Influence of static and dynamic internal actions on elastic nonlinear properties of a granulated unconsolidated medium

We have studied the influence of external static (pressure) and dynamic (caused by an elastic wave with a finite amplitude) actions on the linear and nonlinear elastic properties of a granulated unconsolidated medium, which was simulated by steel spheres with diameters of 2 and 4 mm. We have analyzed the equation of state for such a medium taking into account the presence of weakly and strongly deformed contacts between individual spheres. We have obtained expressions for the elasticity coefficient and second- and third-order nonlinear elastic parameters, and we have experimentally studied the influence of external static pressure on their values. We have measured the dependence of the velocity of elastic waves on external static pressure and the probing signal amplitude. In the studied medium, a number of structural phase transitions were observed, related to rearrangement of the packing of spheres, which were caused by both static and dynamic actions. The structural phase transition was accompanied by an anomalous change in the nonlinear elastic parameters of the medium and the velocity of elastic waves. We have analyzed the results based on the Hertz theory of contact interaction.     

Acoustic radiation of moving dipoles and axial quadrupoles consisting of discrete antiphased monopoles

An acoustic field generated by a moving dipole that consists of two antiphased monopoles and an axial quadrupole represented as a combination of four discrete monopoles with corresponding phases was analyzed. It was revealed that the angular distribution of the pressure amplitude of the distributed source depends on the additional phase incursion between interfering waves due to the anisotropic character of the medium. The angular characteristics of the pressure amplitude of the distributed dipole and quadrupole, which depend on the orientation and velocity of their motion, were mathematically simulated. The results were compared with the conclusions drawn by other researchers.     

Energy analysis during acoustic bubble oscillations: Relationship between bubble energy and sonochemical parameters

In this work, energy analysis of an oscillating isolated spherical bubble in water irradiated by an ultrasonic wave has been theoretically studied for various conditions of acoustic amplitude, ultrasound frequency, static pressure and liquid temperature in order to explain the effects of these key parameters on both sonochemistry and sonoluminescence. The Keller–Miksis equation for the temporal variation of the bubble radius in compressible and viscous medium has been employed as a dynamics model. The numerical calculations showed that the rate of energy accumulation, dE/dt, increased linearly with increasing acoustic amplitude in the range of 1.5–3.0 atm and decreased sharply with increasing frequency in the range 200–1000 kHz. There exists an optimal static pressure at which the power w is highest. This optimum shifts toward a higher value as the acoustic amplitude increases. The energy of the bubble slightly increases with the increase in liquid temperature from 10 to 60 °C. The results of this study should be a helpful means to explain a variety of experimental observations conducted in the field of sonochemistry and sonoluminescence concerning the effects of operational parameters.     

Dependence of the magnetic and magnetoelastic properties of cobalt ferrite on processing parameters

The dependence of the magnetic and magnetoelastic properties of highly magnetostrictive cobalt ferrite on processing parameters has been investigated. The cobalt ferrite samples used in this study were prepared via conventional ceramic processing methods. The processing parameters of interest were sintering temperature, holding time at the sintering temperature and powder compaction pressure. It was observed that the crystal structure, composition and saturation magnetization of the samples studied did not vary with changes in processing parameters but coercive field decreased with increasing sintering temperature. The amplitude of peak to peak magnetostriction was dependent on the holding time and powder compaction pressure. The strain derivative on the other hand was found to depend on powder compaction pressure at any given sintering temperature or holding time. The results show how the magnetoelastic properties of cobalt ferrite can be varied by changing the processing parameters.     

