Mixing and condensation in a wet granular medium

We have studied the effect of small amounts of added liquid on the dynamic behavior of a granular system consisting of a mixture of glass beads of two different sizes. Segregation of the large beads to the top of the sample is found to depend in a nontrivial way on the liquid content. A transition to viscoplastic behavior occurs at a critical liquid content, which depends upon the bead size. We show that this transition can be interpreted as a condensation due to the hysteretic liquid bridge forces connecting the beads, and we provide the corresponding phase diagram.     

Large force fluctuations in a flowing granular medium

We study fluctuations in the force at the boundary of a 2D granular flow. The forces are mainly impulsive at all flow rates. The probability distribution of impulses decays exponentially at large impulses, as do the forces in a static granular medium. At small impulses, the distribution evolves continuously with flow rate with no indication of the transition from collisional flow to intermittently jamming flows. However, the distribution of the time interval between collisions tends to a power law, P(tau) - tau(-3/2), showing a clear dynamical signature of the approach to jamming.     

Fluctuations of acoustic field in a granular medium

Results of an experimental study of sound propagation in a granular medium are presented. It is found that, in the case of excitation of a harmonic signal with a constant amplitude, the acoustic response of a single grain strongly varies in time. The dependence of the harmonic component amplitudes in the response spectrum on the level of signal excitation proves to be nonmonotonic and also strongly varies in time. The most intense fluctuations are observed in the subharmonic component of the propagating signal. The intensity fluctuation spectra of the harmonic components of the response are obtained for the frequency range of 10⁻⁴−10⁻¹ Hz. A possible mechanism that may be responsible for the slow fluctuations of an acoustic field in a granular medium is discussed.

     

Stochastic dynamics of a sheared granular medium

We experimentally investigate the response of a sheared granular medium in a Couette geometry. The apparatus exhibits the expected stick-slip motion and we probe it in the very intermittent regime resulting from low driving. Statistical analysis of the dynamic fluctuations reveals notable regularities. We observe a possible stability property for the torque distribution, reminiscent of the stability of Gaussian independent variables. In this case, however, the variables are correlated and the distribution is skewed. Moreover, the whole dynamical intermittent regime can be described with a simple stochastic model, finding good quantitative agreement with the experimental data. Interestingly, a similar model has been previously introduced in the study of magnetic domain wall motion, a source of Barkhausen noise. Our study suggests interesting connections between different complex phenomena and reveals some unexpected features that remain to be explained.     

颗粒介质中的粘滞系数

将颗粒介质看成是等效均匀介质, 其中的声衰减系数和声速等于该颗粒介质中的相应的量值(它们可由作者的理论给出), 等效静态密度可以用二元混合规则求得. 此外, 根据浓颗粒介质中相互作用的声传播理论, 当入射波为平面波时, 相互作用的次级波仍然是平面波. 在这样的情况下, 可以将三维非线性方程组简化为一维情况, 从而算得浓颗粒介质中的粘滞系数, 结果表明, 颗粒介质中的粘滞系数不仅依赖于颗粒的体积分数而且还与频率有关. 根据推导过程可知, 对比于爱因斯坦理论所能应用的限制, 本文的结果可以更广泛地应用于实际介质.     

Starting to move through a granular medium

We explore the process of initiating motion through a granular medium by measuring the force required to push a flat circular plate upward from underneath the medium. In contrast with previous measurements of the drag and penetration forces, which were conducted during steady state motion, the initiation force has a robust dependence on the diameter of the grains in the medium. We attribute this dependence to the requirement for local dilation of the grains around the circumference of the plate, as evidenced by an observed linear dependence of the initiation force on the plate diameter.     

Energy trapping and shock disintegration in a composite granular medium

We report the first experimental observation of impulse confinement and the disintegration of shock and solitary waves in one-dimensional strongly nonlinear composite granular materials. The chains consist of alternating ensembles of beads with high and low elastic moduli (more than 2 orders of magnitude difference) of different masses. The trapped energy is contained within the "softer" sections of the composite chain and is slowly released in the form of weak, separated pulses over an extended period of time. This effect is enhanced by using a specific group assembly and precompression.     

