Flow beneath inland navigation vessels

Abstract

Growing transportation rates and the subsequent growth in inland waterway transport have led to an increase in inland vessel sizes and draught. Due to the fluctuating water levels on rivers and these increasing draughts, the distance between the river bed and the ships are decreasing. Rijkswaterstaat wants to know the effects of sailing at these small underkeel clearances on river beds and ship manoeuvrability. In order to quantify these effects, more knowledge about the flow field beneath sailing vessels is required, as well as the effect of the flow field on erosion of bed material. Currently no methods exist to determine the flow field beneath vessels, only a few formulations for a single maximum velocity value are available, but these are not applicable at small underkeel clearances (h / T < 1.25). Also the effect on the river bed is fairly unknown. To quantify the different effects of small underkeel clearances on the flow field physical model tests (with a length scale of 30) have been performed at Deltares. During these experiments a ship was towed through the flume, and flow velocities and pressures on the bed were measured, as well as forces on the ship. Additional experiments have been performed to investigate the effect on a moveable bed with different bed forms. From the experiments, it was found that the most important parameters that influence the flow field beneath the keel are the bow shape and the underkeel clearance. Barge bows force more flow underneath the keel than conventional bows, and this results in higher bed velocities. Decreasing keel clearances also result in significantly higher velocities at the bed. However, for very small underkeel clearances the boundary layer on the ship will interact with the boundary layer on the bed. This results in flow blockage underneath the keel. As a result, the flow needs to divert to the sides, and the velocities underneath the keel decrease. The diversion of the flow to the sides is also known as the fanning-out effect. This effect has definitely been proved by the measurements from the experiments. The effect (transverse velocities) increases with decreasing keel clearance (due to boundary layer interaction) and also increases with increasing ship widths. During the experiments, erosion of bed material was clearly observed, and its effect increased with decreasing keel clearance. However, the underkeel clearance needs to be very small (h /UKC < 1.1) to give significant bed erosion. Due to the fanning-out effect and turbulence fluctuations, most sediment transport occurred immediately alongside the vessel, rather than underneath the keel. With bed forms such as dunes the erosion increased, due to erosion at the dune tops and deposition in the troughs (10 passages of a conventional vessel over a dune resulted in a decrease in dune height of 20%). For the removal of small shoals this might be interesting, although a small underkeel clearance is necessary. Barges are preferred over conventional vessels due to the higher velocities and increased turbulence intensity. From the measured velocities during the physical model tests a model has been developed to predict the flow field underneath sailing inland navigation vessels. There are separate models for conventional vessels and for barges. The model is able to accurately predict maximum velocities (in sailing direction as well as in transverse direction), as well as a transverse velocity distribution. Compared to the previous prediction methods, the newly developed model is preferable. The results are more accurate, and the model is more extensive, due to the inclusion of transverse velocities and velocity distributions. More validation is required however, due to the lack of other data sets.