Generation of higher acoustic harmonics at a flat rough interface between two solids

The results of experimental studies of the influence of a static pressure applied to a flat rough interface between two solids on its nonlinear elastic properties are presented. The studies were performed by the spectral method on the basis of an analysis of the efficiency of generation of higher acoustic harmonics, which arise upon the reflection of a longitudinal elastic wave of finite amplitude from the boundary and the passage through it. A nonmonotonic dependence of the amplitudes of acoustic harmonics on the value of the external reversible static pressure applied to the interface was revealed: pronounced amplitude maxima for the amplitudes of the second and third harmonics were observed with a decrease in the external static pressure. It was also found that the amplitudes of the second, third, and fourth acoustic harmonics increase with a decrease in the external static pressure (in comparison with their values at the same pressure values during its increase). The experimentally determined power dependence of the higher acoustic harmonics on the amplitude of the first acoustic harmonic significantly differed from the classical indices for these harmonics. The influence of the external pressure on the values of the nonlinear second- and third-order elastic parameters was analyzed. The experimental results were analyzed on the basis of nonclassical acoustic nonlinearity.     

On some properties of the Fabry-Perot etalon with negative permittivities of the medium

Specific features of the Fabry-Perot etalon with negative permittivities of the medium are analyzed. Inhomogeneous waves may exhibit, after passing through such an etalon, a giant amplitude enhancement. They remain inhomogeneous and do not transfer the field energy. Due to absorption and other losses, there may arise, in the inhomogeneous wave, running components and interference effects. The etalon cannot improve the diffraction-limited resolving power of the optical devices detecting traveling waves. The possibilities of application of the etalon in the near-field optics are noted. The giant enhancement of the inhomogeneous wave amplitude occurs due to resonance of the incident wave with eigen (surface) waves of the input and output planes of the etalon. It is shown that, with respect to inhomogeneous waves, the Fabry-Perot etalon with negative permittivities is a narrow-band filter with the peak transmission for the spatial frequency of the Fourier expansion of the boundary values on the order of the inverse wavelength. Under the resonance conditions, the running components, when passing through the etalon, experience aberrational distortions. Polarization properties of the resonance amplification are clarified.     

Dynamics of Stimulated Brillouin Scattering in a Short and High-Finesse Fiber Fabry-Perot Resonator

We theoretically investigate the dynamics of stimulated Brillouin scattering (SBS) in a short fiber Fabry-Perot resonator with reflection mirrors. The analysis is based on the coherent six-wave model including an optical Kerr effect and the linear cavity detunings for the pump and created Stokes waves are taken into account in the formulation. The instabilities including periodic, quasi-periodic and chaotic oscillations can occur owing to the Kerr effect even if the fiber resonator is so short as to ensure a single longitudinal-mode operation. The threshold power for SBS and the boundary power between the stable and unstable regions are numerically calculated as a function of the amplitude reflectance of mirrors for different cavity detunings. The dynamics of SBS and these two critical powers depend on the amplitude reflectance and the cavity detuning. Although an increase in the mirror reflectance decreases the threshold power for SBS, it may limit the utility of SBS since the stable region is narrow and the output power levels of the pump and Stokes waves are low.     

容性耦合射频氩等离子体放电诊断研究及仿真模拟

针对中等气压、中等功率下射频容性耦合(CCRF)等离子体的放电特性,采用基于流体模型的COMSOL软件仿真,建立一维等离子体放电模型,以Ar为工作气体,研究同一气压时不同射频输入功率下等离子体电子温度和电子密度的分布规律。同时依据仿真模型设计制作相同尺寸的密闭玻璃腔体和平板电极,实验测量了不同射频输入功率时放电等离子体的有效电流电压及发射光谱,进而计算等离子体的电子温度及电子密度;利用玻耳兹曼双线测温法,得到光谱法下等离子体的电子温度及电子密度。结果表明:当气体压强为250 Pa、输入功率为100~450 W时,等离子体电压电流呈线性关系,电子密度随功率的增大而增大,而电子温度并未随功率的变化而有明显变化,其与功率无关。运用仿真模拟验证了实验的准确性,通过比较,三种方法所得的结果相近。通过结合等效回路法、光谱法和数值模拟仿真法初步诊断出中等气压下等离子体的放电参数,提出了结合三种方法作为实验研究的方法,使实验结果更具说服力,证明其方法的可靠性,也为进一步的等离子体特性研究提供依据。     

Increase in the efficiency of the shear wave generation in gelatin due to the nonlinear absorption of a focused ultrasonic beam

Experimental results and theoretical estimates are presented to demonstrate the prospects of using the acoustic nonlinearity of a gel-like medium for increasing the efficiency of the shear wave generation in it by a pulsed ultrasonic beam. The experiment is based on the propagation of a focused beam of longitudinal acoustic waves at a frequency of 1.1 MHz in a gelatin sample and on the detection of shear waves by the optical method [1]. It is demonstrated that the amplitude of the shear wave excited by a nonlinear acoustic pulse can be increased by an order of magnitude owing to the formation of shock fronts in the profile of this pulse.     

