Internal streamlines in oscillatory heap formation on a submerged particle bed

We present the results of a two-dimensional experiment designed to test mechanisms responsible for the change in shape of the surface of a saturated granular bed below a submerged, oscillating plate. During the plate’s upward motion, a fraction of the grains in a region of the bed become mobilized, allowing some particles to be dragged by the fluid both vertically and horizontally through the remaining matrix of fixed particles. Because the vertical forces are stronger than the horizontal, vertical displacements are greater than horizontal, and the horizontal flow of the particle-fluid mixture is less resistant than the vertical. Consequently, when we attempt to fit the measured streamlines over many cycles to those observed in the experiment, using the pressure field in the bed that results from the solution of Darcy’s equation, we must employ different horizontal and vertical permeabilities in the Darcy flow relations. Doing this, we find relatively good agreement with the predictions of the mixture flow.