Modelling of sediment suspensions in oscillating grid turbulence

A k–ε model is used to describe the steady state of fine-grained sediments maintained in suspension by purely diffusive turbulence, as generated in oscillating grid turbulence experiments. The behaviour is shown to depend both on the bulk Rouse number Rou0 and the product of the bulk Rouse number and the bulk Richardson number Ri0 Rou0, built on oscillating grid parameters. For Rou0 < 0.01, concentrated suspensions are observed with a homogeneous particle concentration in the suspension layer. An interface, called lutocline, separates the suspension layer from the clear water at a distance zm from the grid. The depth of the suspension layer is found to vary as zm/z0∝(Ri0 Rou0)−1/4. For Ri0 Rou0 ≪ 1 the decay of turbulence is affected by the particle concentration only in a region very close to the interface. In this case the flux Richardson number approaches the value of 1 near the interface. The lutocline is seen to vanish for large values of Rou0. For Rou0 > 0.01 the mean sediment concentration and turbulence decay simultaneously with increasing distance from the grid, and no sharp interface is observed.