| Spur dike is a common hydraulic structure,which is widely used in river training projects,flood control projects,waterway regulation projects and ecological restoration projects.The layout of the spur dike changes the shape of the existing riverbed cross section,causing changes in the surrounding water flow structure and causing a large erosion and siltation change in the local topography of the riverbed.It is of great significance to fully understand the structure of the flow field around the spur dike and to master the local scouring and silting characteristics of the spur dike,both for the flow field environment expected by the construction project and for the safety of the spur dike itself.With the deepening of engineering practice and scientific research,the layout and structural form of spur dike have been continuously developed and improved.However,the destruction of the spur dike still occurred from time to time,posing great threats and damage to the actual project.In addition to the extreme complexity of the actual site environment,the understanding of the characteristics of the flow field around the dam and the local scour mechanism needs to be further improved.The paper is based on the full verification of the model by using the flume experiments and based on the three-dimensional turbulence model,the VOF free surface capture technology,and the sediment scouring and erosion model combined with the multi-phase flow mode suspended sediment transport and the traditional bed load transport.Numerical simulation study on the flow field around the spur dike and the local scouring and silting process of the spur dike was carried out.The main contents and results are as follows:(1)Experimental study on flow field and local scouring and siltation near the single groyne and groyne group with different flow rates and different water levels was carried out.The Acoustic Doppler Velocimetry(ADV)was used to measure the three-dimensional flow velocity of scour pit and scouring pit morphology data.Based on the experiment results of Zhang Hao single groyne and the results of the test of the flume experiment,the turbulence model,sediment model parameters and grid were fully determined and verified.A reasonable turbulent mathematical model and numerical simulation calculation scheme were determined.(2)The flow field characteristics of single spur dike in the fixed bed and movable bed scouring and silting was compared in detail,such as the recirculation zone,spatial streamline,three-dimensional velocity field,turbulent flow field and bed shear stress.The variation rule of relevant characteristic physical quantities along with the development of scour and deposition was analyzed.The dynamic mechanism of the occurrence and development of scouring and silting changes was further clarified.Due to the blocking and diversion of the dam,a local high-speed zone,ie,a high-shear zone,is formed behind the dam head.The scouring is first developed rapidly in this area,and gradually expands to the sub-high velocity region under the action of the two-way vortex.The initial local scouring pit strengthens the submerged flow in the head of the spur dike,causing the scouring to expand to the upstream and side of the spur dike.The bed shear stress of bed surface decreases with the increase of scour depth until a new equilibrium is formed.The high turbulence mainly occurs in the downstream area of the reflux at the boundary of the main flow,and has little correlation with local scour.(3)The flow field and local scour around the positive single spur dike under seven types of river width narrowing ratio(the ratio of groin length to river width)of 0.1-0.4 were simulated.The effects of river width narrowing ratio on free water surface morphology,circumfluence area range,velocity field,turbulent kinetic energy,bed shear stress and local scouring morphology were analyzed.With the increase of river width narrowing rate,the water surface drop hight(weir effect),the range and strength of local high velocity area(high shear stress area of bed surface),recirculation zone range,local scour pit range and maximum scouring depth are all approximately linear,but the relative ratio to the length of groyne is all decreasing.When the river bed width narrowing rate exceeds 0.3,the scouring can affect the opposite bank.(4)The flow field and local scouring morphology around the positive single spur dike under five Froude Numbers and three sediment particle sizes from 0.2-0.6 were simulated.The results show that Froude number is significant influences for the flow field and local scouring of the spur dike.When the Froude number is less than 0.4,the mainstream on the upstream side of the groyne separates in advance due to the support action of the groyne head,and the sub-mainstream zone is formed between the separation of the dam head.When the froude number is greater than or equal to 0.4,the secondary mainstream area disappears due to the action of inertia,and the upstream recirculation zone is formed.When the Froude number is 0.6,the mainstream forms a folding effect on the opposite bank,and the recirculation zone downstream of the spur dike is suppressed,while a recirculation zone is formed on the opposite bank.The scouring depth also increases with the increase of the Froude number.When the Froude number is greater than 0.4,the scouring increases significantly,which is especially noticeable in the case of small particle size(5)The flow field and local scouring characteristics of the grouped spur dikes consisting of four spur dikes with 2L,3L and 4L spur dike group spacing and three sediment sizes was simulated.The distance between the spur dikes in this study is smaller than that in the recirculation zone of the first spur dike,and the mainstream is constrained to the other side of the spur dike.Therefore,the influence of the spur dike spacing on the mainstream structure field around the spur dike group is not significant.The scouring pit of the first spur dike is the largest,and the scouring pit of the subsequent spur dike is gradually decreased.The scouring pit of the spur dike groups with smaller spacing is easy to be connected as a whole,and the scouring of the upper and lower sides of the spur dike is also small.When the spacing increases,the scour pit becomes discontinuous,and the scouring of the upper and lower sides of the spur dike is strengthened. |
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