|Title:||Dispersion of unsorted particle cloud in ambient cross flow|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Reclamation of land
Sedimentation and deposition
|Department:||Department of Civil and Structural Engineering|
|Pages:||1 v. (various pagings) : ill. ; 30 cm|
|Abstract:||For land reclamation or dredging projects large amounts of sediment and mud are often disposed in the designated areas of coastal waters. As a pre-requisite to the assessment of the environmental impact due to these projects, the dynamics and dispersion of the particle cloud should be determined. The distinctive features of the present problem are that the dynamics will be affected by the negative buoyancy induced, the settling velocity of the sediment particles, and the cross-flow velocity. In this dissertation, the dispersion of unsorted particle cloud in ambient cross flow is studied by means of laboratory experiments and numerical models. This dissertation includes 5 chapters. In the first chapter, a systemic review of previous works related to particle cloud and multiphase flows is made. Their main contributions or conclusions, advantages and shortcomings are also described. In the presence of ambient cross-flow, the interaction of the cross-flow and the buoyancy of the particle cloud will be complicated. So in the second chapter, a series of laboratory experiments has been conducted to investigate the cross-flow and mixing of particle cloud in cross-flow with different particle sizes, initial volumes and ambient flow velocities. The continued motion process of the dumped sand in the flow has been recorded by a digital video camera. Then, the longitudinal width, and longitudinal and vertical frontal position of cloud have been measured using the imagine process software and the temporal variation of cloud size and frontal position of the particle cloud have then been obtained. These data have been analysed and compared with the classical thermal results. The measurements suggest a roughly linear relationship between the displacement of the frontal position and the longitudinal width of the sand cloud. Three dimensionless empirical constants (a1, a2 and a3) of sand particle clouds in ambient cross-flow, which represent the relationship between the non-dimensional vertical frontal position Zfn of the particle cloud against non-dimensional time tn, the non-dimensional vertical frontal position Zfn against the non-dimensional longitudinal width Lfn of a particle cloud, and the non-dimensional longitudinal width Lfn of a particle cloud against non-dimensional time tn respectively, are obtained. In the third chapter, a 3D numerical model using the k-e parameterization of turbulence for the fluid phase and a Lagrangian method for the particle phase has been developed. Firstly, a detailed review numerical models and modified turbulence models for multiphase flows are made. Then, basic equations for fluid and particles are presented. Finally, the numerical schemes are described. The governing equations for fluid phase, and the k and e equations for turbulence are solved numerically using a finite-difference method with a split operator approach. At each time step, the equations are split into three steps: advection, diffusion and pressure propagation. In the advection step, the advective terms in the momentum equations as well as in the k-e equations are solved by an efficient four-node Minimax-Characteristics scheme. In the diffusion and pressure propagation steps, the simple forward time, centered space scheme is used in the solution, with the equations in the diffusion step solved explicitly and the equations in the pressure propagation step solved implicitly using the conjugate squared stabilized (CGSTAB) method. In the fourth chapter, the numerical results are presented and compared with the measured results. Satisfactorily agreement between measurements and computations are obtained. In the last chapter, the main conclusions are summarized and further work is suggested.|
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