Effect of driving voltage pulse shape on the luminance of thin-film electroluminescent devices

The dependence of the average luminance on the parameters of the metal-insulator-semiconductor-insulator-metal structures in thin-film electroluminescent devices (TFELDs) and on the excitation conditions is found by solving kinetic equations for the variation in the concentration of excited emission centers in the phosphor layer of TFELDs for different driving voltage pulse shapes (triangular, trapezoidal, sinusoidal, and rectangular with an exponential leading edge). It is shown that for equal amplitudes and pulse repetition rates of a sign-changing symmetric voltage, the average luminance and luminous efficacy of TFELDs increase as the rate of rise of the voltage is increased for different driving voltage pulse shapes in the following sequence: triangular, sinusoidal, trapezoidal, rectangular. The calculations are confirmed by experiment. Zh. Tekh. Fiz. 69, 64–69 (February 1999)     

Nonlinear compressional pulses in a 2D crystallized dusty plasma

Compressional pulses were launched in a two-dimensional Yukawa lattice, a hexagonal monolayer of polymer microspheres suspended in a plasma. The pulsed wave was excited by a laser beam, and nonlinear effects were observed for Mach numbers M>0.07 and for variation of particle number density delta(n)/n>0.1, but no steepening of the pulse was detected. The pulse propagation speed was found to be comparable to the sound speed of compressional waves launched with sinusoidal excitation.     

Numerical simulation of nonlinear propagation of sound waves in a finite horn

Nonlinear acoustic propagation generated by a piston in a finite horn is numerically studied.A quasi-one-dimensional nonlinear model with varying cross-section uses high-order low-dispersion numerical schemes to solve the governing equation.Because of the nonlinear wave distortion and reflected sound waves at the mouth,broadband time-domain impedance boundary conditions are employed.The impedance approximation can be optimized to identify the complex-conjugate pole-residue pairs of the impedance functions,which can be calculated by fast and efficient recursive convolution.The numerical results agree very well with experimental data in the situations of weak nonlinear wave propagation in an exponential horn,it is shown that the model can describe the broadband characteristics caused by nonlinear distortion.Moreover,finite-amplitude acoustic propagation in types of horns is simulated,including hyperbolic,conical,exponential and sinusoidal horns.It is found that sound pressure levels at the horn mouth are strongly affected by the horn profiles,the driving velocity and frequency of the piston.The paper also discusses the influence of the horn geometry on nonlinear waveform distortion.     

Viscosity destabilizes sonoluminescing bubbles

In single-bubble sonoluminescence (SBSL) microbubbles are trapped in a standing sound wave, typically in water or water-glycerol mixtures. However, in viscous liquids such as glycol, methylformamide, or sulphuric acid it is not possible to trap the bubble in a stable position. This is very peculiar as larger viscosity normally stabilizes the dynamics. Suslick and co-workers call this new mysterious state of SBSL "moving-SBSL." We identify the history force (a force nonlocal in time) as the origin of this destabilization and show that the instability is parametric. A force balance model quantitatively accounts for the observed quasiperiodic bubble trajectories.     

平面波与平面行波脉冲的声相互作用

Westervelt在研究两正交平面波的相互作用时曾经推论:在两个波的公共区之外没有散射声。但由于所论的公共区不是一个有限大的区域,故“公共区之外”是没有实际意义的。本文讨论一列平面波与一列平面行波脉冲的声相互作用,显然这两个波不仅都满足波动方程,而且可以用来讨论公共区之外的声散射声问题。计算结果表明,如果介质没有频散效应,则公共区之外不存在服从因果律的声散射声。本文还指出,对于这两个一阶场所构成的公共区来说,应用Lighthill远场积分来讨论这种情况下的声散射声是不妥当的。 关键词：     

Rogue waves in superfluid helium

Rogue waves have been observed in superfluid helium. The experimental system consists of high intensity second sound (temperature-entropy) waves within a resonant cavity. Under steady state conditions, with a constant oscillatory driving force at the resonant frequency, the waves are turbulent and there are fluxes of energy towards both high and low frequencies. Rogue waves appear under the nonequilibrium conditions that prevail shortly after the drive has been switched on, prior to establishment of the steady state. The experiment is described briefly, relevant results are presented and discussed theoretically in terms of nonlinear wave interactions, and possible connections to rogue waves on the ocean are considered.     

