Onset of flow in a horizontally vibrated granular bed: Convection by horizontal shearing

We present experimental observations of the onset of flow for horizontally vibrated 3D granular systems. For accelerations Γ above Γ*, the top layers of granular material flow, leading to convection with motion both parallel and transverse to the shaking; the lower part of the layer moves with the shaker in solid body motion. With increasing Γ, more of the layer becomes liquefied. The initial bifurcation is backward, but a small amount of fluidization by gas flow lifts the hysteresis. A new convective mechanism, which we explore both experimentally and computationally, associated with horizontal shearing at the walls, is identified as the mechanism driving the transverse convective flow.