Title: Threshold of granular erosion in a channel driven by fluid flow

Author (Poster): Anyu Hong, Clark University

Abstract:

We investigate the conditions under which a fluid flowing over a granular bed leads to erosion at low Reynolds numbers. Experiments are conducted with a bed inside a model enclosed cylindrical channel with a rectangular cross section and a constant fluid flux. For a given flow rate and sufficiently high bed height inside the channel, erosion occurs and the bed height decreases with time and reaches a height corresponding to a condition where the shear stress on the bed surface decreases below a critical value. If the flux is then increased slightly, erosion resumes and no significant hysteresis in the threshold is observed. We evaluate the viscous shear, shear lift, and gravitational force acting on a grain at the interface and consider the torque balance at the threshold of motion. Further, we evaluate the critical Shields number given by the ratio of the shear stress and normal stress as a function of fluid flow rate and grain size. We find that it increases linearly with Particle Reynolds number which is obtained using the slip velocity near the surface of the granular bed and the grain size in the laminar flow regime probed by our experiments. We find the values of critical Shields number to be systematically greater than those reported at higher Reynolds flows. Further, we find that it increases linearly with increasing Rep by varying the flux for grains with the same size. We show that this trend is consistent with the definition of critical Shields number and Rep because both can be shown to be proportional to the shear rate at the interface.