Gravity-driven granular drainage from a closed top quasi-two-dimensional rectangular channel

The study elucidates the physics of granular drainage from a closed-top quasi-two-dimensional (2D) rectangular channel through extensive visualization, image analysis, and digital particle image velocimetry measurements. Draining initiates at channel inclination slightly above the material repose angle and occurs as avalanche flow, augmented by bubbling at higher channel tilts. However, close to vertical orientation, avalanche flow ceases, resulting in slower emptying. A wide array of drainage patterns is exhibited as function of channel inclination, width, and grain size. Qualitative and quantitative analyses reveal fascinating self-similarity in the different regimes amidst the apparent randomness. Scaling relationships are proposed for drainage time and velocity profile in the moving layer during quasi-steady drainage. Additionally, a depth-averaged continuum model, based on experimental observations, is formulated for the prediction of the moving layer thickness.