Meiying Hou

Projectile impact and penetration in loose granular bed

Abstract Dynamics of a projectile impacting on a loose granular bed under the acceleration due to gravity g has been studied by fast video photograph. Granular jet formation and projectile penetration are observed in 3D and quasi-2D experiments. It is found that the penetration velocity u can be described by m du/dt=−γu−kz+mg′, where ³ and k are the parameters which characterize the viscous damping and hydrostatic drag forces of the bed, respectively, z is the penetration distance of the projectile, and g′ is a modified gravity term. The viscous damping term is found important in quasi-2D experiments. For 3D, the damping term is only important at the beginning of the impact, and can be neglected during penetration.

Electric field controlled dilute-dense flow transition in granular flow through a vertical pipe

AbstractThe flow of granular nickel particles moving down a vertical pipe from a hopper in the presence of a local, horizontal ac electric field isstudied. It is found that the flow can be retarded by the electric field, and the retardation depends upon the initial conditions of the flow. If theflow is dilute when passing through the field, there exists a critical voltageV 1 , the flow transition from dilute to dense occurs and flow rate isretarded when V is greater than V 1 . For V<V 1 , the steady-state flow rate is practically unaffected. The flow rate decreases abruptly at V 1 .Particle aggregations near the top end of the electrodes are observed. A two-dimensional molecular dynamics (MD) simulation is performed.It confirms the aggregation and intermittence of the flow, and the abrupt change in flow rate at the dilute–dense transition. The MDsimulation results agree qualitatively well with the experimental observations.D 2003 Elsevier B.V. All rights reserved. Keywords: Granular materials; Electric fields; Granular flow; Computer simulation of molecular and particle dynamics

Retardation and transitions of dilute and dense granular flows in a vertical pipe induced by electric fields

Granular flow of nickel particles down a vertical pipe from a hopper is shown to be retarded by a horizontal ac electric field applied to a local region along the pipe. The particles are released from the hopper by pulling out a stopper in the hopper. Two sequences of experiments with different initial flow conditions are performed. In the first sequence, a dilute flow in the pipe is created after a fixed voltage V (⩽4.8 kV) is applied across two short, vertical copper electrodes. The steady-state flow rate Q remains practically constant for V V1, the flow becomes dense; Q decreases with a power law, Q∼V−1. In the second sequence of experiments, V is first set at 4.8 kV; the flow is allowed to start, and soon becomes a dense flow; then, V is reduced to the desired voltage. The new, steady-state Q vs V curve coincides with the previous Q(V) curve of the first sequence, except for V2

Directed clustering in driven compartmentalized granular gas systems in zero gravity

Clustering of shaken fluidized granular matter in connected compartments has been observed and studied in the laboratory. This clustering behavior in granular gas systems is related to the dissipative nature of granular system, and therefore shall not depend on gravity. This clustering phenomenon in compartmental configuration may provide a means for particle depletion and transportation in microgravity environment. In this work we propose different configurations for possible directed clustering in zero gravity. The related experiment has been planned for the Chinese satellite SJ-10.

To understand earthquake from the granular physics point of view-causes of earthquakes, earthquake precursors and prediction

To recognize the seismogenic process, mechanism and behavior of seismic precursors, as well as look for the ways of earthquake prediction the granular physics principle is applied based on the basic structure of the crust and mantle and existing experimental observations, where we treat the earth crust and mantle as large scale discrete matters. Main outcomes are: A granular model of the structure and movement of the earth crust and mantle is established. A formation mechanism of the tectonic forces, which causes the earthquakes, and a model of propagation for precursive signals are proposed. Properties of the seismic precursive signal and its relevance with the earthquake occurrence are illustrated, and principle of ways to detect the effective seismic precursor is elaborated. In the paper the mechanism of palintectic earthquake is also explained by the jamming-unjamming transition of the granular flow. Some earthquake phenomena which were previously difficult to understand are explained, and the predictability of the earthquake is discussed. Due to the discrete nature of the earth crust and mantle, the continuum theory no longer applies during the quasi-static seismogenic process. In all in this paper, based on granular physics, we study the causes of earthquakes, earthquake precursors and earthquake prediction, and a new understanding, different from the traditional seismological point of view, is obtained.

