To-and-fro motion of a submerged plate above an erodible bed

Abstract

The interaction between the oscillatory flow, driven by the to-and-fro translational movement of a vertical surface-piercing plate, and the (planar) granular bed is experimentally addressed. Focus is paid to the problem of the onset of sediment motion due to this interaction. By systematically varying the amplitude, A, and the frequency, f, as well as the gap between the lower tip of the plate and the bed, G, it is found that initiation of grains motion is mainly driven by the jet-like streams, spatially symmetrical, alternatively expelled after half-period through the gap, G. It was observed that grains are set into motion close to the center of the oscillatory motion, where the plate reachs its maximum velocity, Vp=πAf . Experimental points fall around a straight line when plotted in terms of the dimensionless numbers νTc/H2 and πAc/Gc ( ν and H are kinematic viscosity and plate submergence, respectively, and Tc=1/fc ), suggesting that the threshold condition is well defined by a roughly constant value of Vp/G . This findings are explained by means of a simple hydro-mechanical model, in terms of the (maximum) shear stress needed to dislodge a single grain overcoming the resistive force created by the submerged weight. Although limited to hydraulically smooth flow, the results presented here indicate that the phenomenon depends on geometric and strength parameters.