Qingfan Shi

Effect of the ratios of diameter of silo to bead on the pressure screening in granular columns

We present the apparent mass measurements at the bottom of granular packings for different bead and silo sizes. The redirection parameter K in Janssen theory is found to increase with the ratios of the diameter of the silo to the bead. We attribute this feature to the friction between the beads and the confining wall of silo; it is the role of friction that leads to variations in the shielding of vertical stresses as well as pressure screening.

Symmetrically periodic segregation in a vertically vibrated binary granular bed

Periodic segregation behaviors in fine mixtures of copper and alumina particles, including both percolation and eruption stages, are experimentally investigated by varying the ambient air pressure and vibrational acceleration. For the cases with moderate air pressure, the heaping profile of the granular bed keeps symmetrical in the whole periodic segregation. The symmetrical shape of the upper surface of the granular bed in the eruption stage, which resembles a miniature volcanic eruption, could be described by the Mogi model that illuminates the genuine volcanic eruption in the geography. When the air pressure increases, an asymmetrical heaping profile is observed in the eruption stage of periodic segregation. With using the image processing technique, we estimate a relative height difference between the copper and the alumina particles as the order parameter to quantitatively characterize the evolution of periodic segregation. Both eruption and percolation time, extracted from the order parameter, are plotted as a function of the vibration strength. Finally, we briefly discuss the air effect on the granular segregation behaviors.

Bottom stresses of static packing of granular chains

We experimentally measure the static stress at the bottom of a granular chains column with a precise and reproducible method. The relation, between the filling mass and the apparent mass converted from the bottom stress, is investigated on various chain lengths. Our measurements reconfirm that the scaling behavior of the stress saturation curves is in accord with the theoretical expectation of the Janssen model. Additionally, the saturation mass is displayed as a nonmonotonic function of the chain length, where a distinguishing transition of the saturation mass is found at the persistence length of the granular chain. We repeat the measurement with another measuring methodology and a silo with different size, respectively, the position of the peak maintains robust. In order to understand the transition of the saturation mass, the friction coefficient and the volume fraction of granular chains are also measured, from which Janssen parameter can be calculated. Finally, we preliminarily measure the bottom stress for two distinct packing structures of long chains, find the effect of the entanglements on the bottom stress, and argue that the entanglements might be responsible for the transition of the saturation mass.

Experimental observation of the antiferromagnetic resonance linewidth in KCuF3

We report the results of antiferromagnetic resonance (AFMR) measurements conducted on KCuF3 at various frequencies from 3.8 to 10.6?GHz at 4.2?K. The resonance linewidth is first found to depend on the frequency, i.e., the lower the frequency the greater the resonance linewidth, no matter whether the AFMR field is forced on the easy axis or uneasy axis. Moreover, a linewidth peak seems to exist for at about 4?GHz. Based on the model of Yamada and Kato (1994 J.?Phys.?Soc.?Japan 63 289) and considering the Laudau?Lifshitz damping term, the result of numerical calculation for the resonance linewidth is in good agreement with the data of AFMR experiments.

Bouncing behavior and dissipative characterization of a chain-filled granular damper

Abstract We have experimentally investigated the bouncing behavior and damping performance of a container partially filled with granular chains, namely a chain-filled damper. The motion of the chain-filled damper, recorded by a particle tracing technology, demonstrates that the granular chains can efficiently absorb the collisional energy of the damper. We extract both the restitution coefficient of the first collision and the total flight time to characterize the dissipation ability of the damper. Two containers and three types of granular chains, different in size, stiffness and restitution coefficient, are used to examine the experimental results. We find that the restitution coefficient of the first collision of a single-chain-filled damper can linearly tend to vanish with increasing the chain length and obtain a minimum filling mass required to cease the container at the first collision (no rebound). When the strong impact occurs, the collisional absorption efficiency of a chain-filled damper is superior to a monodisperse-particle-filled damper. Furthermore, the longer the chains are, the better the dissipative effect is.

Flux of granular particles through a shaken sieve plate

We experimentally investigate a discharging flux of granular particles through a sieve plate subject to vertical vibrations. The mean mass flux shows a non-monotonic relation with the vibration strength. High-speed photography reveals that two stages, the free flight of the particles’ bulk over the plate and the adhesion of the particles’ bulk with the plate, alternately appear, where only the adhesion stage contributes to the flow. With two independent methods, we then measure the adhesion time under different vibration conditions, and define an adhesion flux. The adhesion flux monotonically increases with increasing vibration strength. By rescaling the adhesion flux, we find that the adhesion flux is approximately determined by the peak vibration velocity of the shaker. The conclusion is examined with other sieve geometries.

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.

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.

Characterization of Elastic Modulus of Granular Materials in a New Designed Uniaxial Oedometric System

A simple uniaxial oedometric system is developed to test the elastic modulus of granular materials. The stress–strain relationship is first measured under conditions of uniaxial compression with additional lateral stress and strain, then the elastic modulus of the material is determined by the linear fitting method. It is found that the modulus is positively correlated to the grain size and negatively correlated to the container size. Arching and dragging are revealed to be the mechanism of such correlations by using the digital image correlation method and the pressure film technology based on the statistical method.

Heaping instabilities in a layered Bi-disperse granular bed

Heap formations in a layered bi-disperse granular bed under vertical vibration with strength Γ have been studied experimentally. With the volume ratio (χ) between the two different types of grains and Γ as control parameters, different heap patterns can be created and details of the heap formation can be investigated. In particular, a periodic state in which the heap is being created and destroyed more or less periodically has been realized. This periodic heaping state is found to be related to two instabilities controlled by χ and Γ; namely the instability of a horizontal granular interface and the instability of a heap in a layered granular system. This latter instability would occur only in a layered system. A nonlinear phenomenological model is developed to understand these two instabilities and the predictions of the model are found to be in good agreement with experiments.