Effects of Spacing Distance between Cylindrical Obstacles on Granular Shock Interactions in Gravity-Driven Experimental Flows

Gravity-driven geophysical granular flows, such as rock avalanches, landslides, debris flows, etc., interact with obstacles (e.g. bridge piers and buildings) as they flow down the slope, causing rapid changes in flow velocity and height in the vicinity to form a granular shock wave in front of the object. The interaction between shock waves will affect the granular-flow field near the obstacles. However, the complex physical processes make some challenges in understanding how the granular material behaves in the influencing area of shock-shock interaction.

In this study, systematic chute experiments were performed with glass particles to investigate the dynamic interaction between granular flow and two circular cylinders with variable spacing distances. The pressure sensors were used to measure the impact pressure of the granular flow on the upstream cylindrical surfaces and a plate equipped flush with the chute bed. The accelerometers were mounted at the bottom of the plate to record seismic signals generated by the granular flow impacting on the bed as well as the cylinders. Flow velocities and depths were determined using an image processing method. The discrete element method (DEM) was utilized to construct a virtual model of the chute system and particles and to simulate the dynamic processes of granular flow interacting with the cylinders. The experimental and the DEM simulated results showed that bow shock waves were generated just upstream of the two cylinders and a granular vacuum zone was formed on the lee side of each cylinder, with the incoming flow velocity being significantly reduced in the granular-shock influencing area. As the spacing decreases, the two shock waves change from being independent to mutual interference. In addition, the effects of spacing distances on the shapes of the granular vacuum and bow shock waves were investigated by experiments and compared to the DEM results, showing a strong interaction between granular shocks. The pinch-off distance which is determined by the length of the granular vacuum also showed a dependence on the spacing distance of the cylinders, indicating a decreasing pinch-off distance with decreasing value of spacing. The impact pressures and acoustic signals generated by granular flow impacting on the chute bed and the surfaces of the cylinders in the shock influencing area for varying Froude numbers were also analyzed.

In summary, the DEM simulations and the recorded signals are helpful to analyze the interaction between granular shock waves. The finding in present study may contribute to better understanding granular shock dynamics and may eventually in improving the design of the protective structure in hazard-prone area.