Question

Why are sediment grains that are resting on the sea floor more likely to become entrained at high Reynolds numbers than low Reynolds numbers? Make sure you also relate your answer to the shape of the boundary layer.

Answer 1

Boundary layers can either be laminar or layered, or turbulent or disordered, which depends on the magnitude of the Reynolds number. The boundary layer acts as laminar for lower Reynolds number, as one moves away from the wall (on the left of the figure), the streamwise velocity changes uniformly. Again, the boundary layer acts as turbulent for higher values of Reynlods number, the streamwise velocity being signified by unsteady eddy flows in the boundary layer. The edge of the boundary layer reacts with the external flow, similar to as it would with the physical surface of any object. So sediments which were intially settled on the sea floor are very likely to start flowing, or get entrained as soon as the flow becomes laminar to turbulent or achives higher Reynolds number.