Experimental Study On Flow Field And The Process Of Scour Caused By Propeller Jet

With the rapid development of waterway transportation,there is a growing concern about the perturbations of bed sand and the impact on water environment during shipping navigation.On the one hand,the large size of ship propellers makes the jet velocity and the jet scope increase gradually.On the other hand,the propeller jet flow of a ship has an effect on waterbody,which may cause bed sand destabilization.The riverbed surface sediments are transported and re-suspended by the perturbations of the water near the bed surface,and the pollutants released into the overlying water may cause water pollution.Therefore,it is necessary to carry out experimental research on the destabilization of riverbed sendiment caused by propeller jet to provide scientific suggestions for navigation environment protection.The flow field of propeller jet was analyzed by Three-dimensional numerical simulation and physical model test.The velocity distribution characteristics of propeller jet near bed surface was analyzed by using Three-dimensional profile acoustic doppler current meter,and the riverbed topography of the scope of scour caused by propeller jet is measured by the three-dimensional laser topographic position finder.The main conclusions discussed in this paper are as follows:1.Under different test conditions such as propeller speed,propeller diameter and distance from bed surface,when the propeller jet flows to the near bed surface,the transverse velocity distribution in the near bottom cross section presents a single peak,and the maximum velocity is located on the axis,and then it declines rapidly to both sides.With the increase of the distance from the axis,the attenuation rate begins to slow down.The fitting obtained the empirical formula of the lateral velocity distribution near the bottom cross section.2.Under different working conditions,the distribution of the axial component of the near-bottom velocity on the axis increases first and then decreases rapidly after it emerges from the propeller surface.When the speed,the diameter of the propeller and the height from the bed increased,the maximum velocity of near bottom was further from the propeller plane.The empirical formulae of the maximum velocity distance from the bottom to the paddle surface under different working conditions are obtained by fitting.3.In the evolution process of scour topography formed by the disturbance of propeller jet on the bed surface,the axial length,transverse width and scour depth of scour topography change with the change of different rotating speed,different propeller diameter and different heights from the bed surface.With the increase of scour time,the scour slope length-to-depth ratio and width-to-depth ratio of the plane morphology of scour terrain,and scour slope length-to-depth ratio and width-to-depth ratio of the feature section at the bottom of scour terrain show a trend of decreasing first and then gradually becoming stable.When the scour terrain gradually becomes stable,the final length-width ratio,length-depth ratio and width-depth ratio of the scour terrain under different working conditions are 1.54,13.52 and 8.88,respectively.4.The flow field of propeller jet calculated by numerical simulation shows a conical distribution with propeller shaft as the center.The velocity distribution on the axis increases first and then decreases when it comes out from the propeller plane.Under the calculation conditions,if V/Vmax is equal to 10%as the diffusion limit of the jet,the maximum axial disturbance range of the jet is about Nine times oar diameter.The cross-sectional velocity distribution was bimodal when the distance from the propeller was one or two times the diameter of the propeller,Then in the cross section three times the diameter of the paddle and farther behind the paddle,the velocity distribution of the cross section changes from the bimodal type to the single peak type.