Three-Dimensional Numerical Simulation Study Of Underwater Jet Scour

As one of the main dredging methods,jet scouring can ensure the safe operation of reservoirs and determine the degree of water conservancy development.On the other hand,it also affects the process of submarine pipeline laying and determines the development of marine engineering.Therefore,it is necessary to analyze the influence of different factors on the jet erosion results.Most of the problems encountered in engineering applications involve three-dimensional moving jet erosion.However,the existing research focuses on two-dimensional fixed jet.In view of the above problems,this paper optimizes and establishes a three-dimensional numerical model.In this paper,Flow-3d software is used to simulate the fixed jet erosion of circular nozzle with a diameter of 10 mm,the fixed jet erosion of mixed nozzle and the moving jet erosion of mixed nozzle.The effects of different jet velocity,target distance and moving speed on scouring effect were investigated by comparing and analyzing three aspects of scouring pit characteristics,flow velocity distribution and suspended sand characteristics.The research results show that:(1)The fixed jet of circular nozzle and the fixed jet of mixed nozzle have basically the same evolution law with time,and the depth and width of the crater show a growth trend.The velocity of the jet water gradually decreases during the descent process.After the mainstream reaches the sand bed.it moves upward along the slopes on both sides of the scour pit.When the jet velocity on the bed surface reaches the sediment start-up speed,the sediment enters the suspension state.(2)When the fixed jet target distance changes,the circular nozzle jet target distance is in the range of 100mm～350mm,and the erosion depth increases first and then decreases,that is,there is a critical target distance.Due to the limitation of its own structure,the target distance of the mixed-flow nozzle is set in the range of 150mm-350mm,and the scour depth gradually decreases with the target distance.Different jet target distances increase the scour depth of the mixed-flow nozzle by 12.9%-83.7%,and the scour width of the mixed-flow nozzle by 6.7%-32.6%.(3)When the jet velocity changes,the scour depth and width increase with the increase of jet velocity,but the trend of high-speed jet is smaller than that of low-speed jet.Different initial jet velocities increase the scour depth of the mixed-flow nozzle by 11.8%-35.0%,and the scour width of the mixed-flow nozzle by 5.6%-13.3%.(4)During the scouring process of the mixed-flow nozzle moving jet,the change of the depth of the scour hole is fast first and then slow,which is similar to the fixed jet of the mixedflow nozzle.The distribution is logarithmic,but the width of the scour hole is linearly distributed with the scouring time,and the width of the scour hole increases uniformly.The scour depth and width increase with the increase of jet velocity,and the influence of jet velocity on the width of scour hole is greater than that on the depth of scour hole.The scour depth and width decrease with the increase of moving speed,and the influence of moving speed on scour depth is greater than that on scour width.