Research On Sediment Exposure Degree And It’s Application In Formula Of Incipient Velocity

Researches on sediment incipient motion are essential ingredient of river dynamics,which refer to the interaction between water flow and river bed. The randomness of water flow, particle shape and particle location at the sediment group make sediment incipient motion extremely complex and random.Based on the theory of 2-D relative exposure degree that is set up by Han Qiwei,proceeding from longitudinally horizontal distance of sediment particles, using methods of theoretical analysis and experimental investigations, this paper makes a detail study on probability density distribution of 2-D exposure degree and exposure angle of uniform non-cohesive particle, applies their into the formula of starting velocity, and discussions their influence on drag force coefficient and lift force coefficient. In addition, the concepts of them is extended to 3-D generalized model of sediment particles. The mainly works are:(1)The 2-D longitudinally horizontal distance, exposure angle without water scour were measured and through a self-designed test device and their probability density distributions were analyzed,from which the probability density distribution of relative exposure degree was derived.(2)A mechanics equilibrium equation for rolling start of sediment particles is established,from which a formula of instantaneous incipient velocity that contains 2-D relative exposure degree was derived, then it was converted into depth averaged velocity. The value calculated was compared with the measured data, the both accord with each other.(3)In addition, numerical simulation method was applied to analyze the flow around a single particle and multi-particles. The result shows that both exposure degree and Reynolds number have an influence on drag force coefficient and lift force coefficient.(4)Three representative locations of bedload sediment particles are put forward to describe the actual river bed. And then a random test of particles stack was performed to explore the probability of the three locations. Based this, the 3-D relative exposure degree was preliminary researched, and it was introduced into the formula of incipient velocity. Finally,the value calculated was compared with the measured data. It shows that the value calculated is a little more than the measured data.