Experimental study on subharmonic and ultraharmonic acoustic waves in water-saturated sandy sediment

Experimental observations of the subharmonic and ultraharmonic acoustic waves in water-saturated sandy sediment are reported in this paper. Acoustic pressures of both nonlinear acoustic waves strongly depend on the driving acoustic pressure at a transducer. The first ultraharmonic wave reaches a saturation value as the driving acoustic pressure increases. The acoustic pressure levels of both nonlinear acoustic waves exhibit some fluctuations in comparison with that of the primary acoustic wave as the receiving distance of hydrophone increases in sediment. The subharmonic and the ultraharmonic phenomena in this study show close resemblance to those produced in bubbly water.     

颗粒样品形变对声波传播影响的实验探究

为了更好地理解颗粒间接触结构的变化对通过颗粒介质中的声波的影响,本文利用单轴压缩实验,通过一系列增加的轴向压力使样品塑性应变不断增大,这在颗粒尺度上对应于颗粒间接触结构的改变.我们测量了此过程中通过颗粒样品的声波变化,结果表明颗粒体系内接触结构的变化对声波波形中的非相干波部分和频率有明显的影响,并且在样品接触结构变化的初始阶段声速是偏离有效介质理论的预测的.     

A modified kinematic model for the discharge of a granular-like material from long vertical cylinders

The rapid and uncontrolled discharge of a large-sized granular-like material from a vertical cylinder is modelled assuming a modified kinematic relationship exists between granular pressure, speed and density. Discharge is driven by the initial Janssen pressure, and a Beverloo-type equation is derived for the initial discharge. A shock occurs at the bottom of the cylinder, and another upwards travelling shock separates static and moving material. The initial discharge is non-constant, with the constant density and discharge case violating the 'entropy' condition. Two sets of characteristics are found : one travelling upwards, associated with the motion of voids; and the other travelling downwards, associated with work performed by the particle pressure. Contrary to hopper models, a low density of solids is predicted about the cylinder exit. The modified kinematic model allows density waves to travel either up or down through the cylinder, but the frequency and speed of the waves is not fixed uniquely by the model. The waves exhibit a saw-tooth behaviour, with a continuously increasing magnitude in flux at the orifice, interspersed with discontinuous decreases.     

Energy law preserving C finite element schemes for phase field models in two-phase flow computations

We use the idea in [33] to develop the energy law preserving method and compute the diffusive interface (phase-field) models of Allen–Cahn and Cahn–Hilliard type, respectively, governing the motion of two-phase incompressible flows. We discretize these two models using a C⁰ finite element in space and a modified midpoint scheme in time. To increase the stability in the pressure variable we treat the divergence free condition by a penalty formulation, under which the discrete energy law can still be derived for these diffusive interface models. Through an example we demonstrate that the energy law preserving method is beneficial for computing these multi-phase flow models. We also demonstrate that when applying the energy law preserving method to the model of Cahn–Hilliard type, un-physical interfacial oscillations may occur. We examine the source of such oscillations and a remedy is presented to eliminate the oscillations. A few two-phase incompressible flow examples are computed to show the good performance of our method.     

On the validity of Hertz contact law for granular material acoustics

We discuss the acoustical behavior of a 1D model of granular medium, which is a chain of identical spherical beads. In this geometry, we are able to test quantitatively alternative models to the Hertz theory of contact between elastic solids. We compare the predictions of the different models to experimental results that concern linear sound wave propagation in the chain submitted to a static force, and nonlinear solitary wave propagation in an unconstrained chain. We use elastic, elastic-plastic and brittle materials, the beads roughness extends on one order of magnitude, and we also use oxidized metallic beads. We demonstrate experimentally that at low static forces, for all types of beads, the linear acoustic waves propagate in the system as predicted by Hertz's theory. At larger forces, after onset of permanent plastic deformation at the contacts, the brass beads exhibit non Hertzian behavior, and hysteresis. Except in the case of brass beads, the nonlinear waves follow the predictions of Hertz theory. Revised: 28 May 1998 / Accepted: 27 July 1998     

Multiple conversion and optical limiting in a subharmonic-pumped parametric oscillator

We demonstrate that multiple coexisting frequency-conversion processes can occur in an externally resonant second-harmonic generator under suitable conditions. Besides the generation of signal and idler waves by subharmonic-pumped parametric oscillation, sum-frequency mixing among the resonant subharmonic (1064-nm), signal, and idler waves was observed, leading to additional emission wavelengths around the harmonic wavelength (532 nm). The output waves both exhibit high frequency stability, with as long as 4 h of mode-hop-free parametric oscillation, and are continuously tunable over 2 GHz. Near degeneracy the parametric oscillator operates as an optical limiter for the harmonic wave.     

