The Research On Local Scour And Hydrodynamic Characteristics Around A Large-Diameter Vertical Cylinders In Combined Waves And Current

As the demand for new energy resources increases,large-diameter cylinders are widely used as the foundation piles for near-shore wind turbines.The interaction between waves and current can cause intense sediment movement in the coastal area,resulting in local scour around the near-shore wind turbines that use large-diameter cylinder foundations,and seriously affecting the stability of wind turbines.The mechanism of local scour around large-diameter cylinder is sediment transport under the influence of flow field around cylinder.Therefore,the research on local scour and hydrodynamic characteristics around a large-diameter vertical cylinders in combined waves and current has improtant engineering significance.Based on the wave flume experimental and numerical simulation,the local scour and hydrodynamic characteristics around the large-diameter vertical cylinder under wave-current conditions were studied.On a flat sand bed with sediment median size of 0.22 mm in a wave flume,a series of experiments were conducted respectively under combined wave-current conditions with a range of KC numbers,Froude numbers and the weighting ratio of current Ucw.Considering the influence of the timescale of scouring,the comprehensive calculation formula of scour depth under small KC number conditions was obtained from the measured datas.Meanwhile,the three-dimensional vortex structure in the scour pit around the large-diameter vertical cylinder was numerical simulated by us:ing a three-dimensional steady Reynolds averaged Navier-Stokes equations with RNG k-? turbulence closure.The evolution process of the vortex structures in the scour pit were obtained,and the influence of the vortex structure around the cylinder on local scour was discussed.The results reveal that,(1)the weighting ratio Ucw is adopted as a criterion to distinguish the wave-dominated and current-dominated combined flow,when Ucw<0.43.the flow belonged to the wave-dominated regime,the scour patterm is dominated by uniform continuous small-scale ripples,and the size of the scour pit around the cylinder is small.When Ucw>0.43,the flow belonged to the current-dominated regime,the scour type changes from the Twin-Hom-Shaped to the Cone-Shaped as the weighting ratio Ucw increases,and the sand ripples is gradually increased.(2)When Ucw>0.43,the scour type is independent on the KC numbers.The continuous sand ripples appears on the upstream bed surface of the cylinder,and the size of sand ripples increases with the increase of the KC numbers.(3)This classificatlion criterion is found to have a specified value of 0.43,at which the nondimensional characteristic timescale T*coincidentally gains a turning point at lower wave Shields numbers.(4)The ealculation formula between the relative of scour depth and Froude numbers is obtained from the measured datas.It indicates that the relative of scour depth approaches its mathematical asymptotic value(Se/D?1.49)as Froude numbers increases.(5)For small KC number conditions,a slight increase of Ucw could result in a significant increase of equilibrium scour depth.(6)Based on the Q criterion,the three-dimensional vortex structure around the cylinder is obtained when the scour is near equilibrium stage for the flow belonged to the current-dominated regime.Diaring the whole process of wave-current propagation,the horseshoe vortex systems in the scour pit always exists,and the horizontal velocity power spectrum distribution at the vortex core is single peak.While three different types of tail vortex systems were found:the steady,transition and breakaway.It indicates that the horseshoe vortex systems in the scour pit is relatively stable,and is different from the instability phenomenon exhibited by the horseshoe vortex at the initial scour stage.(7)As the polar angle ?increases,the horseshoe vortex intensity weakens and the horseshoe vortex core position gradually moves away from the cylinder surface,until polar angle ?=90°,the horseshoe vortex structures in the scour pit basically disappears.In the range of-90°<?<90°,the bed shear stress in the scour pit is smaller than the bed shear stress near the cylinder at the initial scour stage,and the largest values were recorded for polar angles ?=45°.When polar angles ??900,the existence of the scour pit causes a strong upflow behind the cylinder,producing a strong wake vortex,which causes the bed shear stress behind the cylinder is much larger than that at the initial scour stage.(8)By comparing the bed shear stress distribution around the cylinder,it was found that when the scour is near equilibrium stage,the bed shear stress on different polar angle planes in the scour pit is still larger than the critical shear stress of the sediment movement.It indicates that although the scour rate around the cylinder is very slow,it does not reach the final scour equilibrium stage,and the scour is still in progress.It is further explained that many scholars neglected the influence of the timescale of scouring during the physical tests,which led to the calculated value of the scour depth was too small.