The resistance of a plate moving through mud - experiments and simulations

Safe navigation in restricted areas is ensured, among others, by setting a minimum under keel clearance (UKC), which is the distance between the ship’s keel and the bottom. However, in many ports and waterways around the world the seabed is covered by mud and the position of the bottom is not longer clearly defined. In this case, the depth is determined based on the nautical bottom, defined as “the level where physical characteristics of the bottom reach a critical limit beyond which contact with a ship’s keel causes either damage or unacceptable effects on controllability and manoeuvrability”.
A complete implementation of the nautical bottom concept therefore requires a good understanding of the possible effects of muddy bottoms on the navigation of marine vessels. Although some model-scale and full-scale trials have been carried out in the past decades, results are difficult to generalise because of the large number of parameters involved (UKC, mud layer thickness, rheological properties, ship’s geometry and speed, fairway cross-section, etc). Thus, for practical reasons, port authorities define the nautical bottom as the level where the mud reaches either a critical
density (e.g. 1200 kg/m³) or a critical rheological property, based on the experience acquired over the years. These criteria, however, may be too conservative in some cases, which either lead to unnecessary maintenance and environmental costs, or to excessive restrictions on the allowed draughts of the vessels.
A research project was thus started with the aim to build a Computational Fluid Dynamic (CFD) framework that would allow more systematic studies on this topic. One of the main difficulties of the CFD approach, however, is that mud exhibits a very complex non-Newtonian rheology. Nevertheless, for engineering purposes, mud is often modelled as a Bingham fluid as it is the simplest rheological model capable of capturing one of the main features of mud, i.e. viscoplasticity. These type of fluids start to flow only when the level of shear stress exceeds a certain threshold, called yield stress. Below the yield stress, viscoplastic fluids behave as solid-like materials. Other examples of these fluids are gels, drilling fluids, cosmetic and food products (e.g mayonnaise).
The main goal of this research is to establish whether, in spite of its simplicity, the Bingham model is suitable for prediction of the forces acting on marine vessels moving through mud. Since experimental data for such scenarios are rather difficult to obtain, a simpler problem is considered here. In this paper, an experimental and numerical study on the resistance of a thin plate moving through homogeneous mud in laminar regime will be presented.