Experimental Study On The Scour Properties Of A Sedimentary Bed By Steady And Oscillating Jets

Jet scour,induced by hydraulic structures or artificial actions,is common in the world.The process of jet scouring can be described as the overcome results of jet-induced shear stress to the bed resistance,dislodging the sediments from the bed surface,and transporting the sediment downstream.However,due to the variability of the jet flow field and the diversity of the sediments,problems such as unstable turbulences and inaccurate estimation of bed resistance often puzzle the jet-scour researchers.In terms of the flow fields,on account of the vortex-induced vibrations,flow pulsations,etc.,the jet flow in engineering practice is often accompanied with oscillating shears during its underwater evolution.While in terms of the bed resistance,if the bed is composed of cohesive sediments,the resistance to sediment transport has to include the influence of the internal cohesion of the cohesive sediments.However,as a result of the complexity of physical,chemical and biological properties of cohesive sediments,there are many factors affecting the internal cohesion.How to measure the resistance characteristics of cohesive sediments is still a challenge.In addition,according to sediment rheology,cohesive sediments have the characteristics of shear thinning and thixotropic,leading to a time-dependent cohesion under the external oscillating shear.In this scenario,the cohesive sediments appertain to a non-Newtonian fluid,and would behave rheological properties.Hence,based on the theory of mechanics of turbulent jet,sediment movement and sediment rheology,this dissertation experimentally investigates the scour properties of cohesive sedimentary bed by the steady and oscillating submerged jets.The details are as follows:Firstly,based on the knowledge of sediment rheology:“The cohesive sediments possess the rheological properties such as shear thinning and thixotropy,resulting in the unstable internal cohesion,and unstable resistance to the flow”,the experiments of sediment rheology under mechanical vibrations are carried out.The rheological behavior of underwater cohesive sedimentary bed under the vibrating shear is measured.The time-dependent dynamic soil pressure,dynamic pore water pressure and acceleration in the cohesive sedimentary bed are measured.Comparing to the rheology curve?shear stress-time curve?,the time-dependent mechanical characteristics are detailly discussed.The fluidization mechanism of underwater cohesive sediments under vibration shears is further analyzed.The distribution of dynamic soil pressure amplitude with vibration distance is observed.The effect of vibration intensity on the cohesive sediment yield strengthis discussed.It is found that under the action of vibration shear loads,the varying trend of the dynamic soil pressure,dynamic pore water pressure and acceleration in underwater cohesive sediments is synchronized well with the sediment rheological curve?shear stress-time curve?.At the initial stage of vibration shear load,the underwater cohesive sediments show the properties of the material mechanics without motion.After failed by the vibration shear load,the underwater cohesive sediments show the properties of kinematic mechanics that its acceleration increases with the increase of the force.At last,the force and the movement properties of the underwater cohesive sediments reach equilibrium.In the spatial distribution of the vibration shear load,the peak value of the dynamic soil pressure during the equilibrium stage of the underwater cohesive sediments decreases negative-exponentially with the increase of the vibration distance.The degree of reduction in sediment yield strength is logarithmically related to the vibration load intensity.Secondly,from the perspective of sediment motion resistance,the jet scouring is investigated by divided into scouring the non-cohesive bed and scouring the cohesive bed.In terms of scouring the non-cohesive bed,408 experimental data are collected from the previous relevant researches,and twelve experiments are carried out.The prediction formula of the dimensionless maximum sizes for the equilibrium jet scour hole considering the particle size-hydraulic radius ratio is proposed.It is revealed that both the particle gravity and form resistance of large-scale particles should be considered when estimating the resistance of the non-cohesive sediments.The dimensionless maximum depth of the equilibrium jet scour hole varies linearly with the density Froude number,and non-monotonically with the particle size-hydraulic radius ratio.In terms of the cohesive sediment scouring,the jet scouring properties of the cohesive sediments with sediments of different initial yield strengths is observed.It is found that the maximum depth of the cohesive sediment jet scour hole increases logarithmically with time.Within 10 min of jet scouring,the shape of the cohesive sediment scour hole remains smooth,and nearly no obvious collapse occurs during this period.With different initial yield stresses,the cohesive sediment bed behaves different scouring rates.The jet scouring rate of the cohesive sediment bed shows a negative exponential growth with the increase of the yield strength.Then,the hydraulic properties of the oscillating submerged jet are observed in laboratorial flume,including:instantaneous flow field characteristics of the free jet zone and the impact zone;pulsating pressure of the bed surface of the impact zone;near-wall velocity profile,turbulence characteristics and decay characteristics along the bed of the oscillating wall jet zone.It is found that in the free jet zone,the distribution of the velocity along the axial direction of the oscillating jet is not completely symmetric with respect to the jet centerline,and the velocity gradient of the free jet region is denser on the front side of the swing than on the back side of the swing.In the jet impact zone,the critical oblique angle of the oscillating jet which leads to the upstream recirculation after the jet impinged on the bed is greater than the steady oblique jet.The impact pressure on the bed surface behaves different forms of fluctuation at different location.In the oscillating wall jet zone,the velocity turbulence varies slightly with phase and frequency,but its velocity distribution in the near-wall flow region is not substantially different from the steady wall jet.The velocity of the oscillating wall jet appears in a wave form,which can be resolved as both the current-contributed part and wave-contributed part exhibit a decay characteristic along the path.At the same time,according to the empirical turbulence hypothesis,the formula to estimate the bed shear stress ?^y of the oscillating wall jet is proposed.Finally,the flume experiments are carried out to study the scour properties of cohesive sediment bed by the oscillating jet.In the experiments,three kinds of cohesive sediments are scoured by oscillating wall jets with different frequencies and amplitudes.By comparing the morphological characteristics of the scour hole surface under different oscillating frequencies,the influence of the oscillating shear load on the scouring process is analyzed based on the sediment rheology.The effect of the oscillating shear intensity on the yield strength changes is explored.The scour properties of the oscillating wall jet on the cohesive sediment bed are explored by introducing the effect of the oscillating shear load intensity on the yield strength and sediment motion resistance.It is found that the underwater cohesive sediments behave obvious shear thinning under the action of high frequency oscillating wall jet.The scouring rate of cohesive sediment bed under the shear load of the oscillating wall jet is negatively correlated with the initial yield strength of the sediments,and increases logarithmically with the oscillation intensity.