Periodic and disordered structures in a modulated gas-driven granular layer

Experiments with a thin gas-fluidized granular layer revealed a sequence of well-defined transitions as the amplitude and frequency of the gas flow modulation are varied. The observed patterns include subharmonic squares and stripes, quasiperiodic and disordered structures. The wavelength of subharmonic patterns increases with the mean flow rate and decreases with the modulation frequency.     

器壁滑动摩擦力对受振颗粒体系中冲击力倍周期分岔过程的影响

颗粒物质由离散的固体颗粒组成, 受到周期性振动时可以表现出复杂的动力学行为. 这些行为往往受众多因素的影响, 如空气阻力和器壁摩擦力等. 针对受振颗粒体系中冲击力的倍周期分岔现象, 通过抽真空或将容器底镂空消除空气阻力, 单独研究器壁滑动摩擦力的影响. 结果表明在仅有器壁摩擦力作用的情况下, 倍周期分岔过程仅受约化振动加速度的控制, 与颗粒的尺寸、颗粒层数及振动频率无关. 将器壁摩擦力处理成一个大小恒定、方向与颗粒和器壁相对速度反向的阻力, 并包含到完全非弹性蹦球模型中, 能够对所观察到的现象给出很好的解释. 通过对倍周期分岔点测量平均值的拟合, 得到器壁滑动摩擦力的大小约为颗粒总重量的10%. 关键词： 颗粒物质 器壁摩擦力 倍周期分岔 冲击力     

流化床内超细颗粒的流动

基于气体分子运动论和颗粒动理学,建立超细颗粒气固两相湍流流动模型,模型考虑了气相与颗粒聚团之间以及颗粒聚团之间的动量和能量的传递和耗散。建立超细颗粒固相粘性系数、超细颗粒压力等物性参数计算模型。超细颗粒的聚团改变了单颗粒碰撞动力学以及颗粒相压力、粘性系数等输运特性。模型模拟计算颗粒聚团直径分布与Zhaolin等[1]实测值相吻合。     

Impact of acoustic pressure on ambient pressure estimation using ultrasound contrast agent

Local blood pressure measurements provide important information on the state of health of organs in the body and can be used to diagnose diseases in the heart, lungs, and kidneys. This paper presents an approach for investigating the ambient pressure sensitivity of a contrast agent using diagnostic ultrasound. The experimental setup resembles a realistic clinical setup utilizing a single array transducer for transmit and receive. The ambient pressure sensitivity of SonoVue (Bracco, Milano, Italy) was measured twice using two different acoustic driving pressures, which were selected based on a preliminary experiment. To compensate for variations in bubble response and to make the estimates more robust, the relation between the energy of the subharmonic and the fundamental component was chosen as a measure over the subharmonic peak amplitude. The preliminary study revealed the growth stage of the subharmonic component to occur at acoustic driving pressures between 300 and 500 kPa. Based on this, the pressure sensitivity was investigated using a driving pressure of 485 and 500 kPa. At 485 kPa, a linear pressure sensitivity of 0.42 dB/kPa was found having a linear correlation coefficient of 0.94. The second measurement series at 485 kPa showed a sensitivity of 0.41 dB/kPa with a correlation coefficient of 0.89. Based on the measurements at 500 kPa, this acoustic driving pressure was concluded to be too high causing the bubbles to be destroyed. The pressure sensitivity for these two measurement series were 0.42 and 0.25 dB/kPa with linear correlation coefficients of 0.98 and 0.93, respectively.     

竖直振动颗粒物厚层中冲击力分岔现象

实验研究了竖直振动颗粒物厚层中颗粒对容器底部的压力. 发现这种压力是脉冲式的，并表现出受振动加速度控制的倍周期分岔现象. 在颗粒层底部观察到颗粒密堆积在一起的聚集态. 聚集态内颗粒的自由程较小，并像一个整体一样运动. 关键词： 颗粒物质 混沌 倍周期分岔 非弹性碰撞     

Air-Driven Segregation in Binary Granular Mixtures with Same Size but Different Densities

We investigate the segregation effect of binary granular mixtures with the same size but different densities under vibration at different air pressures. Our experiments show that the segregation state is seriously dependent on the air pressure and there is a new type of partially segregated state at high air pressure, which has the characteristic that the lighter grains tend to stay at the bottom and form a pure layer, while heavier grains and remained lighter ones tend to rise and to form a mixed layer on the top of the system. We redefine the order parameter to study the variation of the segregation effect with the air pressure and vibration parameter in detail. Finally, the mechanism of the air-driven segregation is illustrated by the faster acceleration due to the airflow through the granular bed for lighter particles.     

