Temporal development of unconfined propeller scour in waterways

Abstract

A ship propeller jet causes bed scouring and deposition of sediments around the scoured area. Although the bed topography changes induced by the ship propeller are one of the main issues in a harbor, these effects are relevant in other contexts concerning the ship maneuvering zone. An unexplored problem is the scour induced by the propeller jet in waterways. In this case, the still water condition, commonly accepted in several research studies, is not justifiable owing to the effect of an additional flow to the propeller jet, which can induce alterations in the scouring process. Thus, in this paper, an experimental campaign on the combined effects of a propeller jet and a uniform flow on a mobile bed is presented. Specifically, different propeller submergence depths and water discharges were tested. Numerical simulations were performed to highlight the different behaviors observed in the experimental runs with and without a uniform flow in addition to the propeller swirling jet. Furthermore, the presence of the channel flow was investigated in terms of scoured longitudinal and transverse bed profiles, as well as of the entire scour hole bathymetry. A regression equation was obtained to compute the maximum scour depths and the scoured volume for the investigated conditions. Finally, a spectral analysis method is proposed to characterize the complex bed bathymetry induced by the propeller jet and the uniform flow.