有限长号筒中非线性声传播的数值模拟研究

研究了大振幅活塞声源经有限长号筒向外辐射声波的非线性声学问题。采用具有频散保持特性的高精度计算格式求解了适用于变截面管道的一维非线性声场模型,并考虑到声波的非线性畸变及管口处的声反射,加入了宽频时域声阻抗边界条件。宽频阻抗模型的共轭复数系数采用优化拟合方法近似求解,并采用递推卷积算法快速求解出时域声阻抗。在弱非线性条件下模拟指数形号筒中的声传播取得了与已有实验相一致的结果,表明模型能够描述声波非线性畸变带来的宽频特性。进而采用本模型数值模拟了大振幅活塞声源在双曲形、锥形、指数形和正弦形号筒中的非线性声传播问题,结果表明号筒出口声压级受活塞振动速度、频率以及号筒形状的影响,并分析讨论了波形畸变与号筒几何形状之间的关系。      

Numerical simulations of the aspherical collapse of laser and acoustically generated bubbles

The details of nonlinear axisymmetric oscillations and collapse of bubbles subject to large internal or external pressure disturbances, are studied via a boundary integral method. Weak viscous effects on the liquid side are accounted for by integrating the equations of motion across the boundary layer that is formed adjacent to the interface. Simulations of single-cavitation bubble luminescence (SCBL) and single-bubble sonoluminescence (SBSL) are performed under conditions similar to reported experimental observations, aiming at capturing the details of bubble collapse. It is shown that any small initial deviation from sphericity, modeled through a small initial elongation along the axis of symmetry, may result in the formation and impact of two counter-propagating jets during collapse of the bubble, provided the amplitude of the initial disturbance is large enough and the viscosity of the surrounding fluid is small enough. Comparison between simulations and experimental observations show that this is the case for bubbles induced via a nano-second laser pulse (SCBL) during a luminescence event. In a similar fashion, simulations show that loss of sphericity accompanied with jet formation and impact during collapse is also possible with acoustically trapped bubbles in a standing pressure wave (SBSL), due to the many afterbounces of the bubble during its collapse phase. In both cases jet impact occurs as a result of P(2) growth in the form of an afterbounce instability. When the sound amplitude is decreased or liquid viscosity is increased the intensity of the afterbounce is decreased and jet impact is suppressed. When the sound amplitude is increased jet formation is superceded by Rayleigh-Taylor instability. In the same context stable luminescence is quenched in experimental observations. In both SCBL and SBSL simulations the severity of jet impact during collapse is quite large, and its local nature quite distinct. This attests to the fact that it is an energy focusing mechanism whose importance in generating the conditions under which a luminescence event is observed should be further investigated.     

The wave equation with viscoelastic attenuation and its application in problems of shallow-sea acoustics

A suitable tool for the simulation of low frequency acoustic pulse signals propagating in a shallow sea is the numerical integration of the nonstationary wave equation. The main feature of such simulation problems is that in this case the sound waves propagate in the geoacoustic waveguide formed by the upper layers of the bottom and the water column. By this reason, the correct dependence of the attenuation of sound waves in the bottom on their frequency must be taken into account. In this paper we obtain an integro-differential equation for the sound waves in the viscoelastic fluid, which allows to simulate the arbitrary dependence of acoustic wave attenuation on frequency in the time domain computations. The procedure of numerical solution of this equation based on its approximation by a system of differential equations is then considered and the methods of artificial limitation of computational domain are described. We also construct a simple finite-difference scheme for the proposed equation suitable for the numerical solution of nonstationary problems arising in the shallow-sea acoustics.     

Numerical simulations of random sound waves

We perform one-dimensional numerical simulations of both driven and impulsively generated sound waves propagating through a medium whose mass density admits time-independent, random fluctuations. While the amplitude of both types of wave is always attenuated, driven sound waves can be either retarded or speeded up depending on their wavenumber and amplitude and on the strength of the random field. The speed of a pulse propagating in the random medium is also altered, in agreement with the findings for the driven waves. The concomitant action of nonlinearity and randomness results in wave speeding for wavenumbers which are of the order of the size of an average random density fluctuation, whereas it gives retardation for larger wavenumbers.     

