Three-Dimensional Numerical Simulation Of The Influence Of Spur Angle On The Surrounding Flow Field And Local Scour

Spur dikes are the most common hydraulic structures in river improvement,which can effectively change the flow structure of the river,and play an active role in bank slope protection,water transportation and navigation,and improvement of aquatic habitat environment.But at the same time,the scouring at the dam head will also have a greater impact on the stability of the spur dike.According to the different angles between the spur dike and the incoming flow direction,it can be divided into the upward cantilever type,the forward cantilever type and the downward cantilever type.The different cantilever angles of the spur dike have different effects on the flow around the spur dike and the erosion of the dam head.Based on Ansys Fluent software,this paper uses RNG turbulence model and SIMPLEC algorithm,combined with Fluent dynamic grid technology,to carry out three-dimensional numerical simulation of the flow field and scour near the 60°,90° and120° pick angle non-submerged spurs,and the following conclusions are obtained:(1)In the case of the same river width and narrow rate,the backflow zone range of the three kinds of angled spur dikes are basically the same,but the position of the vortex center of the backflow zone moves downstream with the decrease of the angled angle.Compared with the calculation result of Li Guobin’s reflow range formula,the relative error of the simulated value of reflow length is 11.7%,and the relative error of the reflow width is1.3%.(2)The velocity field,turbulent kinetic energy,turbulent kinetic energy dissipation rate and the distribution of bed shear stress near each spur dike were compared.The results show that the flow velocity and turbulent kinetic energy distribution of spur dikes with different pick angles are basically the same,and the maximum velocity is slightly different,of which the 90° pick angle corresponds to the largest value.According to the vertical velocity distribution of each section in the recirculation zone,it can be seen that as the distance from the dam increases,the average velocity of the section first increases and then decreases.The larger values of the turbulent kinetic energy and the turbulent energy dissipation rate are both distributed at the junction of the mainstream area and the backflow area behind the dam,and the maximum values appear in the downstream area of the dam head.The distribution range of the turbulent kinetic energy low energy area near the dam head increases with the challenge The angle increases,but the range of the high-energy zone decreases with the increase of the angle.Due to the guiding effect of the 60° angle spur spur on the water flow,the shear stress of the downstream bed surface is relatively larger.(3)Based on the Fluent dynamic grid technology,the sediment transport equation,river bed erosion and sedimentation equation,and sand slip model are incorporated into UDF files to achieve local scouring of the spur,and compared with previous flume test results,simulation results The accuracy is high.Among them,the relative error of the maximum stroke depth of the 90° spur dike is 11.8%,the relative error of the punch length is 6.7%,and the relative error of the punch width is 18.8%.The relative error between the simulated drawing depth of the three angles and the formulas of Zhang Yiqing and Tian Weiping is relatively small.(4)The local scour simulation results of different angle spur dikes show that the maximum scour depth is located near the head of the dam,and the scour pit is in an inverted conical shape as a whole.With the increase of the pick angle,the maximum scouring depth and the length of the scouring pit in front of the dam gradually increase,while the length of the scouring pit behind the dam gradually decreases,and the position of the maximum scouring depth also moves upstream with the increase of the pick angle.Under the influence of the pressure gradient,a clockwise vortex was generated in the scouring pit in front of the dam,and there was also a certain development on the outside and downstream of the spur dam.(5)The distribution of bed surface shear stress and critical starting shear stress of sediment in the longitudinal section of the scour pit are compared and analyzed.The distribution of critical starting shear stress along the slope shows a double peak pattern,and the bed surface shear stress increases first and then decreases,and the maximum value is0.78 Pa,which is smaller than the critical starting shear stress of sediment under the condition of flat slope.