Experimental study of the critical criteria for incipient sediment movement using a simple model eddy

Experiments have been performed to study the interaction between a sediment layerand a vortex ring (used, here, as an idealised model for an isolated turbulent eddy). Attentionwas focussed on studying the interaction under “critical” conditions, in which motion of sedimentgrains was only just induced by the flow field of the vortex ring. Near-spherical monodisperse sedimentgrains were used throughout with relative density 2.5 and diameters (dp) ranging between0.053mm and 1.0 mm. Measurements of the vortex-ring flow field were obtained, during theperiod of interaction, using two-dimensional particle imaging velocimetry (PIV). The conditionsfor incipient sediment motion were investigated in terms of the critical Shields parameter (µc), definedin terms of the peak tangential velocity measured directly above the bed surface and obtainedfrom the PIV data. Effects due to increases in bed slope were studied by tilting the sediment bedat various angles between the horizontal and the repose angle for sediment. The measured valuesof µc are compared and discussed in terms of the particle Reynolds number (based on the particlesettling velocity and dp), the ratio dp=± (where ± is the viscous sublayer thickness), and the bedslope.