Granular Flows in Low Gravity: Silo Discharge and Clogging

Models applicable on Earth tend to fail in low gravity. Beyond a critical gravitational acceleration, cohesive forces become predominant, resulting in a shift in macroscopic behavior. For granular processing in space, notably to sustain human presence on the Moon, this can have disastrous consequences; yet, the influence of (low) gravity on granular flows is not accounted for in most existing models. We present results from hopper discharge experiments, conducted in low gravity using an active drop tower and parabolic flights. To access partial gravity, a centrifuge is placed inside the plane, allowing us to generate lunar gravity inflight. The flow rate, Q, and clogging probability, Pc, are measured for different granular materials. We find an increased clogging probability and deviations from the generally accepted gravity-scaling of the Beverloo equation, Q - √g. We generalize the granular behavior in low gravity for all materials studied, and propose an explanation based on dominant cohesive interparticle forces in low gravity.