Unconfined propeller scour in waterways: The role of flow intensity

Abstract

Flow intensity represents an important input to be investigated in the ship propeller jet scouring process in ship maneuvering areas, waterways path, and mooring in navigation channels. Although the still water condition is not realistic, since it induces modifications in the canonical propeller jet scouring process in waterways, several studies were based on this assumption and only a few papers focused on the influence of the flow intensity are present in the technical literature. In this work, an experimental campaign aimed at exploring the flow intensity effects on the propeller jet and related scour was carried out. In particular, three flow intensities combined with different propeller submergence depths were tested. The experimental results are shown in terms of longitudinal and cross-sectional scoured bed profiles, and empirical equations to compute the main scour hole dimensions are proposed. Moreover, scour hole bathymetries were analyzed by spectral analysis to estimate characteristics length scales. In order to increase the detail on the fluid mechanics analysis to explain the influence of the flow intensity on the evolution of the scour hole and obtain an insight into the influence of the boundary effect on the flow field, Reynolds-Averaged Navier–Stokes simulations were performed over the three-dimensional scoured bathymetry, acquired at the equilibrium stage, and over the horizontal boundary, representing the earliest stages of the scouring process. The good agreement between the numerical results and the measured velocities at the equilibrium stage allowed also the simulations over a flat bed in the same hydraulic conditions. These represent the beginning of the scour process in the case in which the propeller blades are close to the bottom. As to the authors' knowledge, the last configurations were never simulated before, owing to the difficulties in managing the computation of a moving mesh close to a solid boundary. The results show, for the first time numerically, the plane boundary effects on the characteristics of propeller jets with and without an incoming flow.