The effect of static pressure on the inertial cavitation threshold

The amplitude of the acoustic pressure required to nucleate a gas or vapor bubble in a fluid, and to have that bubble undergo an inertial collapse, is termed the inertial cavitation threshold. The magnitude of the inertial cavitation threshold is typically limited by mechanisms other than homogeneous nucleation such that the theoretical maximum is never achieved. However, the onset of inertial cavitation can be suppressed by increasing the static pressure of the fluid. The inertial cavitation threshold was measured in ultrapure water at static pressures up to 30?MPa (300 bars) by exciting a radially symmetric standing wave field in a spherical resonator driven at a resonant frequency of 25.5 kHz. The threshold was found to increase linearly with the static pressure; an exponentially decaying temperature dependence was also found. The nature and properties of the nucleating mechanisms were investigated by comparing the measured thresholds to an independent analysis of the particulate content and available models for nucleation.     

Effect of static pressure on acoustic energy radiated by cavitation bubbles in viscous liquids under ultrasound

The effect of static pressure on acoustic emissions including shock-wave emissions from cavitation bubbles in viscous liquids under ultrasound has been studied by numerical simulations in order to investigate the effect of static pressure on dispersion of nano-particles in liquids by ultrasound. The results of the numerical simulations for bubbles of 5 μm in equilibrium radius at 20 kHz have indicated that the optimal static pressure which maximizes the energy of acoustic waves radiated by a bubble per acoustic cycle increases as the acoustic pressure amplitude increases or the viscosity of the solution decreases. It qualitatively agrees with the experimental results by Sauter et al. [Ultrason. Sonochem. 15, 517 (2008)]. In liquids with relatively high viscosity (～200 mPa s), a bubble collapses more violently than in pure water when the acoustic pressure amplitude is relatively large (～20 bar). In a mixture of bubbles of different equilibrium radius (3 and 5 μm), the acoustic energy radiated by a 5 μm bubble is much larger than that by a 3 μm bubble due to the interaction with bubbles of different equilibrium radius. The acoustic energy radiated by a 5 μm bubble is substantially increased by the interaction with 3 μm bubbles.     

Pressure effects on the kinetic properties and phase transitions in lithium

The pressure dependences of the electrical resistance and thermal electromotive force of lithium were measured at room temperature. The results substantiated the occurrence of a phase transition caused by increasing pressure (6.7 GPa). A phase transition was detected when pressure was decreased (6.4 GPa). Temperature effects on the pressures of these transitions were studied near room temperature. At pressures above 4 GPa, the pressure dependences of thermal electromotive force and of the velocity of ultrasonic shear waves in BCC lithium exhibited anomalies. The suggestion was made that applying pressure increased the role played by electron-phonon and phonon-phonon interactions in lithium.     

Physical features of ultrasound assisted enzymatic degradation of recalcitrant organic pollutants

This work has attempted to provide answer to the interaction of sonolysis and enzymatic treatment on degradation of recalcitrant dyes in a combined treatment. The model system comprises of two dyes, acid red and malachite green as model pollutants, along with horseradish peroxidase as a model enzyme and ultrasound of 20 kHz frequency. A dual approach of coupling experimental results with simulations of cavitation bubble dynamics has been adopted. Utilization of oxidation potential of horseradish peroxidase has been found to be a function of convection level in the medium. Cavitation phenomenon is found to have an adverse effect on enzyme action due to generation of high amplitude shock waves, which denature the enzyme. Degradation of dye at high static pressure increases due to absence of cavitation and high energy interaction (or collisions) between enzyme and dye molecules, which are beneficial towards enzymatic oxidation of the latter. High intensity convection generated by ultrasound also obviates need for an external shielding agent such as PEG that prevents attachment of the phenoxy radicals to enzyme that blocks the active sites of the enzyme.     

