Stress distribution and surface instability of an inclined granular layer

Static granular materials may avalanche suddenly under continuous quasi-static drives. This phenomenon, which is important in many engineering applications, can be explained by analyzing the stability of the elastic solutions. We show this for a granular layer driven by its inclination angle in gravity, where the elastic problem can be solved generally and analytically. It is found that a loss of stability may occur only at the free surface of the layer. This result is considered to be relevant for understanding surface avalanches and the flows observed experimentally.     

颗粒固体弹性势能的声波性质

基于大量静止颗粒堆积而成的物体属于非线性弹性体的观点,计算了其弹性势能的声速表达式(考虑圆柱对称和沿主应力方向传播的平面波情形).通过与己报道的一些实验数据的对比,认为精密的声学方法能够测定势能中的所有材料参数,因此是研究这类材料非线性弹性行为细节的一个重要观测手段,但需要对样品的所有应力、密度和均匀程度进行全面仔细的监控.     

不同形状的棒在颗粒物质中受到的静摩擦力

通过实验对横截面周长相同、形状不同的棒在颗粒物质中受到的最大静摩擦力进行了研究.实验结果表明:相同条件下,圆棒、方棒和扁棒受到的最大静摩擦力各不相同.用水平面上颗粒分层、对称分布模型对实验结果进行了定量分析.     

Dissipative effects on internal gravity waves in geophysical fluids

Summary In the present paper we develop a systematic analysis of the dispersion and dissipation effects on internal gravity waves in fluids of geophysical interest. Dissipation induces a cut-off in the short wavelengths, leading to an admissible range of waves. Defining a suitable criterion for detection of waves, an observable wave range results, in which the frequency dispersion is nearly the same as for the inviscid case, whereas the amplitude is damped. The peculiar behaviour displayed by the dispersion relationship for Prandtl number equal to one is discussed.     

Effects of granular size on stress transmission in a granular column

We investigate the deflection and the fluctuation of stresses due to different sizes of granular material in a cylindrical column. It is experimentally observed that the saturation mass systemically increases with granule diameter . The results indicate the shielding of vertical stress in silo is varying. We show that the ratio between the horizontal and the vertical stresses, K, decreases with the increase in grainule diameter D. In addition, it has also been found that the presence of larger granules leads to stronger stress fluctuation on the bottom plate of silo.     

Directed segregation in compartmentalized bi-disperse granular gas

A bi-disperse granular gas in an asymmetrical two-compartment system is studied experimentally. The presence of asymmetry within the range of our experimental parameters results in a directed segregated state and a directed clustering state. This deterministic system does not depend on the initial conditions. A modified flux model based on Lohse's flux model for bi-disperse granular gases is derived. The modified flux model explains qualitatively the experimental results.     

Directed segregation in compartmentalized bi-disperse granular gas

A bi-disperse granular gas in an asymmetrical two-compartment system is studied experimentally.The presence of asymmetry within the range of our experimental parameters results in a directed segregated state and a directed clustering state.This deterministic system does not depend on the initial conditions.A modified flux model based on Lohse’s flux model for bi-disperse granular gases is derived.The modified flux model explains qualitatively the experimental results.     

Evidence of rayleigh-hertz surface waves and shear stiffness anomaly in granular media

Using the nonlinear dependence of sound propagation speed with pressure, we evidence the anomalous elastic softness of a granular packing in the vicinity of the jamming transition. Under gravity and close to a free surface, the acoustic propagation is only possible through surface modes guided by the stiffness gradient. These Rayleigh-Hertz modes are evidenced in a controlled laboratory experiment. The shape and the dispersion relation of both transverse and sagittal modes are compared to the prediction of nonlinear elasticity including finite size effects. These results allow one to access the elastic properties of the packing under vanishing confining pressure.     

Effect of number density on velocity distributions in a driven quasi-two-dimensional granular gas

The motion of mono-disperse spherical steel particles in a vibration driven quasi-two-dimensional (2D) square cell is studied. The cell is horizontally vibrated to eliminate the effect of gravity compaction. The velocity distributions at different particle number densities are studied and found to obey the form exp[-β(|vy|/σy)α], in which v y and σ y are velocity and its variance in the transverse direction, and α and β are fitting parameters. The value of α is found to decrease with the number density of particles increasing. To investigate the effect of the bottom plate, the molecular dynamics simulation without considering any bottom friction is performed. The accordance between the simulation result and the experimental result shows that the influence of bottom plate friction force on the high energy tail of the velocity distribution can be neglected.     

Experimental compaction of anisotropic granular media

We report on experiments to measure the temporal and spatial evolution of packing arrangements of anisotropic and weakly confined granular material, using high-resolution -ray adsorption. In these experiments, the particle configurations start from an initially disordered, low-packing-fraction state and under vertical solicitations evolve to a dense state. We find that the packing fraction evolution is slowed by the grain anisotropy but, as for spherically shaped grains, can be well fitted by a stretched exponential. For a given type of grains, the characteristic times of relaxation and of convection are found to be of the same order of magnitude. On the contrary, compaction mechanisms in the media strongly depend on the grain anisotropy.     

光衍射法研究水表面波的性质

将水的表面波视作一种正弦相位型光栅,导出了水表面波衍射光斑间距与其波长的关系,从而利用水表面波对激光的衍射测定了水波波长,确定了表面波成分,即在所用实验频率范围内毛细波占主导地位,验证了表面波的色散关系.     

