Grid stirred turbulence: applications to the initiation of sediment motion and lift-off studies

A laboratory experimental set - up for studying the behaviour of sediment in presence of a turbulent field with zero mean flow is presented. Particular interest is shown on the initiation of sediment motion and in the sediment lift - off. Some examples of the results obtained with this set-up are shown.

A turbulent velocity u′ lower than that u estimated by the Shield diagram is required to start sediment motion. The minimum u′ required to start sediment lift - off, is a function of sediment size, cohesivity and resting time. The lutocline height depends on u′, and the vorticity at the lutocline seems constant for a fixed sediment size. Combining grid stirring and image analysis, sediment vertical fluxes and settling speeds could be measured.