Characteristics of electrodeposited bismuth telluride thin films with different crystal growth by adjusting electrolyte temperature and concentration

Bismuth telluride (Bi₂Te₃) thin films were prepared with various electrolyte temperatures (10°C–70 °C) and concentrations [Bi(NO₃)₃ and TeO₂: 1.25–5.0 mM] in this study. The surface morphologies differed significantly between the experiments in which these two electrodeposition conditions were separately adjusted even though the applied current density was in the same range in both cases. At higher electrolyte temperatures, a dendrite crystal structure appeared on the film surface. However, the surface morphology did not change significantly as the electrolyte concentration increased. The dendrite crystal structure formation in the former case may have been caused by the diffusion lengths of the ions increasing with increasing electrolyte temperature. In such a state, the reactive points primarily occur at the tops of spiked areas, leading to dendrite crystal structure formation. In addition, the in-plane thermoelectric properties of Bi₂Te₃ thin films were measured at approximately 300 K. The power factor decreased drastically as the electrolyte temperature increased because of the decrease in electrical conductivity due to the dendrite crystal structure. However, the power factor did not strongly depend on the electrolyte concentration. The highest power factor [1.08 μW/(cm·K²)] was obtained at 3.75 mM. Therefore, to produce electrodeposited Bi₂Te₃ films with improved thermoelectric performances and relatively high deposition rates, the electrolyte temperature should be relatively low (30 °C) and the electrolyte concentration should be set at 3.75 mM.     

驻波型热声发动机的数值模拟

通过对驻波发动机热声边界条件的分析 ,给出了一种用“打靶法”进行迭代运算的循环模式 ,从而避开繁重的矩阵运算 ,计算效率得到有效地提高。最后以一个实例证明本方法计算结果与实验吻合较好。     

Influence of external magnetic field on dust acoustic waves in a capacitive RF discharge

This paper reports experiments on self-excited dust acoustic waves (DAWs) and its propagation characteristics in a magnetized rf discharge plasma. The DAWs are spontaneously excited in dusty plasma after adding more particles in the confining potential well and found to propagate in the direction of streaming ions. The spontaneous excitation of such low-frequency modes is possible due to the instabilities associated with streaming ions through the dust grain medium. The background E-field and neutral pressure determine the stability of excited DAWs. The characteristics of DAWs strongly depend on the strength of external magnetic field. The magnetic field of strength B < 0.05 T only modifies the characteristics of propagating waves in dusty plasma at moderate power and pressure, P = 3.5 W and p = 27 Pa, respectively. It is found that DAWs start to be damped with increasing the magnetic field beyond B > 0.05 T and get completely damped at higher magnetic field B ∼ 0.13 T. After lowering the power and pressure to 3 W and 23 Pa respectively, the excited DAWs in the absence of B are slightly unstable. In this case, the magnetic field only stabilizes and modifies the propagation characteristics of DAWs while the strength of B is increased up to 0.1 T or even higher. The modification of the sheath electric field where particles are confined in the presence of the external magnetic field is the main cause of the modification and damping of the DAWs in a magnetized rf discharge plasma.     

Stimulus dependencies of the gerbil brain-stem auditory-evoked response (BAER). I: Effects of click level, rate, and polarity