Numerical 3D flow simulation of ultrasonic horns with attached cavitation structures and assessment of flow aggressiveness and cavitation erosion sensitive wall zones

As a contribution to a better understanding of cavitation erosion mechanisms, a compressible inviscid finite volume flow solver with barotropic homogeneous liquid–vapor mixture cavitation model is applied to ultrasonic horn set-ups with and without stationary specimen, that exhibit attached cavitation at the horn tip. Void collapses and shock waves, which are closely related to cavitation erosion, are resolved. The computational results are compared to hydrophone, shadowgraphy and erosion test data. At the horn tip, vapor volume and topology, subharmonic oscillation frequency as well as the amplitude of propagating pressure waves are in good agreement with experimental data. For the evaluation of flow aggressiveness and the assessment of erosion sensitive wall zones, statistical analyses of wall loads and of the multiplicity of distinct collapses in wall-adjacent flow regions are applied to the horn tip and the stationary specimen. An a posteriori projection of load collectives, i.e. cumulative collapse rate vs. collapse pressure, onto a reference grid eliminates the grid dependency effectively for attached cavitation at the horn tip, whereas a significant grid dependency remains at the stationary specimen. The load collectives show an exponential decrease towards higher collapse pressures. Erosion sensitive wall zones are well predicted for both, horn tip and stationary specimen, and load profiles are in good qualitative agreement with measured topography profiles of eroded duplex stainless steel samples after long-term runs. For the considered amplitude and gap width according to ASTM G32-10 standard, the analysis of load collectives reveals that the distinctive erosive ring shape at the horn tip can be attributed to frequent breakdown and re-development of a small portion of the tip-attached cavity. This partial breakdown of the attached cavity repeats at each driving cycle and is associated with relatively moderate collapse peak pressures, whereas the stationary specimen is rather unfrequently stressed at the end of each subharmonic oscillation cycle by the violent collapse of the complete cavity.     

振动颗粒物质中倍周期运动对尺寸分离的影响

实验研究了竖直振动颗粒床中,倍周期运动对尺寸分离的影响.实验中,当振动加速度足够大时,系统中出现稳定的对称对流,进一步增大振动加速度到某个临界值时,还会出现倍周期运动.观察表明,背景颗粒的对流运动对分离过程起主导作用,对流速度决定着分离过程的快慢,而在2倍周期和4倍周期分岔之后,分离时间有所减慢.对引起对流运动的起因进行了分析,以此为基础分析了倍周期运动产生影响的物理机理,并对分离时间进行了定量计算,结果与实验值符合很好. 关键词： 颗粒物质 “巴西果”效应 倍周期分岔 对流     

Fundamental study on subharmonic imaging by irradiation of amplitude-modulated ultrasound waves

The second harmonic and subharmonic components, the frequencies of which are twice and one half the fundamental frequency, are included in echoes from contrast agents. An imaging method, which employs a second harmonic (second harmonic imaging), is widely used in medical diagnoses. On the other hand, subharmonic is expected to provide a higher contrast between biological tissues and blood flow because echo signals are generated only from blood containing the contrast agents. However, the subharmonic component echo signal power from contrast agents is relatively low. This has resulted in little progress in the field of subharmonic imaging. In this study, a new imaging method is proposed using amplitude-modulated waves as transmitted waves combined with the pulse inversion method to enhance subharmonic echo signals. Two optimal frequencies are set, including the modulated waves, F(1) and F(2), so that the subharmonic frequency of F(1) and the second harmonic frequency of F(2) may result in the same value. This allows a more powerful signal at the frequency band because the second harmonic and subharmonic components are integrated. Furthermore, a B-mode ultrasound image of an agar phantom that imitated biological tissue and showed the effectiveness of our method was reconstructed. As a result, the echo power of the subharmonic component was enhanced by approximately 11.8 dB more than the conventional method and the signal to noise ratio showed an improvement of 7.6 dB.     