Deconvolution of sinusoidal rapid EPR scans

In rapid scan EPR the magnetic field is scanned through the signal in a time that is short relative to electron spin relaxation times. Previously it was shown that the slow-scan lineshape could be recovered from triangular rapid scans by Fourier deconvolution. In this paper a general Fourier deconvolution method is described and demonstrated to recover the slow-scan lineshape from sinusoidal rapid scans. Since an analytical expression for the Fourier transform of the driving function for a sinusoidal scan was not readily apparent, a numerical method was developed to do the deconvolution. The slow scan EPR lineshapes recovered from rapid triangular and sinusoidal scans are in excellent agreement for lithium phthalocyanine, a trityl radical, and the nitroxyl radical, tempone. The availability of a method to deconvolute sinusoidal rapid scans makes it possible to scan faster than is feasible for triangular scans because of hardware limitations on triangular scans.     

含气泡液体中的非线性声传播

考虑到分布在液体中的气泡是声波在含气泡液体中传播时引起非线性的一个很重要的因素,本文研究了声波在含气泡液体中的非线性传播.将气体含量的影响引入到声波在液体中传播的方程中,从而得到声波在气液混合物中传播的数学模型.通过对该模型进行数值模拟发现,气体含量、驱动声场声压幅值及驱动声场作用时间均会影响到气液混合物中的声场分布及声压幅值大小.液体中的气泡会"阻滞"液体中声场的传播并将能量"聚集"在声源附近.对于连续大功率的驱动声场来说,液体中的气泡会"阻滞"气液混合物中声场及其能量的传播.     

Effect of time profile of laser pulse on the excitation of sound signals in a liquid with a series of laser pulses

We have studied the sound generation with high repetition rate pulsed laser. We have solved the inhomogeneous wave equation for acoustic pressure in a liquid generated by a laser, using Green’s function formalism and convolution technique. To obtain the maximum pressure of the sound waves, we found the conditions on repetition rate and on period of laser pulse of various shapes. Our analysis shows that the sound generated in a liquid with a series of laser pulses is highly affected by the time profile of the pulses besides other parameters, namely laser beam diameter, laser beam optical wavelength, repetition rate and period of laser pulse. This effect is pronounced particularly in frequency domain. We found that the noise of higher harmonics in the generated sound can be greatly removed with the proper choice of the time profile of the laser pulses. It is found that the pressure is generated around the fundamental frequency for the half-sine and rectangular pulses, with the proper choice of repetition rate and period of pulse. The application of the present analysis for underwater communication is pointed out.     

基于BLT方程的微带线电磁耦合终端响应

为定量研究电磁波与微带线的耦合终端响应问题,提出一种基于BLT方程的电磁波与微带线耦合分析方法,相比传统方法计算效率高、占用内存少。将PCB板上微带线等效为有耗传输线模型,对辐照平面波进行矢量分析并求解等效激励源,运用BLT方程研究不同入射方式、脉冲波形对微带线终端的影响,并对入射波与终端响应进行了时域、频域分析。研究表明:当入射波频率与微带线长度满足一定关系式时对微带线的耦合最强,耦合电压峰值达2.4 mV;电场平行微带线入射比垂直PCB入射时的终端电压峰值大一倍;且同幅同脉宽的矩形脉冲较高斯脉冲和三角脉冲对微带线的耦合峰值电压大;不同波形的脉冲通过调节脉宽都可对微带线耦合终端电压达到mV量级。     

Wave field synthesis of moving virtual sound sources with complex radiation properties

An approach to the synthesis of moving virtual sound sources with complex radiation properties in wave field synthesis is presented. The approach exploits the fact that any stationary sound source of finite spatial extent radiates spherical waves at sufficient distance. The angular dependency of the radiation properties of the source under consideration is reflected by the amplitude and phase distribution on the spherical wave fronts. The sound field emitted by a uniformly moving monopole source is derived and the far-field radiation properties of the complex virtual source under consideration are incorporated in order to derive a closed-form expression for the loudspeaker driving signal. The results are illustrated via numerical simulations of the synthesis of the sound field of a sample moving complex virtual source.     

