Research On The Characteristics Of Jet Flow Fields Generated By Twin Ship Propellers

With the rapid economic development and continuous technological innovation,the ship engine power has been greatly improved,and ships are developing with larger size and higher speed.However,while achieving high transportation efficiency,these ships produce a more significant effect of disturbance to river flows during navigation.Relevant departments and the public are concerned that such disturbance to water generated by propeller rotation may damage the stability of riverbed surface sediment,resulting in secondary water pollution by pollutants adsorbed on the bottom sediment.It is a precondition and basis for answering and clarifying this concern to study the distribution rule of propeller wake flow velocity and the influence range of jet flow disturbance.For this purpose,an experiment of jet flow generated by ship model propellers which in the direction parallel to the flow direction was performed in a flume;particle image velocimetry(PIV)was used to test and measure the influence of water flow velocity,water depth and calm water speed on the jet flow fields on both vertical and horizontal sections.Main conclusions obtained from analysis are described as follows:(1)From the results of flow field measurement on the longitudinal section,The ship is in the stationary state,the axial and vertical development distance of jet flow gradually decreases as the water flow velocity increases.The vertical influence depth of propeller jet flow when the ship is greater than the value when the ship is in the navigation state at different incoming flow velocities.When the vessel is ready to set sail,the hydrostatic speed is 1.67 times the average velocity of cross section,the vertical influence depth is maximum,its value is 2.56 times the diameter of the propeller,with the increase of a ship's speed,the vertical influence depth is gradually decreased,When the Vjh/U is 0.5,the vertical influence scope is minimum,its value is 0.87 times the diameter of the propeller.With the increase of Vjh/U,the vertical influence depth increases gradually.And the vertical influence depth first increases slowly and then gradually becomes stable when the ship is in the navigation state.(2)From the results of flow field measurement on the horizontal sections,The ship is in the stationary state,the horizontal influence width of jet disturbance at low flow rate is greaterthan 10 times the diameter of propeller,the velocity of flow increases by 33%,and the influence of the disturbance is reduced to 8.12 times;when the ship is in the navigation state,the same change rule as that in the stationary state is observed,but the horizontal influence width when the ship is in the stationary state is greater than that in the navigation state.When the vessel is ready to set sail,the hydrostatic speed is 1.67 times the average velocity of cross section,the horizontal influence width is maximum,its value is 9 times the diameter of the propeller,with the increase of a ship's speed,the horizontal influence width is gradually decreased,When the Vjh/U is 0.5,the horizontal influence width is minimum,its value is 4.87 times the diameter of the propeller.With the increase of Vjh/U,the horizontal influence width increases gradually.(3)When the ship propeller speed,water depth and flow velocity remain unchanged,different flow velocity peaks are observed on different horizontal sections when the ship is in the stationary state.As the vertical distance from the propeller axis increases,the axial development distance and the horizontal influence depth both decrease.At great water depth,when v/U is 1.67 in the stationary state,the axial development distance of jet flow reaches8.56 times of the propeller diameter within the horizontal section which is 0.5d away from the propeller axis,the influence of propeller jet flow basically disappears.