Evolution process of center discharge flow pattern and pressure fluctuation of silo wall in a flat-bottomed silo

Silos are commonly used for the bulk storage of grain in agriculture. The pressure acting on the silo wall during silo discharging is one of the most important indicators during the structure design. The flow state of the material in the silo is the key influencing parameter on the distribution of discharging pressure. This study aims to explore the evolution of granular material flow patterns and the wall pressure distribution during silo discharging. A discharging test was performed on a 3D test silo to explore the flow of granular material with an average particle size of 3.5 mm. The plexiglass silo was designed semi-cylindrically with seven pressure sensors along the silo wall. The inner diameter of the test silo was 500 mm, the wall thickness was 20 mm, the height was 1 100 mm, and the diameter of the outlet was 50 mm. Both the internal and the external flow of the granular material were observed to simultaneously record the wall pressure distribution. Meanwhile, the particle trajectory test was carried out to explore the mechanism of force transmission and the motion of granular particles. Furthermore, the discrete element method (DEM) was adopted to analyze the evolution of the force chain network, velocity vector and porosity of the granular material in store and during discharging. The results showed that the strong force chain fracture led to the increase of porosity (from 0.15043-0.20030 before discharging to 0.15136-0.23223 at discharging instant). The wall pressure increased sharply at the discharging instant (discharging rate 0%-1%), and the wall pressure increased the most(the pressure of the silo wall at the depth of 0.2, 0.4 and 0.6 m increases sharply to the peak value of 1.65, 3.63 and 4.05 kPa). The storage zone during discharging was divided into three zones, i.e., the flow, the (quasi) static, and the boundary zone between them. These zones evolved, as the discharging developed. The intersectional interface between the flow and static zone was located at 0.329 m (about 1/3 height) from the silo bottom. Four flow patterns included the mass, funnel, mixed with funnel and tubular flow, as well as the tubular flow. The radius of the flow channel during tubular flow gradually decreased from 270 to 50 mm. An explanation was then proposed for the phenomenon of the sudden increase in the wall pressure at discharging instant. Specifically, the broken chains of strong force near the outlet were represented to release the strong forces among particles. By contrast, the weak force chains failed to restrain the particles in the entire storage area. Thus, all granular materials immediately moved to the wall and the bottom of the silo, resulting in a sudden increase in the force chain that transmitted to the silo wall, and finally, the wall pressure along the silo suddenly increased in discharging instant. The flow development was also summarized for the diagram. These findings can provide useful information for the flow during discharging, particularly for the wall pressure from granular materials.