The Study Of Onset Motion Of Non-coheresive Uniform Particles Under Laminer Flow

Study of onset motion for non-cohesive uniform particles under laminar flow has great significance for investigating the viscose sub-layer and the particle motion in high viscose fluid. Based on the critical drag force theory, by using experimental observation, theoretical analyzing and numerical simulation, the threshold principle for non-cohesive uniform particles under laminar flow was studied. Experimentally, it was divided into two parts: the traditional straight rectangular flume experiment and the cone-and-plate flume experiment which can consider the long term evolvement of particles motion. The technique used here including PIV(Particle Image Velocimetry), RIM(Rrefractive Index Matching), CCD(Coupled Charged Device) imaging and so on, which can reduce the system error to the maximum content. For the theoretical analyzing, according to the experimental results, the rolling model was selected. Further, based on the characteristic of the granular surface, the non-dimensional drag force equation for onset motion of non-cohesive uniform particles was derived. For the numerical simulation part, the drag coefficient and life coefficient of a particle under different structure types was calculated. The main results of this paper are as follows:(1) The flow observation shows that the flow field at the transition area obeys exponential function. The granular bed influence on the flow field was mainly at the first layer of the granular surface, below which the flow decays into Darcy flow. The slip velocity at the boundary increases as the flow Reynolds number increases. But the thickness of the transition layer is relative to the particle size, about half of the diameter, and keep constant over all the testing Reynolds number.(2) Take the ceastion of erosion at the granular surface as the threshold condition in the straight flume study, it shows that the distribution of Shields curve has a systemically band trend. With the development of granular surface armoring, the Shields number increase correspondingly. For same particle, the Shields number at armored state can be twice of the loosely packed bed. Further, by changing the grain shape and bed slope, it has the same amoring trend. Espically for the mixing case, when the coarse particle’s volume fraction larger than 70%, the threshold condition will be cotrolled by the rearrangement of the coarse particle.It indicates that the feature of granular surface has direct impact on the critical drag force. Also for some bed with bad gradation, it should take both the rearrangement amoring and sizing coarsing into account.(3) The long term evolution of granular bed in the cone-and-plate flume shows that there are two threshold conditions for the onset motion of the investigated granular bed. At the initial threshold, particles start moving, but the movements are not stable stopped after running for a while. At the saturated threshold, particles start move and form a stable particle flow. And the critical drag force at the saturated threshold is much higher that the initial threshold. The results reveals that onset of uniform non-cohesive particles is a continuous process. The results are consistent with the straight flume test, and the onset process is recorded directly.(4) The non-dimensional drag force equation shows that when the particle protrusion changes, the critical drag force varies and the shape of critical drag force curve changes at the same time. In the real threshold situation, with the devolpment of surface armoring, the particle protusion decrease. Most of the experiment results lay in the region of particle protrusion from 0.2 to 1. When the protrusion fell below 0.2, there’s no movement. It indicates that the structure of the granular surface have huge impact on the onset motion of particles, the former empirical formula cannot reveal the principle of the onset motion.(5) Based on the experiment results and therotical analyzing, the impact of granular surface structure on the onset motion was stuied using Fluent. The Fluent simulation of the drag coefficient and life coefficient indicate that the surrounding shield from other particles will reduce the facing area of the target particle and lower the drag force. For the same particle structure, when the facing flow direction changes, the drag force will also be changed. But for the life force, the surrounding shield from other particles will increase the lift force of the target particle. At the threshold condition, with the surface amoring, the drag force and lift force on the onset particle will change. The results are consistent with the experiment results and therotical analyzing. According to the research above, it shows that the surface structure of granular bed has direct impact on the critical drag force, and the Shields curve has a brand disturbution trend in laminar rigime.