Numerical studies of a granular gas in a host medium

The nonlinear dissipative Boltzmann equation for an granular gas diffusing in a elastically scattering host medium is investigated and numerically solved by means of direct stochastic simulation. The procedure requires an appropriate treatment of the two collision integrals involved, and of their different features. The algorithm is first tested versus exact results for macroscopic moments worked out in the Maxwellian-pseudo-molecules approximation, and then it is applied to the more realistic collision model of hard spheres for both elastic and inelastic encounters. When collisions with background are dominant, reliability of some hydrodynamic closures is then discussed by comparison of their outputs to the kinetic results achieved by the present DSMC approach.     

Nonlinear electrical conductivity in a 1D granular medium

We report on observations of the electrical transport within a chain of metallic beads (slightly oxidized) under an applied stress. A transition from an insulating to a conductive state is observed as the applied current is increased. The voltage-current (U-I) characteristics are nonlinear and hysteretic, and saturate to a low voltage per contact (0.4 V). Our 1D experiment allows us to understand phenomena (such as the Branly effect) related to this conduction transition by focusing on the nature of the contacts instead of the structure of the granular network. We show that this transition comes from an electro-thermal coupling in the vicinity of the microcontacts between each bead - the current flowing through these contact points generates their local heating which leads to an increase of their contact areas, and thus enhances their conduction. This current-induced temperature rise (up to 1050 C) results in the microsoldering of the contact points (even for voltages as low as 0.4 V). Based on this self-regulated temperature mechanism, an analytical expression for the nonlinear U-I back trajectory is derived, and is found to be in very good agreement with the experiments. In addition, we can determine the microcontact temperature with no adjustable parameters. Finally, the stress dependence of the resistance is found to be strongly non-hertzian due to the presence of the surface films. This dependence cannot be usually distinguished from the one due to the disorder of the granular contact network in 2D or 3D experiments.Received: 7 November 2003, Published online: 28 May 2004PACS: 45.70.-n Granular systems - 72.80.-r Conductivity of specific materials     

Infinite penetration of a projectile into a granular medium

An object falling in a fluid reaches a terminal velocity when the drag force and its weight are balanced. Contrastingly, an object impacting into a granular medium rapidly dissipates all its energy and comes to rest always at a shallow depth. Here we study, experimentally and theoretically, the penetration dynamics of a projectile in a very long silo filled with expanded polystyrene particles. We discovered that, above a critical mass, the projectile reaches a terminal velocity and, therefore, an endless penetration.     

阻塞态颗粒介质的慢速阻力

报导了不同尺寸的大球缓慢压入颗粒床过程中所受阻力随深度变化的实验测量结果,发现阻力曲线在不同的深度区域有不同的增长规律,存在凹—凸转变.阻力在较浅的区域满足1.3次方的幂率增长,而在较深的区域趋向0.3次方的幂率增长.通过研究凹凸转变中拐点的性质,发现这种阻力增长速度趋缓的转变不是来源于前人认为的器壁支撑导致的介质压强饱和,而是来源于侵入物自身的体积效应.此外,适用于颗粒介质快速阻力的静水压力描述并不适用于阻塞态颗粒介质的慢速阻力行为,实验表明慢速阻力对深度的依赖关系不是线性,且测得的静水压力系数也远大于理论估算值. 关键词： 颗粒介质 阻力 力链重组     

Upward penetration of grains through a granular medium

We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing, we use Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like polydispersity and the initial packing fraction of the host granular system. It is found that the anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of the packing fraction of the host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into the host granular medium, we observe size segregation patterns.     

Experimental study of nonlinear acoustic effects in a granular medium

Results of a series of experimental studies of nonlinear acoustic effects in a granular medium are presented. Different effects observed in the experiments simultaneously testify that the nonlinearity of granular media is governed by the weakest intergrain contacts. The behavior of the observed dependences suggests that the distribution function of contact forces strongly increases in the range of forces much smaller than the mean force value, which is inaccessible for conventional experimental measuring techniques. For shear waves in a granular medium, the effects of demodulation and second harmonic generation with conversion to longitudinal waves are studied. These effects are caused by the nonlinear dilatancy of the medium, i.e., by the nonlinear law of its volume variation in the shear stress field. With the use of shear waves of different polarizations, the anisotropy of the nonlinearity of the medium is demonstrated. The observation of the cross-modulation effect shows that the nonlinearity-induced modulation components of the probe wave are much more sensitive to weak nonstationary perturbations of the medium, as compared to the linearly propagating fundamental harmonic. The nonlinear effects under study offer promise for diagnostic applications in laboratory measurements and in seismic monitoring systems.     

