PIV Experimental Study On Velocity Structure Of Non-uniform Flow In Open Channel With Smooth Bed

Velocity structure is an important factor affecting the resistance and sediment transport of open channel riverbed.There are few achievements on velocity structure of non-uniform flow at now,which need to be analyzed urgently.Based on the PIV flume test data of non-uniform flow in open channel with smooth bed,the distribution law of time-averaged velocity,turbulence characteristics and coherent structure along the vertical section was analyzed.The variation characteristics of velocity structure along the width of the flume were discussed.The trend of influence of inflow conditions on velocity structure was studied.The research results were as follows:(1)Average velocity of non-uniform flow could be divided into three characteristic regions along water depth: near-wall region,logarithmic region and near-surface region.Near-wall area was mainly logarithmic distribution,less linear distribution;middle area was logarithmic distribution;near-surface area along the channel width mainly presents four distribution forms,wake function could not be fully applied,it need to be combined with linear distribution.Vertical average velocity could effectively reflect the characteristics of vertical velocity.To constructed logarithmic distribution formula instead of frictional resistance velocity could not only avoid the shortcomings of relatively large fluctuation of Reynolds stress in non-uniform flow and small linear distribution range,but also the fitting coefficient of the formula could more sensitively reflect the velocity growth rate.There was a clear linear negative growth relationship between logarithmic coefficient and constant coefficient.(2)The statistic value of turbulence characteristics of non-uniform flow still kept strip distribution along the flow,but the fluctuation of distribution along water depth was greater than that of uniform flow.Increasing the velocity of cross-section flow or increasing the bottom slope were liable to damage the strip structure along the flow.The relative turbulence intensity of non-uniform flow no longer gathers on the same smooth curved along the relative water depth,and the vertical turbulence intensity tended to weaken along the course.The vertical relative turbulence intensity near the water surface tended to increase when the influence of the side wall was enhanced,and the Reynolds stress and turbulence energy distribution near the water surface gradually bent to the right,showing an increasing trend.Reynolds stress distributed in the shape of "S" along the water depth near the side wall.The variation trend of turbulence characteristics along the channel width was related to the position of the maximum vertical velocity,and it was obviously affected by the confinement of the side wall when the depth was about one time from the side wall.(3)The strip structure of non-uniform flow distributed in the whole depth range,and the scale range was large,mostly nested,and the fluctuating velocity inside was larger than outside.The probability of sudden events decreases with the decrease of section velocity,and increased with the increase of bottom slope.The enhancement of side wall effect would reduce the probability of Q1 and Q3 events near the water surface,and increased the probability of Q2 and Q4 events.The probability of sudden events increased first and then decreases along the slot width,while the relative Reynolds stress and turbulent kinetic energy decreased first and then increased.(4)The transverse vortices of non-uniform flow mainly occurred near the bed,and the transverse vortices of different vorticity valued vary greatly along the water depth.With the increased of cross-section velocity,the density of forward and reverse vortices tended to decrease,whereas the density of reverse vortices tended to increase.Vortex structures with vorticity greater than 0.8 were less affected by the bound edge wall.Vortex structures with vorticity less than 0.6 tended to increase near the water surface,and with the increase of the influence of the edge wall,the increase of the number of reverse vortices was more than that of the number of forward vortices.