dynamic-light-scattering method makes it possible to reduce the error in determining the average particle size by this method to no more than 11%.     

Control of the pulse duration in one- and two-axis passively Q-switched solid-state lasers

We study theoretically and experimentally different methods to control the pulses emitted by solid-state lasers passively Q-switched by a saturable absorber. We explore one- and two-axis laser schemes allowing to control the pulse duration, which is ruled by the saturation powers of the transitions in the absorber and in the gain medium. In one-axis lasers, it is shown that the adjustment of the pump and laser beam sizes in the active medium and in the absorber provides an efficient means to control the pulse temporal shape and duration. Furthermore, a two-axis laser cavity supporting so-called forked-eigenstate operation permits to freely adjust the parts of the mode power which circulate in the gain medium and in the absorber. In this case, a lengthening of the pulse duration up to 500 ns is obtained with an increase of the average output power. The theoretical results obtained by using rate equations adapted to each cavity geometry are in close agreement with experiments performed on a diode-pumped Nd³⁺:YAG laser Q-switched by a Cr⁴⁺:YAG saturable absorber. The relevance of the different techniques to control the pulse durations in the framework of potential applications is discussed. Received 3 December 2001     

Dependence of the ultrasound velocity on the acting stress during plastic flow of polycrystals

The dependence of the propagation velocity of ultrasound in polycrystalline materials on the magnitude of the stress acting during deformation is measured. It is established that the dependence has three stages, which are apparently due to the formation of dislocation ensembles of various characteristic sizes in the deformed material. Zh. Tekh. Fiz. 69, 100–101 (December 1999)     

Geometrical optics of moving media

The propagation of a monochromatic light wave in a medium moving at a spatially nonuniform velocity is described in terms of geometrical optics. An eikonal equation is derived to the first order in the parameter v/c (where v is the velocity of the medium and c is the velocity of light). It is shown that the nonuniformity of the motion of the medium leads to a shift and bending of the light rays and to a rotation of the plane of polarization. The estimates obtained demonstrate the feasibility of observing the revealed effects in real experiments. These effects can be used to analyze the distributions of the velocity of motion of fluids and, possibly, gases.     

Calculation of the velocity of sound in rubber containing cylindrical voids with allowance for dynamic correction

The problem of the velocity of sound in a medium containing cylindrical voids is solved with the use of the theory describing coupled vibrations of a tube of finite height, which serves as a model of the medium. The expression for the velocity of sound is derived with allowance for the dynamic correction similar to the Rayleigh correction for an infinitely long rod. Good agreement is obtained between the calculations (by the formulas derived for the velocity) and the experimental characteristics of the reflection and transmission coefficients of multilayer samples made of rubber with cylindrical voids, which were measured in the Low-Frequency Acoustic Pipe system in the frequency range 100–5000 Hz under hydrostatic pressures up to 30 kg/cm².     

Bifurcation of the Biot slow wave in a porous medium

The propagation of the Biot slow wave in a fluid-saturated porous medium at low frequencies is investigated by asymptotic methods. It is proven that the Biot wave has a bifurcation behavior depending on its wave number. The bifurcation occurs in a neighborhood of the critical value k(cr), which depends on the permeability of a medium and the viscosity of a fluid. The P2 wave is fully attenuated if its wave number is smaller than k(cr) and it becomes propagatory with wave numbers bigger than k(cr). Asymptotic formulas for the phase velocity and attenuation of the Biot wave are derived.