Cascade diffusion of water molecules in a porous granular medium

Water vapor inleakage to a porous granular adsorbent was studied by diode laser spectroscopy. Stepping in the temporal variation of the diffusivity, probably associated with porous matrix space fractality, was detected. The H₂O molecular sensing technique is proposed as a tool for experimental study of the structure of porous media and processes occurring in them.     

Features of nonlinear wave propagation in a layer of a granular medium

Computer simulation was performed to study compression wave propagation in a granular medium in a gravity field. It is shown that in a layer with spherical granules, periodic wave structures can be formed and their parameters depend on the granule size and layer thickness. If the layer is compressed, the wave structures fail to emerge and wave attenuation is much weaker. Wave processes in a layer of granular medium with nonspherical granules were also studied. It is shown that the medium is characterized by very strong wave attenuation and by the absence of wave structure formation and is highly sensitive to its initial stress state.     

The scaling and dynamics of a projectile obliquely impacting a granular medium

In this paper, the dynamics of a spherical projectile obliquely impacting into a two-dimensional granular bed is numerically investigated using the discrete element method. The influences of projectile’s initial velocities and impacting angles are mainly considered. Numerical results show that the relationship between the final penetration depth and the initial impact velocity is very similar to that in the vertical-impact case. However, the dependence of the stopping time on the impact velocity of the projectile exhibits critical characteristics at different impact angles: the stopping time approximately increases linearly with the impact velocity for small impact angles but decreases in an exponential form for larger impact angles, which demonstrates the existence of two different regimes at low and high impact angles. When the impact angle is regarded as a parametric variable, a phenomenological force model at large impact angles is eventually proposed based on the simulation results, which can accurately describe the nature of the resistance force exerted on the projectile by the granular medium at different impact angels during the whole oblique-impact process. The degenerate model agrees well with the existing experimental results in the vertical-impact cases.     

Impact-induced vibrations of a simple viscoelastic column model

A two-degree-of-freedom model for an almost-axially impacted viscoelastic cantilever column is analyzed. The impact load is produced by a mass striking the free end of the column. Under the assumption of small displacements two second-order non-linear ordinary differential equations for the coupled longitudinal and transverse vibrations of the column are derived. In the absence of damping these equations of motion are reduced to Mathieu's equation through the use of a perturbation method. The excitation parameters are (i) the natural frequency of small amplitude transverse vibrations of the undamped column and (ii) the initial velocity of the end of the column. The boundaries of two unstable regions are obtained. In the stable regions the solution of Mathieu's equation for the transverse displacement is close to that of the original non-linear equations of motion. In the first unstable region there is agreement only for early time. With increasing damping the peak of the maximum transverse displacement in the first unstable region decreases or even vanishes.     

Dynamics of drag and force distributions for projectile impact in a granular medium

Our experiments and molecular dynamics simulations on a projectile penetrating a two-dimensional granular medium reveal that the mean deceleration of the projectile is constant and proportional to the impact velocity. Thus, the time taken for a projectile to decelerate to a stop is independent of its impact velocity. The simulations show that the probability distribution function of forces on grains is time independent during a projectile's deceleration in the medium. At all times the force distribution function decreases exponentially for large forces.     

The propagation of a nonlinear sound wave in an unconsolidated granular medium

The propagation of a high-intensity sound wave in an unconsolidated medium is considered. Dissipation effects are taken into account on the basis of Buckingham’s theory of a relaxation mechanism of sound attenuation in a saturated sediment. The nonlinear evolution equation for the relaxing medium is obtained, and the solutions of this equation are analyzed. The second-harmonic generation in such a medium decays, as does the linear sound wave of the same frequency. The stationary weak shock profile has a specific form due to relaxation effects.     

运动物体在颗粒介质中的阻力形式

物体由重力驱动在颗粒介质中的运动过程，从动力学上可以用等效重力、颗粒床的黏性阻力及静压阻力来描述.通过求解此动力学模型，找到了一个能够控制颗粒系统处于不同阻尼状态的参量Γ，Γ的表达式直接反映了黏性阻力项和静压阻力项的竞争.这种竞争使得颗粒介质能够处于不同阻尼状态，表现出不同的表观阻力行为.根据理论分析结果设计实验，实现了对颗粒介质体系阻尼状态的调节，验证了理论模型给出的运动物体在颗粒介质中受到的阻力形式. 关键词： 颗粒体系 阻力 动力学过程