Effects of obstacle's curvature on shock dynamics of gravity-driven granular flows impacting a circular cylinder

Abstract

Granular flows, such as rock avalanches and debris flows, are gravity-driven and often hit engineering structures that are placed in their paths, generating dynamic impact pressures and potentially causing hazard to infrastructures. To improve the design of protective structures and the hazard assessment, it is important to understand the physical mechanism of such granular flows impacting on obstacles with variable sizes and shapes. In this study, the dynamics of granular shock waves generated by experimental flows impacting on a circular cylinder varying in diameter and on a closed barrier were investigated by conducting systematic laboratory experiments. Pressure sensors were mounted at the bottom of the experimental chute and the upstream cylinder surface to measure the dynamic impact pressures in the granular shock area. Accelerometers that were installed at the underside of the chute bed were used to record the seismic signals of granular flows during the entire impacting process. The velocity and depth of a granular flow impacting (or just before impacting) on a cylinder were estimated using an image processing method. The results showed that the dimensionless standoff distance DDstandoff of the granular shock wave decreases nonlinearly with increasing Froude number ranging from 5.8 to 12.3. The values of DDstandoff tend to change less for the larger investigated Froude numbers, approaching a limit of about 0.19. The dimensionless runup and the pinch-off distance correlated linearly with increasing Froude number and can be affected by the radius of curvature (RC) of the obstacle at the stagnation point. The impact pressures grow linearly as Froude number increases, and the RC with a smaller value usually excites a larger impact pressure. Additionally, the effect of RC on the dimensionless pressures was observed to be weakened by an increase in Froude number. The mean frequency of the seismic signals produced by the impacts of a granular flow also indicates a dependency on RC. The findings of this study contribute to the understanding of the dynamic signal response generated by a granular flow, which may result in improving the design of the protective structures.