SEDIMENT TRANSPORT ASSESSMENT IN SUSPENSION FLOW BASED ON COHERENT STRUCTURES CHARACTERISTICS

摘要

The sediment entrainment ability of coherent flow structures is investigated by comparing higher order statistical properties of shear stress and of turbulent mass fluxes in suspension, open-channel flow under capacity charge conditions. The quadrant repartitions of these quantities as a function of the corresponding threshold levels are estimated using a higher order cumulant discarded probability density distribution of the time fluctuating velocity and concentration fields. Good agreement between the third order model and the experimental results is found for all investigated quantities in the wall and intermediate flow regions. The quadrant distributions of the relative horizontal and vertical mass fluxes are dominated by the same two quadrants as the shear stress. The suspended sediment transport capacity of coherent structures is directly quantified from the estimation of the conditionally sampled terms of the sediment diffusion equation. Coherent structures of a burst cycle are found to be important contributors in the mass transport mechanism under highly turbulent flow conditions in open-channel flows. Direct estimation of the time scales of coherent structures permitted to correct a novel formulation of the near bed equilibrium concentration proposed recently by Cao (1999).