Experimental Study On Flow Structure In Open Channel With Dispersed Roughness

The flow structure has a great influence on the accuracy of the river model test.The roughness of the bed surface is a universal method to ensure the similar resistance in the river model test.It is one of the most commonly used bed roughness methods.Therefore,it is of great theoretical and practical significance to carry out the research on the related problems of the flow movement in the rough bed surface..Based on the test results of PIV measurement,the velocity structure,turbulence characteristics and roughness resistance of the smooth bed surface and 4 kinds of rough beds are studied.The following results have been obtained:?1?The hydrodynamic characteristics of open channel with spacing and roughness can be divided into grain zone and intergranular zone,and the flow parameters of the two regions are different.Both the friction velocity and the theoretical bed surface location have obvious partitioning characteristics: the particle area is generally smaller,and the grain area is generally larger.The theoretical bed surface of the particle area is changed within0.15d⁰.35 d range below the top of the particle,and the theoretical bed surface of the grain zone is located in the smooth bed surface of the particle bottom.?2?Based on the experimental data of PIV,the distribution law of average velocity under different rough conditions was studied.The results show that the velocity flow field presents obvious strip distribution,and the velocity distribution has a great relationship with particle spacing,particle diameter and arrangement mode.The smaller the distance between particles,the smaller the flow velocity.The particle diameter increases and the flow velocity decreases.The flow velocity is smaller than that of the rectangular and roughened bed surface,so the shape of the plum flower has a great influence on the flow velocity.In the upstream of the granule,the distribution of the vertical distribution of the vertical velocity is more obvious,and the middle and lower reaches of the region show the trend of stabilizing and increasing.?3?The longitudinal and transverse pulsating flow velocity u and v of smooth bed face and rough bed face are clear high,low speed band structure.With the increase of water depth,the average probability of all kinds of turbulent events increases first and then decreases.The effect of roughing particles on turbulent event probability is mainly concentrated in the area below the top of the particle.Bed surface and roughness,the probability density distribution of vertical fluctuating velocity change is small,and the probability density distribution of longitudinal fluctuating velocity change is larger,distribution curve becomes more steep,the velocity fluctuation range is more concentrated.?4?The rule changes of turbulence intensity,Reynolds stress and turbulent kinetic energy under different flow conditions and rough conditions are described.The results show that the longitudinal turbulent intensity shows a zonal distribution,and the closer to the bottom of the trough,the greater the longitudinal turbulence intensity is,and the maximum is reached within 1d above the top of the particles,and then gradually decreases.The larger the particle diameter,the smaller the longitudinal turbulence intensity.The Reynolds stress shows the trend of increasing first and then decreasing.The larger the particle diameter,the smaller the Reynolds stress.The variation law of turbulent kinetic energy is basically consistent with the longitudinal turbulence intensity,and the maximum value appears within 1d below the top of the particle.The larger the diameter of the bed,the smaller the turbulent kinetic energy.The turbulence intensity,Reynolds stress and turbulent kinetic energy of the rectangle under the same flow condition are larger than that of the plum-shaped bed.?5?Relative equivalent roughness k_s0 is related to particle size,spacing,arrangement mode and flow conditions,etc.When the flow rate is the same,the larger the particle diameter or the smaller the spacing,the greater the average value of k_s0.The higher the flow rate,the smaller the average value of k_s0.The mean value of k_s0 in the plum arrangement is greater than that of the rectangle.?6?The resistance coefficient f in the particle area and the intergranular area is all subject to the power function formula.When h/d is small and the bed surface is close to the large scale roughness,the incremental change of f with k_s0 is obvious.With the increase of h/d,the incremental change of f with k_s0 gradually decreases.In the same case with k_s0,the smaller h/d,the more obvious the increase of f;In the case of h/d and k_s0,the flow resistance coefficient of the particle area is greater than that of the intergranular zone.