On The Spatial Structure Of Low-speed Streaks And Particle Motion In The Near Wall Region Of Turbulent Open Channel Flow

For dilute solid-liquid two-phase flow, the mechanism of particle motion driven by fluid is the main subject, especially the particle motion in the near wall region. The bursting phenomenon and coherent structure of turbulence play an important role on the motion of particles in the near wall region. Therefore, studies on the interaction between particles and turbulence are of essential significance for fluid mechanics and river dynamics. In this paper, the spatial structure of the low-speed streaks and particle motion in the near wall region of turbulent open channel flow were observed on the side and bottom of a water flume, respectively, using flow visualization and image processing techniques.A measurement system was developed based on flow visualization and image processing techniques, which can be used to investigate the spatial structure of low-speed streaks in the near wall region of turbulent open channel flow. A large number of data can be collected using this system because of the real-time sampling on the computer. It is a reliable instrument for the measurement of the coherent structure in turbulence. An improved 2-D PTV was developed based on previous research experiences, which can be used to measure the trajectories of particles motion. For the experimental study on the particle motion in the dilute solid-liquid two-phase flow, especially for the motion characteristics and dynamic characteristics of coarse particles in water, this kind of system was proved to have great advantages.A series of experiments in a wider range of Reynolds number were conducted for the investigation on the spatial and temporal distribution of the low-speed streaks in the near wall region of turbulent open channel flow. For the first time, the relationships between the spacing, the width, the length scale in the streamwise direction and the duration time of low-speed streaks and the Reynolds number based on shear velocity were revealed. The present results indicate that the quantitative description of the non-dimensional mean lateral streak spacing + differs from the results previously published. The spacing + is not a constant value of 100. It becomes larger as the Reynolds number in terms of shear velocity increase. In the smooth regime the spacingincreases with the quadratic power of Re., but has a linear relationship with Re, inthe transitional regime. The dividing point is located at about Re, = 900 . The probability density distribution of + conforms to the lognormal behavior. Thevariation of the width, the length scale in the streamwise direction and the persisting time of low-speed streaks with Re, have similar relations to the spacing.The distribution of streak width along the lateral direction shows that the width attains its maximum value at the central line and becomes narrower toward the side wall, and its distribution is symmetrical about the central line of the flume. The ratio of the streak spacing to the width is about 2.0, which implies that the high and low speed streaks takes up almost the same space in the lateral direction. A special experiment was performed to investigate the formation process of streaks. A new vortex model was presented to interpret some experimental results in present work based on the mechanism of streaks production.A series of experiments on the motion characteristics of light polystyrene particles and heavy glass particles were carried out in open channel flow. The test results show that it is essential to divide the particle motion into three typical types that is rising, falling and translating motion, because the motion characteristics and dynamics characteristics are apparently different when the particles moving in different types, especially in the near bed region of the open channel flow. The mean velocity profile ofparticles in streamwise direction follows the log-law, namely U p = Kp lny+ + Bpwith KP = 0.29 ~ 0.43 . The value of Bp is dispersed. The particle velocity instreamwise direction as the particle rise is larger...