Cohesive sediment transport research: the science of mud

Mud in the natural environment is a mixture of clays, silt, sand, water and organic matter. It can be found in large quantities in estuaries and along neighbouring coasts, as is the case for Belgium. The usually high content of fine-grained particles (clays and some silts) explain the cohesive nature of mud, which is the net result of electrostatic forces from the clay particles and the sticky organic substances produced by bacteria and other micro-benthos. Therefore, cohesive particles consist of flocs of aggregates of the primary particles, which structure and density are determined not only by the internal properties, but also by those of the ambient water and the external mechanic forces (especially turbulent shear and particle interactions). When cohesive particles settle, they form a layer of slowly consolidating fluid mud. The siltation of navigation channels, docks and aquaculture farms is a well known problem. Managing authorities rely on model predictions in order to estimate the related economic cost and environmental impact of cohesive sediment transport and the effects of human interference by dredging and the construction of structures. However, the accuracy of these models is very low. The Hydraulics Laboratory of the KU Leuven investigates possibilities to improve the modelling capacities of presently used engineering software for sediment transport. The present focus of the research is on fluid-particle interactions, high-concentration effects, mudwave interaction, flocculation and bottom erosion resistance. The model improvements are strongly physically-based and supported by experimental data

Modeling flocculation processes: continuous particle size distribution method

The flocculation process of cohesive sediment suspended in water consists of aggregation of the fine particles and breakup of the large flocs. The population balance equation (PBE) is a statement of continuity for particulate systems, and it is used to model the flocculation process and predict the particle size distribution (PSD). Different numerical methods are developed to solve the PBE, however most of the methods have difficulties in representing the continuous PSD or improving computational efficiency. In this research, the B-spline FEM and Galerkin FEM are studied to simulate the continuous PSD. The B-spline FEM solves the PBE over the whole domain, which is truncated to finite domain; the open non-uniform B-splines are used as basis function to approximate the PSD; the curve of PSD is required to be smooth enough. The Galerkin FEM discretizes the PBE on each sub-domain (the whole domain is split to several sub-domains), and it is used to solve less-smooth problems. The adaptive technique is applied to readjust the computational grid (particle size domain) to improve computational efficiency and the accuracy, and it is also applied in varied time step to get suitable time step to improve the stability. The analytical solutions of the PBE in special conditions and the experimental data are used to validate both B-spline FEM and Galerkin FEM, and the results are compared with that of the classical DPBE method. It shows that both B-spline FEM and Galerkin FEM can solve the PBE and simulate continuous PSD accurately and efficiently

Interaction of suspended cohesive sediment and turbulence

This paper describes the work done in the COSINUS project, carried out within the framework of the European MAST3 research programme, on the interaction between suspended (cohesive) sediment and turbulence, with particular emphasis on its modelling. Specific attention is given to the modelling of buoyancy damping effects and turbulence production due to internal waves. Finally, some experimental results are presented on the effect of advected turbulence to the entrainment of fluid mud

Design features of the upcoming Coastal and Ocean Basin in Ostend, Belgium, for marine renewable energy applications

The new Coastal and Ocean Basin (COB) located at the Greenbridge Science Park in Ostend, Belgium is under construction since February 2017. The laboratory will provide a versatile facility that will make a wide range of physical modelling studies possible, including the ability to generate waves in combination with currents and wind at a wide range of model scales. The facility is serving the needs in Flanders, Belgium, in the fields of mainly offshore renewable energy and coastal engineering. The COB will allow users to conduct tests for coastal and offshore engineering research and commercial projects. The basin will have state-of-the-art generating and absorbing wavemakers, a current generation system, and a wind generator. It will be possible to generate waves and currents in the same, opposite and oblique directions. The basin is expected to be operational in 2019. This paper presents an overview of the basin’s capabilities, the ongoing work, and selected results from the design of the COB

Validation of macroscopic modelling of particle-laden turbulent flows

Large-scale engineering sediment transport problems require advanced closures for the description of particle-turbulence interactions, in order to improve the predictive capacities of the numerical modelling tools, which are based on the continuous-phase approach. Recently developed closures have revealed new information on the suspension capacity of shear flows. Qualitative validation by comparison between model results and flume data confirm these new features and show the need for a more detailed modelling approach for the supersaturated near-bottom layer

Modelling the thixotropic behaviour of dense cohesive sediment suspensions

A dense cohesive sediment suspension, which contains primarily clay particles, is a thixotropic non-ideal Bingham fluid with a true yield stress. Its time-dependent rheological behaviour can be described by the structural kinetics theory in which the yield stress is taken as a measure for the structural parameter. This theory allows the construction of a more general equation of state, which is independent of the rate equation. The applicability of the model is demonstrated by examples of the prediction of constant structure curves and of transient behaviour. The thixotropy model is incorporated into a Navier-Stokes solver to stimulate the flow behaviour in a Couette viscometer

Modelling of turbulent flow with suspended cohesive sediment

Traditional (cohesive) sediment transport models contain several simplifications which are no longer justifiable when sediment concentrations or stratification effects become significant. This paper gives a rather technical overview of various modifications to a sediment transport model with k-8 turbulence closure, used as research tool, in order to improve the physics described by the model. The attention is focussed on the modelling of sediment-turbulence interactions