Effect of self-stratification of sediment suspensions in turbulent plane couette flow.

摘要

The research of this dissertation is motivated by the problem of the formation and breakup of lutoclines in tidal flows. Sediment delivered by rivers into sea spread on the continental shelves, and are resuspended by waves or tidal currents. The sediment concentration near the bottom can be enhanced to a fairly high level, and a sharp concentration gradient known as a lutocline forms between a bottom layer and an upper layer, which dampens turbulence and inhibits mixing. As a result, most sediment will be confined in the lower layer, which is referred to as a fluid mud layer. The turbulence structure, velocity profile and suspended sediment concentration profile is remarkably different depending whether or not a lutocline is present. In tidal environments, a lutocline can be formed as sediment settles out during stagnant high or low tide, and then broken up by strong flow velocities generated by rising or ebbing tide. A preliminary to the study of lutocline formation and breakup by studying the stable stratification in steady, equilibrium sediment-laden plane turbulent Couette flow is presented in the dissertation. Analysis has been made based on a mixing length hypothesis, and numerical simulations have been conducted with an algebraic turbulence closure model and a stratification-corrected k-? Reynolds Averaged Navier-Stokes Model (RANS). The results show that in sediment-laden turbulent plane Couette flow where density stratification effect can be neglected, the relation between the mean velocity and the roughness height takes a form similar to the Keulegan relation for open channel flow, differing only by a constant. The eddy viscosity profile can be fitted to a quadric polynomial, similar to the parabolic profile in open channel flow. Based on this, the equilibrium sediment concentration profile for cases of negligible stratification effect can be derived in a similar way to the Rousean profile for open channel flow. In the case of sediment-laden flow, two dimensionless parameters have been identified to govern the self-stratification of the sediment-laden turbulent plane Couette flow: a shear Richardson number Ri_tau and a dimensionless fall velocity vs.;. Increase in either parameter has a similar effect ofstrengthening sediment stratification, which is manifested by a sharpening of concentration gradient near the bottom, and a preferential dampening of turbulence in the lower layer of the flow. The relation between the friction and the layer-averaged flow velocity is determined with regression, showing increase in either Ri_tau or vs.;result in reduction of resistance. Similarly,the relation between the layer averaged concentration and the parameters Ri_tau and vs.;is determined with regression and reduces to the relation for non-stratifiedflow as the stratification effect diminishes. Although the setting of the numerical model constrains that the sediment can not settle out of the computation domain, in cases of sufficiently high sediment stratification or insufficient shear stress, sediment is sequestered in a thin layer near the bed, which in fact represents a condition that not all sediment can be held in suspension. A maximum value of the product of Ri_tau and vs.;is given as a criterionabove which sediment could not be held in suspension.