稠密颗粒射流撞击壁面颗粒膜表面波纹特征

采用高速摄像仪对稠密颗粒射流撞击有限尺寸壁面的流动过程进行了实验研究,重点研究了颗粒膜及其表面波纹特征,考察了颗粒粒径、射流速度和固含率等因素对颗粒膜形态和表面波纹的影响.研究结果表明,随着颗粒粒径增大,稠密颗粒撞壁流由颗粒膜向散射模式转变.与液体射流撞壁液膜相比,颗粒膜扩展角较大,射流速度对其影响不显著.稠密颗粒射流撞壁颗粒膜表面波纹存在明显的叠加现象,颗粒膜表面波纹频率比液膜大约低一个数量级.颗粒膜表面波纹主要由射流脉动引起,表面波纹频率与射流脉动频率具有相同的数量级.     

Nonlinear dynamics of density waves in granular flows through narrow vertical channels

We study granular flows through narrow channels driven by gravity in the framework of the kinetic theory for dissipative dense gases. We derive equations of motion for quasi-one-dimensional systems. In a certain range of flow density, the steady homogeneous regime is found to be unstable against the formation of density waves. We show moreover that near the onset of the instability, the governing equation for the flow density is a mixture of the Korteweg-de-Vries equation, which leads to soliton, and the Bürger equation which exhibits spatio-temporal chaos. The competition between chaos and solitons may lead either to regular spatially ordered density waves or to chaotic dynamics. We argue that these two types of dynamics can be encountered experimentally according to the channel width and the dissipative properties of the granular media. Received: 11 March 1998 / Revised and Accepted: 3 July 1998     

考虑气蚀效应的气液两相冲压发动机数值模拟

针对一种新型水下气液两相冲压发动机,综合考虑了湍流效应、气液两相之间的拖曳作用及传热与传质,利用计算流体力学方法研究了气液两相冲压发动机内流场的流动特性随发动机工作条件的变化规律,重点研究了气蚀效应对发动机工作性能的影响.主要结论为:当航行速度大于32 m/s,气液两相冲压发动机入口附近会产生气蚀并造成严重的总压损失,导致扩张段下游产生流动分离,此时发动机无法产生正推力;通过增大气体质量流率,气液两相冲压发动机内流场的压力将会随之升高,气蚀效应被抑制;提高注入发动机气体的温度,发动机的推力及比冲均增大,但是发动机效率急剧降低.      

PRESSURE FLUCTUATION AND FLOW REGIMES OF AIR-WATER FLOW IN A SMALL TUBE

Previous studies have shown that flow regimes of gas-liquid flow in small tubes and channels are related to pressure drop fluctuations along the flow passages, and some flow regimes exhibit characteristic pressure fluctuation patterns. This has led to the possibility of using pressure fluctuations to identify flow patterns. This article presents a test program to show experimentally the relationship of pressure fluctuations to gas bubbles, liquid slugs, and waves on a gas-liquid interface in small tubes and channels. In the experiment, a high-speed video camera was used to examine flow mixture passing a pressure tap. Pressure fluctuation at the tap was measured simultaneously using a pressure transducer. The results have shown that high-frequency pressure fluctuation is related to gas bubbles, liquid slugs, or waves on a gas-liquid interface passing the pressure tap. It has found that low-frequency pressure waves are related to flow regimes in the test tube. This research has provided further information for flow regime identification using pressure fluctuation traces.     

The metre

A musical wind instrument transforms a constant pressure input from the player's mouth into a fluctuating pressure output in the form of a radiating sound wave. In reed woodwind and brass instruments, this transformation is achieved through a nonlinear coupling between two vibrating systems: the flow control valve formed by the mechanical reed or the lips of the player, and the air column contained by the pipe. Although the basic physics of reed wind instruments was developed by Helmholtz in the nineteenth century, the application of ideas from the modern theory of nonlinear dynamics has led to recent advances in our understanding of some musically important features of wind instrument behaviour. As a first step, the nonlinear aspects of the musical oscillator can be considered to be concentrated in the flow control valve; the air column can be treated as a linear vibrating system, with a set of natural modes of vibration corresponding to the standing waves in the pipe. Recent models based on these assumptions have had reasonable success in predicting the threshold blowing pressure and sounding frequency of a clarinet, as well as explaining at least qualitatively the way in which the timbre of the sound varies with blowing pressure. The situation is more complicated for brass instruments, in which the player's lips provide the flow valve. Experiments using artificial lips have been important in permitting systematic studies of the coupling between lips and air column; the detailed nature of this coupling is still not fully understood. In addition, the assumption of linearity in the air column vibratory system sometimes breaks down for brass instruments. Nonlinear effects in the propagation of high amplitude sound waves can lead to the development of shock waves in trumpets and trombones, with important musical consequences.