Acoustic characteristics of pulse detonation engine with ellipsoidal reflector

Acoustic characteristics of a pulse detonation engine(PDE) with and without an ellipsoidal reflector are numerically and experimentally investigated. A two-dimensional(2 D) non-splitting unstructured triangular mesh Euler solver based on the space-time conservation element and solution element(CE/SE) method is employed to simulate the flow field of a PDE.The numerical results clearly demonstrate the external flow field of the PDE. The effect of an ellipsoidal reflector on the flow field characteristic near the PDE exit is investigated. The formation process of reflected shock wave and reflected jet shock are reported in detail. An acoustic measurement system is established for the PDE acoustic testing. The experimental results show that the ellipsoidal reflector changes the sound waveform and directivity of PDE sound. The reflected shock wave and reflected jet shock result in two more positive pressure peaks in the sound waveform. The ellipsoidal reflector changes the directivity of PDE sound from 20 to 0. It is found that the peak sound pressure level(PSPL) and overall sound pressure level(OASPL) each obtain an increment when the PDE is installed with a reflector. The maximum relative increase ratio of PSPL and OASPL are obtained at the focus point F2, whose values are 6.1% and 6.84% respectively. The results of the duration of the PDE sound indicate that the reflecting and focusing wave generated by the reflector result in the increment of A duration and B duration before and near focus point F2. Results show that the ellipsoidal reflector has a great influence on the acoustic characteristic of PDE sound. The research is helpful for understanding the influence of an ellipsoidal reflector on the formation and propagation process of PDE sound.     

Edge wave on axis behind an aperture or disk having a ragged edge

Diffraction by a circular aperture or disk having a ragged edge is investigated. Theory and measurements are reported. The ragged edge is modeled as N arcs, of differing radii a(i), each of which contributes a scattered signal to the edge wave on axis behind the aperture or disk. The amplitude of each scattered signal is proportional to the angle of the arc, and the corresponding time delay is square root of [(ai)2 + (s0)2/c0], where s0 is the axial distance from the aperture plane and c0 is the sound speed. Kirchhoff theory is used to make the calculation. A formula is derived for the rms pressure of the edge wave in terms of the rms pressure and autocorrelation function of the incident wave. The formula can be evaluated for incident waves that are sinusoidal, random (e.g., noise), or transient. Predictions agree reasonably well with underwater measurements made with a spark-generated pulse incident on various apertures. The main result is that making the edge ragged reduces the rms pressure of the edge wave. Indeed, an edge profile is presented that, for a given frequency and axial observation point, eliminates the edge wave completely.     

Characteristics and application of laser-generated acoustic shock waves in air

When a high-power laser beam is focused at a point, the air at the focal point is heated to temperatures of thousands of degrees within several nanoseconds and breaks down. This generates a spark that, in turn, is accompanied by an acoustic shock wave. The acoustic shock waves generated by focussing the beam from a pulsed laser with a 1064 nm wavelength and a power of 800 mJ per pulse have been measured using 1/4″ and 1/8″ B&K microphones. Nonlinear sound levels are observed up to 1.5 m from the laser-induced sparks. Beyond a certain region close to the source, levels are found to decrease in a manner consistent with spherical spreading plus nonlinear hydrodynamic losses. Analysis of the waveforms shows that the acoustic pulses associated with the laser-induced sparks are more repeatable and have higher intensity than those from an electrical spark source. Laser-generated acoustic shock waves are ideal for simulating a blast wave or a sonic boom in the laboratory and for studying the associated propagation effects. To illustrate this application, the propagation of the laser generated shock waves over a series of different hard, rough surfaces has been investigated. The results show the distinctive influences of ground roughness on the propagation of the shock wave.     

Edge Effects on Growth of Ordered Stress Relief Patterns in Free Sustained Aluminum Films

An unusual form of ordered stress relief patterns is observed in a nearly free sustained aluminum film system deposited on liquid substrates by the therm~l evaporation method. The edge effects on the growth of the ordered patterns are systematically studied. It is found that the patterns initiate from the film edges, preexisting ordered patterns, or other imperfections of the film. When the patterns extend in the film regions, they decay gradually and finally disappear. If they develop along the boundaries, however, the sizes are almost unchanged over several millimeters. The stress relief patterns look like rectangular waves in appearance, which are proven to evolve from sinusoidal to triangular waves gradually. The morphological evolution can be well explained by the general theory of buckling of plates.     

New interior solution describing relativistic fluid sphere

Nonlinear low-frequency electrostatic waves in a magnetized, three-component plasma consisting of hot electrons, hot positrons and warm ions have been investigated. The electrons and positrons are assumed to have Boltzmann density distributions while the motion of the ions are governed by fluid equations. The system is closed with the Poisson equation. This set of equations is numerically solved for the electric field. The effects of the driving electric field, ion temperature, positron density, ion drift, Mach number and propagation angle are investigated. It is shown that depending on the driving electric field, ion temperature, positron density, ion drift, Mach number and propagation angle, the numerical solutions exhibit waveforms that are sinusoidal, sawtooth and spiky. The introduction of the Poisson equation increased the Mach number required to generate the waveforms but the driving electric field E ₀ was reduced. The results are compared with satellite observations.