极化电场对可激发介质中螺旋波的控制

螺旋波在不同的物理、化学和生物系统中普遍存在.周期外场,比如极化电场,尤其是具有旋转对称性的圆极化电场可对螺旋波动力学产生重要影响.本文综述了极化电场对可激发介质中螺旋波的控制,包括共振漂移、同步、手征对称性破缺、多臂螺旋波的稳定、次激发介质中的螺旋波、三维回卷波湍流态的控制、心脏组织中螺旋波的去钉扎、心脏组织中螺旋波湍流态的控制等.     

颗粒物质中的空洞效应

通过实验研究了颗粒体系内部的空洞对整个颗粒体系的影响.当在颗粒体系中向上提拉探测棒时,棒下端即刻产生空洞.由于空洞的存在,将导致整个颗粒体系内部结构的崩塌,崩塌的情况可通过测定棒受到的摩擦力来研究.实验发现整个体系崩塌的时间t与空洞产生的深度h间存在着一个指数的对应关系,即t正比于exp(h/λ),并且这个关系不随颗粒尺寸的变化而改变.同时,还进行了二维的辅助实验,对颗粒体系内部的情况进行了较为细致的观察.     

Elastic waves in noncohesive frictionless granular crystals

An ordered structure of noncohesive spherical beads constitutes a phononic crystal. This type of media combines the properties of wave propagation in phononic crystals (dispersion due to the geometrical periodicity) with the properties of wave propagation in granular media (nonlinearities, rotational degree of freedom) and gives the opportunity to have interesting features as tunable frequency band gaps for example.In this work, the acoustic bulk modes of a hexagonal close packed (hcp) structure of beads, considered as rigid masses connected by springs, are theoretically evaluated and their associated resonance frequencies are compared to experimental results. When friction is neglected, the elastic interaction between the beads are reduced to a normal spring interaction given by the Hertz theory. According to this theory, the rigidity of the contact depends on its static loading. The theory predicts the existence of elastic transverse and longitudinal acoustical-type modes and transverse and longitudinal optical-type modes.The acoustic transfer function of a hcp crystal slab built with stainless steel beads is measured and its resonance frequencies are compared to the theoretical predictions. Despite some differences between theory and experiments, which could come for instance from the disordered character of the contact loads, the developed theory and the experimental results show relatively good agreement.     

Air-coupled detection of nonlinear Rayleigh surface waves to assess material nonlinearity

This research presents a new technique for nonlinear Rayleigh surface wave measurements that uses a non-contact, air-coupled ultrasonic transducer; this receiver is less dependent on surface conditions than laser-based detection, and is much more accurate and efficient than detection with a contact wedge transducer. A viable experimental setup is presented that enables the robust, non-contact measurement of nonlinear Rayleigh surface waves over a range of propagation distances. The relative nonlinearity parameter is obtained as the slope of the normalized second harmonic amplitudes plotted versus propagation distance. This experimental setup is then used to assess the relative nonlinearity parameters of two aluminum alloy specimens (Al 2024-T351 and Al 7075-T651). These results demonstrate the effectiveness of the proposed technique – the average standard deviation of the normalized second harmonic amplitudes, measured at locations along the propagation path, is below 2%. Experimental validation is provided by a comparison of the ratio of the measured nonlinearity parameters of these specimens with ratios from the absolute nonlinearity parameters for the same materials measured by capacitive detection of nonlinear longitudinal waves.     

The viscous effects on shear horizontal surface acoustic waves in semi-infinite superlattices

In this paper, by a semi-analytical method, the propagation characteristics of shear horizontal surface acoustic waves in semi-infinite superlattices containing viscous materials are investigated. The factors that influence the attenuation and phase velocity of the surface waves are analyzed in detail. The results may be useful for the design of acoustic wave devices.     

A new statistical mechanics approach to dense granular media

A new statistical mechanics approach to dense granular media is presented. The thermodynamic formalism is set out directly in terms of elastic potential energy, such that the configurational temperature (the intensive property which defines the steady state) relates to a quadratic function of the stresses (rather than other linear functions used in recent developments). Dense granular media are considered as canonical ensembles of noninteracting clusters, which can be identified with repeatable equilibrium configurations. Then, particles can be located in a new proposed phase space (conceived to separate the elastic potential energy levels). Although the importance of this paper lies in the method itself, it has been illustratively applied to the simple case of two-dimensional (2D) dense granular media (an arrangement of frictionless monodisperse elastic disks under isotropic horizontal stress compression). In this case, the temperature is directly replaced by the squared external pressure, and the packing ratio of the most probable microstate is close to the reported value of random close packing. Moreover, some interesting general conclusions arise.     

Free surface waves in wall-bounded granular flows

We report free-surface waves in granular flows near boundaries in an inclined chute. The chevron-shaped traveling waves spontaneously develop at inclinations close to the angle of repose for both steady and accelerating flows. Two distinct regimes are characterized by internal angle and frequency variations. Experimental measurements indicate that subsurface circulation driven by velocity gradients near frictional walls plays a central role in the pattern formation mechanism, suggesting that wave generation is controlled by the granular analog of a fluid boundary layer.