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Item type:Publication, Access status: without files , Mechanism of anomalous sinking of an intruder in a granular packing close to incipient fluidizationObjects released into a granular packing close to incipient fluidization may float or sink depending on their density. Contrary to intuition, Oshitani et al. [Phys. Rev. Lett. 116, 068001 (2016)10.1103/PhysRevLett.116.068001] reported that under certain conditions, a lighter sphere can sink further and slower than a heavier one. While this phenomenon has been attributed to a local fluidization around the sinking sphere, its physical mechanisms have not yet been understood. Here, we studied this intriguing phenomenon using both magnetic resonance imaging and discrete particle simulation. Our findings suggest that local fluidization around the sinking sphere and the formation and detachment of gas bubbles play a critical role in driving this anomaly. An analysis of forces acting on the intruder revealed that the upward-directed fluid force acting on a sphere is almost fully counterbalanced by the sum of the net contact forces and the gravitational force acting downward, when the sphere density is close to the bulk density of the granular packing (ρsphere/ρbulk≈1). At the time when bubbles detach from the sphere, the gas pressure gradient experienced by the sphere is slightly attenuated and the sphere is pushed downward by the particle cap located on top of the sphere. Because the deviations from the force equilibrium are small, the sphere sinks slowly. Even after the sphere has reached its final stable depth, local fluidization in combination with bubble formation remains in the proximity of the sphere.Publicationtype: Journal Article50 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Real-Time magnetic resonance imaging of bubble behavior and particle velocity in fluidized bedsSnapshots of particle concentration and velocity fields in bubbling gas-solid fluidized beds were acquired using magnetic resonance imaging. Using a recently developed multichannel radiofrequency receiver coil in combination with fast readout techniques, adapted from medical MRI protocols, the temporal resolution was 7 and 18 ms for two-dimensional images of particle concentration and velocity fields, respectively. A cylindrical bed with 190 mm diameter and 300 mm height was filled to heights of 100, 150, and 200 mm with spherical 1 and 3 mm diameter particles and fluidized at ratios of superficial gas velocity to minimum fluidization velocity (U/Umf) of 1.2, 1.5, 2.0, 3.0, and 4.0. The effects of these varying parameters on the number of bubbles, bubble diameter, bed height, and particle speed are investigated. It is hoped that these data sets will become important benchmarks against which computational, analytical, and empirical models can be validated.Publicationtype: Journal Article98 9 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the Mechanism of Anomalous Sphere Sinking in Apparently Fixed Particle Beds (Discrete Particle Simulation and Ultra-Fast MRI Measurement)It is well known that a particle bed behaves as a fluid in the case it is aerated with gas velocity higher than the minimum fluidization velocity Umf. This is called as a fluidized bed and widely used in a large number of industrial processes. Under Umf, on the contrary, gravity force working on a particle overcomes fluid force and a particle bed behaves as a solid. Oshitani et al. (Phys. Rev. Lett., 116, 068001, 2016) reported anomalous sinking of intruder spheres in granular beds aerated under but close to Umf. They observed deep and slow sinking of spheres in the case density of sphere is close to the bulk density of the bed. They attributed this phenomena to local fluidization around the spheres while its detailed mechanism was not clear due to opaque nature of granular beds. In the present study, anomalous sinking and stopping mechanism of intruder spheres in an apparently fixed bed aerated just below Umf were explored and revealed in detail by means of discrete particle simulation and ultra-fast MRI measurement. Our numerical simulation and MRI measurement enable us to observe detailed flow dynamics and forces working on the spheres with sufficient spatial and temporal resolutions. Fluid force, contact force from surrounding particles and the gravity force working of the sphere are almost balanced in the case sphere density matches with the bulk density of a bed and small deviations from the equilibrium make slow sinking. The deviation is due to the detachment of small bubbles from the sphere surface. Bubble detachment from the sphere surface loosens local gas pressure gradient that supports the sphere weight and the sphere falls at the time.Publicationtype: Conference Paper34 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Real-time probing of granular dynamics with magnetic resonanceGranular dynamics govern earthquakes, avalanches, and landslides and are of fundamental importance in a variety of industries ranging from energy to pharmaceuticals to agriculture. Nonetheless, our understanding of the underlying physics is poor because we lack spatially and temporally resolved experimental measurements of internal grain motion. We introduce a magnetic resonance imaging methodology that provides internal granular velocity measurements that are four orders of magnitude faster compared to previous work. The technique is based on a concerted interplay of scan acceleration and materials engineering. Real-time probing of granular dynamics is explored in single- and two-phase systems, providing fresh insight into bubble dynamics and the propagation of shock waves upon impact of an intruder. We anticipate that the methodology outlined here will enable advances in understanding the propagation of seismic activity, the jamming transition, or the rheology and dynamics of dense suspensions.Publicationtype: Journal ArticleScopus© Citations 58 92 130