Research On Hydraulic Characteristics Of Different Rough Bed Surfaces

The uniform turbulent flow structure of the open channel is affected by the roughness of the bed surface.The current research on this aspect is almost blank,and experimental research is urgently needed.In this paper,the longitudinal section of uniform turbulence in open channels along the water flow direction under different rough bed conditions is measured by the PIV system with high temporal and spatial resolution,and twodimensional instantaneous flow field data is established.Based on the two-dimensional flow field data,the flow velocity distribution,turbulence intensity,Reynolds stress,transverse vortex and other hydraulic elements are analyzed,and the laws of water flow structure under different rough bed surfaces are studied.The research results are as follows:(1)The greater the roughness of the bed surface,the greater the roughness coefficient;under the same water flow conditions,the wall shear stress and friction velocity increase with the increase of the bed surface roughness;under the same roughened bed surface condition,Wall shear stress and friction flow velocity decrease with the increase of section flow velocity.(2)90% of the working conditions of the velocity distribution near the wall are I-2 type,and the rest are I-3 type.In the flow velocity distribution,the proportion of the water depth near the bed surface is related to the water flow conditions.The larger section flow velocity,the larger the proportion of the I zone,which has nothing to do with the roughness of the bed surface.The velocity distribution in the near water surface area is logarithmic extension under most working conditions,and a few working conditions have Type III-1,Type III-3 and Type III-4.The roughness of the bed surface has little effect on the velocity distribution near the water surface.The A value fitted by the logarithmic area flow velocity distribution formula is between 5.3?6.5,and the B value is between 0?6.(3)The longitudinal turbulence intensity of different rough bed surfaces all increase with the decrease of y/H;the vertical turbulence intensity and the absolute value of Reynolds stress first increase and then decrease with the decrease of y/H,and reach the maximum when y/H=0.1?0.2;the turbulence intensity and Reynolds stress are related to the cross-sectional velocity.The greater the cross-sectional velocity,the greater the absolute value at the same water depth.Under the same flow rate gradient condition,the turbulence strength and Reynolds stress of different rough bed surfaces have small differences,and there is no increasing or decreasing trend with the bed surface roughness.(4)The spanwise vortex density is related to the water depth and the velocity of the cross-section.It reaches the maximum at y/H=0.1?0.2,and then decreases and decreases with the increase of y/H.The spanwise vortex density decreases at a faster rate near the water surface.The mean value of the spanwise vortex density increases with the increase of the section velocity.Under the same flow rate gradient condition,the spanwise vortex density under different rough bed surfaces is basically the same amount,and there is no obvious correlation.The difference is the largest in the region of y/H<0.1.