Turbulent Structure In Open-Channel Non-Uniform Flows

Some researchers have focused on turbulence structure in long term.However,to date,people do not know enough about the turbulence structure of non-uniform flow in open channel due to the comprehensive factors including water surface gradient,bed slope,river width,tailrace height and so on.In addition,it is unlikely that the research results of uniform flow in open channel could be directly applicable to non-uniform flow.Consequently,there is a particular need to deepen the understanding of the non-uniform flow turbulence structure because the majority of natural rivers or channels are non-uniform and the point is that these achievements can play an important role in engineering practice.Therefore,carrying out the study of turbulence structure is of great not only practical significance but also theoretical value on enriching the basic theory of turbulence structure of non-uniform flow in open channel.Here,to make acknowledge about non-uniform flow turbulence structure clear,we explore the hydraulic characteristic of non-uniform flow in open channel and the characteristics of turbulent coherent structures combining theoretical analysis and high precision PIV flume test.Major findings are as follows:(1)Non-uniform flow turbulent statistical characteristics: Combined with theoretical derivation and flume test analysis,the results show that: Turbulence-intensity and Reynolds stress profiles decrease in accelerating and increase in decelerating flows(compared with the ones in uniform flow).In the depth direction the distribution of Reynolds stress in the accelerated flow presents a concave curve distribution,on the contrary,in the decelerated flow it displays a convex curve distribution.The Carmen constant has a nonlinear relationship with the change rate of non-uniform flow water depth,and the integral constant is linear with the change rate of nonuniform flow water depth.(2)Scale characteristics of non-uniform flow: Absorbed the advantages of velocity gradient tensor and mode matching method,combined with the regional growth algorithm,an improved method was proposed for identifying and extracting vortex structures.Based on power spectrum and uniform momentum zones and the improved vortex identification method,the distribution of the scales about very large scale motions and large scale motions and vortex structure are analyzed,the results indicate: the scales of very large scale motions and large scale motions and vortex structure go down with increasing non-uniform flow parameter.The vortex structure is elliptical shape,and the ratio between the major axis and the minor axis of vortex is in the range of 1.4~1.7.The ratio and density of the vortex structure goes up with the increase of non-uniform flow parameter.(3)Motion characteristics of non-uniform flow: Based on cross-correlation algorithm to form the large-scale structure of the tracking technology and method of the proper orthogonal decomposition,the motion of very large scale structure,large scale structure and vortex structure are analyzed,the results indicate: The scale of mainly contain energy structures of very large scale motions go down with increasing non-uniform flow parameter.The large scale motions appear in a Single form cluster or mixed form cluster,The length of cluster go down with increasing non-uniform flow parameter.The interaction of high speed and low speed motions forms vortex structure.An overall model of non-uniform flow turbulence structure is proposed.The main driving force of the turbulent structure is the downwelling motions in water surface.The very large scale motions,large scale motions are triggered by the downwelling motions.The vortex structure,hairpin vortex package and boiling phenomena are triggered by high speed and low speed motions.