| The non-uniform glass-sphere array bed is a common type of bed surface in mountain rivers.The flow structure near the rough particles is disordered and the spanwise vortex structure is easy to be generated.The spanwise vortex plays an important role in turbulent maintenance,material transport and energy transfer in flow field.However,existing vortex structures are mainly focused on smooth bed surface,and few studies have been conducted on spanwise vortex structures under loose rough bed surface.Therefore,the study of turbulent spanwise vortex structure in open channel on non-uniform glass-sphere array bed can enrich the understanding of vortex structure and has practical significance for related engineering applications.In this paper,based on PIV flume test data,the applicability of different vortex identification methods was studied to determine the spanwise vortex identification method under the non-uniform glass-sphere array bed.The shape characteristics and evolution characteristics of the spanwise vortex structure were analyzed,and the statistical characteristics of the spanwise vortex structure were discussed.The following research results were obtained:(1)Through the analysis of the morphological characteristics of the lateral vortex structure in the instantaneous flow field sequence,it is found that the lateral vortex structure often exists with the strong directional adjoint structure,and together form the coherent structure with obvious structural characteristics.The vortex tracking method based on object attribute vector is used to track the transverse vortices frame-by-frame.The survival time of the transverse vortices under the loose rough bed surface is smaller than that on the smooth bed surface.Under the same flow conditions,the larger the rough particle size,the shorter the survival time of the transverse vortices.(2)By observing the lateral vortex displacement trajectory,it is found that the region affected by particles(y/h<0.3)the maximum relative displacement of the spanwise vortex is only 0.06 h,and the longitudinal and vertical relative displacement of the region away from the influence of particles(y/h=0.6～0.8)can reach 0.5h and 0.2h,respectively.In the region y/h=0.1～0.3,the Angle between the lateral vortex displacement trajectory and the horizontal axis is large,with an average of about 50°.In addition,the variation of vortex intensity over time can be summarized into three types:first increase and then decrease,small fluctuation and gradually decrease.(3)Under the same flow condition,the density of reverse vortex is less than that of forward vortex in the same water depth;The vortex density far away from the particle influence region is smaller than that of the particle influence region.The vertical rotation strength of the spanwise vortex is stronger in the particle region and weaker in the interparticle region,and it changes periodically along the flow direction,especially near the center of the flume.On the same vertical line,the maximum absolute strength of the reverse vortex is smaller than that of the forward vortex.When the flow and boundary conditions are the same,the vortex intensity increases with the flow.The vertical distribution of spanwise vortex shape parameters presents a "trapezoid" along the water depth,and the vortex shape in the middle area of the flume is oval,which is more flat at the water surface and trough bottom.(4)Due to the influence of rough particles,the frequency of spanwise vortex structure is higher at the bottom of the flume,and the "high frequency region" is distributed in a strip along the flow direction between particles,and in a point or vertical strip near particles.It is found that the probability density curves of the long axis and short axis scales of the vortex structure are lognormal distribution,and the probability density curves of the shape parameters of the vortex structure are normal distribution. |
