Study On Velocity Distribution Of Ship Stern Flow And Stability Calculation Method Of Rubble Bed Stone

As the foundation of gravity wharf and vertical breakwater, rubble bed plays an important role in maintaining the stability of the two kinds of structures. In recent years, to satisfy the demand for larger scale and higher velocity of ships, the thrust force of propeller and the velocity of propeller jet are continuously increasing. The velocity of propeller jet exceeds the design pick-up velocity of rubble, which results in the unstable failure of rubble bed. Currently, the cases that rubble beds at home and abroad are scoured destructively by high-velocity flow have crucial effect on working life and using safety of wharfs. However, the effect of ship stern flow on rubble bed stability is not taken into consideration in the code of harbor and coastal engineering design in China. It has become a very urgent issue in the field of harbor design and construction.The purpose of this paper is to study the rubble bed stability under the effect of ship stern flow, the main work are as follows,(1) Through comparing and analyzing the physic model experiment result, theory and empirical formulae for propeller jet, this paper summarizes the distribution law of propeller jet velocity. A set of empirical formulae are proposed to describe the velocity distribution of ship stern flow.(2) The CFD software Fluent is used to establish 3-D numerical model for propeller jet flow field, calculate and analyze the distribution condition of propeller jet velocity. The calculation results are fitted to the empirical formula to validate the feasibility of numerical simulation.(3) This paper presents a numerical model for rubble bed under the effect of propeller jet. By changing the location of propeller, the distribution law of propeller jet velocity with the influence of rubble bed is analyzed. The maximum velocity on the surface layer and the corresponding location are determined.(4) This paper compares the numerical calculation results with the stability calculation method based on empirical formulae, found that the numerical calculation results take into consideration the effect of quay wall and rubble bed on the velocity distribution of propeller jet, as well as the jet reflux scouring action. According to the numerical calculation results, it is found that inflow has weaker influence on rubble stability, while the reflux is the key factor resulting the rubble unstable and picked up. The scour mechanism of rubble bed under the effect of ship stern flow is summarized and the effective measures to prevent scour action are introduced.