Three experiments evaluating the effects of various stimulus manipulations on the click-evoked gerbil brain-stem auditory-evoked response (BAER) are reported. In experiment 1, click polarity and level were covaried. With increasing click level, there is a parallel decrease in the latency of the first five BAER peaks (i-v) and an increase in BAER peak amplitudes. Mean wave i amplitude was greater for rarefaction than condensation clicks at high click levels; mean wave v amplitude was greater for condensation clicks at higher click levels. Experiment 2 covaried click rate and polarity. The latency of the BAER peaks increased with increasing click repetition rate. This rate-dependent latency increase was greater for the later BAER peaks, resulting in an increase in the i-v interval with increasing click rate. As rate increased, the amplitudes of waves i and v decreased monotonically, whereas the amplitudes of waves ii-iv were largely uninfluenced by click rate. As in experiment 1, mean wave i amplitude was greater for rarefaction clicks, whereas mean wave v amplitude was greater for condensation clicks. The magnitude of these polarity dependencies on waves i and v amplitude decreased with increasing click rate. Experiment 3 evaluated the effects of click polarity on BAERs to high-intensity (100 dB pSPL) clicks presented at a rate of 10 Hz. In eight of ten gerbils evaluated, wave i amplitude was greater to rarefaction clicks, and, in all ten animals, wave v amplitude was greater to condensation clicks. The effects of click level and rate on BAER peak amplitudes, latencies, and interwave intervals are reminiscent of stimulus dependencies reported for the human BAER. The effects of click polarity on the amplitudes of waves i and v of the gerbil BAER have also been reported for the human BAER.     

Ionization wave as an Instability — Evolution and Nonlinear Effect

Evolution from the linear growth to the nonlinearly saturated state of ionization waves (moving striations) is investigated from a viewpoint of an instability which appears in a plasma. The experiments were performed observing backward waves excited below the upper critical current I_c in xenon, argon, and argon-mercury gases at pressures of the order of Torr. It is found that (1) the behavior of the evolution obeys the Landau amplitude equation; (2) the saturated amplitude does not depend on an initial value, but only on plasma parameters; (3) the linear growth and squared saturation level are proportional to the excess over I_c, and when frequency-controlled, they have a parabolic dependence on the frequency; and (4) nonuniform axial changes in the dc state due to the nonlinear effects appear in a form of enhanced ionization in the plasma where the electron temperature and density are increased. The way of these behavior can be applied to a large number of spatially or temporally unstable modes.     

Thermal stabilization of the discharge in a nitrogen laser

The effect of cathode temperature upon power output from a transversely excited N₂ laser has been studied. An increase of 60% in output power has been observed for 75° C increase in cathode temperature. This degree of thermal enhancement did not depend significantly upon pulse repetition rate so it was possible to increase the maximum average power by the same factor of 60%. As commutation of the Blumlein was accomplished without the use of spark gaps, relatively higher pulse repetition rates were accessible. By externally heating the cathode to 100°C an average output power of 150 mW was obtained at 100 Hz for a charging voltage of 15 kV.     

高Q Kerr介质腔中非关联双模相干态光场与V型三能级原子相互作用系统中光场的等阶Y压缩效应

本文利用多模压缩态理论,详细研究了高Q Kerr介质腔中非关联双模相干态光场与V型三能级原子相互作用系统中双模光场的一次和二次等阶Y压缩效应,结果表明:1)等阶Y压缩特性强烈地依赖于Kerr介质的三阶非线性极化系数(x)和双模光场中各模的平均光子数n¹、n²;当x<1,n¹=n²<1时,不产生等阶Y压缩效应;而当x=2.5、5、10以及n¹=n²=2、5时,光场可呈现出强烈的一次及二次等阶Y压缩效应;2)在x以及n¹、n²相同的条件下,不同压缩阶次的等阶Y压缩效应的时间演化曲线的压缩周期与压缩阶次成反比,压缩度的幅度则随压缩阶数的升高而迅速下降;3)在平均光子不变(即n¹=n²恒定),但Kerr介质不同(即x变化)的条件下,相同压缩阶数的等阶Y压缩效应的Y压缩度曲线的时间演化周期与Kerr介质的三阶非线性化系数x成反比,压缩度的幅度则随Kerr介质的三阶非线性极化系数x的增大而增强;4)等阶Y压缩效应的持续时间以及等阶Y压缩度的大小等强烈地依赖于Kerr介质的非线性程度和光场强度;一般而言,Kerr介质的非线性程度越高(即x越大),并且光场越强(即n¹=n²取值越大),等阶Y压缩效应持续时间就越短,等阶Y压缩度就越大.