Long range boundary effect of 2D intermediate number density vibro-fluidized granular media in micro-gravity

We present a micro-gravity experimental study of the statistical properties of intermediate number density vibro-fluidized inelastic spheres in a rectangular container. It is found that although when taking all the particles into account, the probability distributions of velocities both along and perpendicular to the vibration direction are exponential and symmetric, when dividing particles along the vibration direction into different bins, the local velocity distributions are found to deviate measurably from a symmetric distribution for the velocity component in the vibrating direction. The skewness analysis of the local distribution profiles for vx and vy shows that the local distribution of vx remains symmetric, however, the skewness of the distribution profile in vy changes nearly linear from positive to negative with skew = 0 near the center bin. This indicates a long range boundary effect of the asymmetry in vy. We further studied the hydrodynamic profiles granular pressure px and py, and temperature Tx and Ty in positive and negative components such as p+x and p−x, and Tx+ and Tx−, in accordance with the sign of velocity components. The profiles for the two components are found different along the y direction. Along vibration direction granular medium is found inhomogeneous and anisotropic not only in the particle number densities, but also in vy, py and Ty. This suggests new hydrodynamical modeling is needed for such vibro-fluidized granular systems.

Criticality of the dilute-to-dense transition in a 2D granular flow

This work investigates the criticality of the dilute-to-dense transition in an inclined quasi-2D granular channel flow. At fixed inflow rate Q0 and exit opening size d, the waiting time t before the transition occurs after a dilute flow is initiated at t = 0 is recorded. The histogram f(t) of the number of times counted that the transition occurs at a time t is plotted as a function of t for each d. It is found that the probability function C(t) for the flow remaining dilute at a waiting time t decays exponentially, and its characteristic time α−1(d) can be fitted well to a power law a(dc−d)−γ with dc the critical opening size beyond which the transition will never occur.

Factors of the Transition from Dilute Flow to Dense Flow in Two-Dimensional Channel

Granular flow of steel beads from dilute flow to dense flow on an inclined two-dimensional channel is studied. The initial inflow Q0 is always a dilute flow. A transition from dilute to dense is observed when D≤Dc. In the dense flow the outflow rate (Q) depends only on opening (D), given by . For different inflow rate Q0 in the dilute flow case, the relation between the transition critical exit width (Dc) and the channel width (W) is given. And in larger inflow, the critical exit width is approximately in proportion to the square root of inflow rate, .

Experimental study of segregation patterns in binary granular mixtures under vertical vibration

We report the experimental observations of the segregation patterns in initially well mixed copper and glass spheres subjected to a vertical sinusoidal vibration at different values of acceleration Γ and frequency f. A segregation phase diagram is obtained, which includes Brazil nut (BN), Reversed-BN (RBN) and “sandwich” segregation patterns at different Γ and f. The stable RBN segregation is experimentally found for the first time, in which large heavy particles move down to the bottom and form the lower-layer while small particles rise to the top and form an upper-layer. The boundary values (Γ, f), which separate regions of different patterns, depend on system’s initial condition, i.e. hysteresis exists.

Asymmetric local velocity distribution in a driven granular gas

Purpose – The purpose of this paper is to investigate the local feature of driven granular gases in event-driven molecular dynamic simulation, in order to achieve spatial profiles of local velocity distribution and granular temperature, and the local state with various coefficients of restitution. Design/methodology/approach – Event-driven molecular dynamic simulation is performed to study a vibro-fluidized granular gas system. Triangular-wave vibration is adopted in the simulation. The authors focus on the steady state of a driven granular gas. Findings – The simulation finds the local velocity distribution is asymmetric along vibration direction in this driven granular gas system, which agrees with the experimental results obtained in micro-gravity. A nonlinear spatial profile of the skewness of local velocity distribution in vibration direction is found in the simulation. Furthermore, it is found that the value of skewness increases with the system dissipation. It is also found that the two temperature...