The settling and hydrodynamic properties of 3-D fractal flocs in quiescent water are investigated with a numerical model based on the Lattice Boltzmann Method （LBM）, with considering the settling velocity, hydrodynamic drag force and infra-floc flow. The comparisons of floc settling velocities and effective densities indicate that the numerical results present good agreements with observations in field and at laboratory. The results show that the drag force Fo increases with the floc size dr according to the relationship FD ∝ df^3. Moreover, the intra-floc flow field and movement of the pore water provide a better understanding of the intra-floc flow from the microscopic viewpoint. The results also indicate that the lattice Boltzmann method is a promising approach to reveal the mechanisms of the flocculation in aquatic environments.
According to the mechanism of sediment suspension under waves, namely, the main reason of sediment suspension changes from the turbulent mixing in the bottom boundary layer to the periodic motion of the water particle near the free water surface, a three-layer model of sediment concentration distribution due to waves is presented along the whole water depth based on the concept of the finite mixing length. 1he determination of the parameters in the model is discussed and an empirical formula is suggested. Comparisons between the calculated results and the measurements indicate that the resuits of the model agree well with the data from both the large and small scale flume experiments.
Linear surface gravity waves on Maxwell viscoelastic fluids with finite depth are studied in this paper.A dispersion equation describing the spatial decay of the gravity wave in finite depth is derived.A dimensionless memory(time)number θ is introduced.The dispersion equation for the pure viscous fluid will be a specific case of the dispersion equation for the viscoelastic fluid as θ=0.The complex dispersion equation is numerically solved to investigate the dispersion relation.The influences of θ and water depth on the dispersion characteristics and wave decay are discussed.It is found that the role of elasticity for the Maxwell fluid is to make the surface gravity wave on the Maxwell fluid behave more like the surface gravity wave on the inviscid fluid.
A two-layer model, with the upper layer being the perfect fluid and the lowerlayer being the pseudo-plastic fluid describing water wave attenuation over mud bed, wasestablished. A simplified method based on the principle of e-quivalcnt work was applied to solve theboundary value problems. The computational results of the model show that the two-layer perfectfluid model and the perfect-viscous fluid model are all special cases of the present model. Thecomplex nonlinear properties of wave attenuation over mud bed, can be explained by the presentmodel, e. g., the wave dissipation rale decreases wilh the wave height in certain cases, while thesmall wave propagates over mud bed with less energy dissipation and large wave attenuates rapidly inother cases. Other factors influencing the wave attenuation were also discussed.
