Study On Coherent Structure Of Turbulent Flow In Open Channel With Step-shaped Spur Dike

Beach regulation is an important means to improve channel conditions,and the spur dike is one of the most common regulation structures.In order to reduce sediment deposition during the flood period,the water receding period scour as soon as possible,it is necessary to adopt the combination of flood and low water control,so the step-shaped spur dike at the right moment.Because of the complex flow conditions around the step-shaped spur dike,the coherent structure is bound to occur,which will affect the navigation of ships,aquatic animals and aquatic plants.Existing research is mainly focused on the single spur dike,and the coherent structure of step-shaped spur dike is less discussed.This paper is based on PIV test results,determining the horizontal vortex identification method,analyzing the step-shaped spur dike area along the horizontal apparent characteristics of vortex,discussing the characteristics of transverse vortex characteristic parameters of spur dike area under different flow,comparing the characteristics of different forms of spur dike under the action of horizontal vortex.The results are as follows:(1)A single vortex structure usually has the same accompanying flow field with its steering,and a multi-vortex structure is usually a nested form of "large vortex within small vortex".Among them,the steering of large vortices is basically one,while the steering of small vortices may be the same and different,which is related to the inflow characteristics and surrounding boundary conditions.The main reason for the different inclined directions of the downstream vortex is the difference of the flow characteristics and the flow field around the vortex itself.(2)The spatial autocorrelation structure cloud map is basically elliptical around the measuring point.There are obvious positive and negative correlation zoning phenomena in the cross-correlation structures around the two measuring points in the downstream of the spur dike,and the characteristics of each zoning also evolve along the width of the dike.(3)Under the action of step-shaped spur dike,the vortex density decreases with the larger flow rate.The position where the greatest influence of the spur on the upstream and downstream vortex density is not symmetrically distributed.The distribution is closer and smaller to dike in the upriver,while the distribution is farther and larger to dike in the downriver.In the area close to the flume side wall,the maximum vortex density of step-shaped spur dike appears earlier than that of single spur dike.(4)In the upstream area near the dike,the disturbance of the stepped spur dike to the consequent vortex area is mainly reflected in the area where it intersects the spur dike.However,in the downstream area near the dike,the closer to the trough wall on the side of the spur dike,the greater the influence is,and the influence is most significant in the non-coincidence area of the first and second order spur dikes.Compared with the single type,the step-shaped spur dike can make the shape parameters of the vortex outside the spur dike change along the way closer to that inside the spur dike,and keep the shape of the vortex relatively uniform.(5)The effect of step-shaped spur dike on vortex shape parameters is partitioned along the width of the dike.At a small flow rate,the inner side of the stepped spur dike is more circular and the outer side is more flat.At large flow rates,the difference is not significant.(6)The eddy rotation intensity is an exponential function distribution and gradually decreases along the passage.The rotation strength of a single spur dike changes smoothly along the passage and the presence of a step-shaped spur dike can slow down the decrease of the rotation strength of the transverse vortex.The strength of the reverse vortices is about 5 times that of the consequent vortex.