Experimental Study On Local Scour Around Large-diameter Submerged Cylinders Under Combined Waves And Current Action

With the rapid development of social and economic construction,and most types of Marine construction require pile foundations,large-diameter submerged structures such as underwater caissons,energy dissipation measures,drilling platform foundations and fish habitats are widely used.In the offshore environment,waves and tidal currents often exist simultaneously,the local scouring around the cylindrical foundation under the combined action of wave and current seriously affects the stability of the structure.Therefore,it is of great scientific value and theoretical significance to study the local scour experimental around the large diameter submerged cylinder under the combined action of waves and current.In this paper,a flat sand bed is laid in the test section of wave-current flume with prototype sand of median diameter size d50-0.22mm,and simulating local scour experiment of large diameter submerged cylinder on flat bottom seabed under the action of waves and current.Based on the wave-flume test,the local scour mechanism,the relationship between pore water pressure and scour depth and the influence factors of equilibrium scour depth around a large diameter submerged cylinder were observed and measured under different conditions of wave-current combinations and submerged rates.The experimental results show that under the combined action of waves and current,the time development of scour depth around the large diameter submerged cylinder is consistent,and the scour hole shape around the cylinder are mainly composed of semi-circular scour hole in front of the cylinder,the slope of scour hole and saddle-shaped sand dune behind the cylinder.Compared with the non-submerged cylinder,the scour rate and the depth of equilibrium scour hole of the submerged cylinder are smaller,and it takes longer to reach the scour equilibrium.With the increase of submerged rate Sr,the time development of scour,the radius and depth of scour hole are decreased.The shape of the equilibrium scour hole is equivalent to 180 degrees rotation of the scour hole when under the wave with opposing current conditions,but the depth,radius and sedimentation height of the scour hole are smaller than those when under the wave with following current conditions.Due to the reflection and diffraction of the large diameter cylinder,the scour hole shape and scope is quite different from that of the small diameter cylinder.The pore water pressure around the cylinder presents periodic sinusoidal variation,and the amplitude of the instantaneous pore water pressure increases with the increase of wave height.The pore water pressure exerts an upward permeability on the sand under the series of wave troughs,which is also beneficial to the mobility and scour of the bed.The relevant dimensionless parameters(?.Ucw?KC and Fr)have great influence on the depth of equilibrium scour hole.For the KC number is constant,the equilibrium scour depth s/D increases with the increase of Ucw and Fr,and the scour depth will tended to be stable as Fr increases to a certain extent(Fr>0.8).The scour depth increases with increasing KC number,and the growth rate slows down gradually for a fixed value of Ucw.Under the same waves and current conditions,the scour depth decreases significantly with the increase of submergence rate Sr,while hc/D>1,with the cylinder height increases,the scour depth grows faster until the critical value is reached.