Propeller Jet Scour And Vertical Movement Of Pollutants In Substrate Sediment Under Stillwater Conditions

With the rapid development of economic society,international and domestic port trade,inland river shipping industry has increasingly become an important pillar industry.The jet generated by high-speed rotating propeller will increase the influence range of riverbed.When the water depth decreases to a certain extent,it will have an impact on bottom mud and overlying water body.When the depth of the river is small and the propeller is close to the bed,the jet action scour the riverbed surface and may cause the resuscitation of the surface sediment.At this time,the pollutants attached to the sediment move up to the water body and affect the water quality and the habitat of aquatic organisms in a local scope.Therefore,the research on the influence mechanism of propeller jet on the resuspension and diffusion of sediment pollutants is carried out to scientifically demonstrate whether ship navigation has an impact on water quality,which is beneficial to provide a scientific basis for the decision-making of the management department.This study carried out in indoor use high-precision circulation channel physical model experiment,the propeller diameter,different speed,blade propeller jet flow function under condition of different distance from the ground to scour the influence law of terrain,under different working conditions,and the propeller jet flow for sediment heavy metal lead in the sediment-overlying water interface between the vertical migration.The main conclusions are as follows:1.Under the action of propeller jet,the topography change of scour is related to the influence factors such as propeller speed,propeller diameter,near-bottom velocity,height of propeller tip and particle size of sediment.On the propeller axis,the axial near-bottom velocity Vx increases at first and then decreases.The peak value of the transverse near-bottom velocity in the cross section is just below the propeller axis,and the velocity decreases rapidly on both sides of the axis,showing a single peak distribution.The scour pit disturbed by propeller jet is related to the distribution of near-bottom velocity,and the sediment will accumulate as the longitudinal velocity decreases.2.By comparing the particle size of 8.146 microns of cohesive soil and particle size of 0.4 mm of non-cohesive sediment scouring pit pattern,found that the static water conditions,the 0.4mm of cohesive soil plane shape of the pit are similar to ellipse,under different conditions,there are differences in the length and width of the oval,but the oval length-width ratio range between 1.45 to 1.66,with an average of 1.53,under different working conditions,and the scour pit of cohesive soil plane form close to round.3.When the propeller jet scour the bed surface,the sediment on the bed surface will move upward to the overlying water,and the suspended sediment concentration in the water will change.The suspended sediment concentration(TSS)is related to the turbulence intensity and sediment particle size of the propeller jet.Under the same conditions(D=9cm,N=1000r/min,H_j=2cm;D=4cm,N=700r/min,H_j=2cm),the clay with the median particle size of 8.146 micron and the non-cohesive sand with the median particle size of 0.4 mm were taken as the experimental subjects to conduct the disturbance experiment.The results show that at X/D=2,the suspended solid concentration(TSS)generated by the bottom mud of the cohesive soil after disturbance is larger than that of the non-cohesive sand.This is because when the propeller perturbed the bottom mud of the cohesive soil,the suspended sediment particles are more and the suspended height is higher.However,the particles of the cohesive soil are smaller and the sediment settling speed is slower.During the disturbance,the suspended sediment particles are more,so the suspended solids content in the water body of the disturbed cohesive soil is larger.4.The propeller disturbance moves the sediment-water boundary downward.When the propeller speed and propeller diameter are increased,the water turbulence intensity is increased,and the sediment suspension is promoted.At the same time,the clearance from the propeller tip to the bottom is reduced,and the percentage of heavy metal pollutants released at the interface between the new sediment and the overburden water is increased.