Research On Incipient Motion Velocity Based On Relative Exposure Degree Ofuniform Sediment

Sediment starting problem is one of the basic problems of sediment transport mechanics, The research on incipient motion velocity has a great significance on both theoretical analysis and practical application.Relative exposure degree reflects the relative position of sediment particles on the bed, the size of the values, decided the bed surface sediment particle by force and the size of the starting torque, finally determines whether sediment starting. Based on the concept of relative exposure degree that established by Han Qiwei, the exposure angle and absolute exposure degree of the uniform glass ball and cobble were measured through a self-designed test device, and the relative exposure degree and its distribution rules were calculated and analyzed. adopt the method of theoretical derivation, the exposure and drag coefficient, lift force coefficient and the relation between the incipient motion velocity was studied, the main research results are as follows:(1)The results show that the relative exposure degree of glass ball ranges from 0 to 0.5, the exposure angle meets for normal distribution and the relative exposure degree meets for skewed normal distribution. Based on the measured data of the exposure angle, the coefficient of the normal distribution formula was determined, and the probability density function of the relative exposure degree was acquired. The relative exposure degree of cobble also meets for skewed normal distribution, but it doesn’t coincide with that of glass ball, which shows that there is a significant influence on the probability density function of relative exposure degree for the complex shape, size and core of cobbles.(2)the probability of drag force coefficient value ranging from 1 to 1.22 is greater than 92%, with a mean value of 1.11; The average ratio of drag force coefficient and lift force coefficient changes with the sand Reynolds number, but it will tend to be constant 1.28 when the particle Reynolds number is greater than 25000. Moreover, a formula of incipient velocity of non-cohesive sediment considering relative exposure degree is derived by applying the drag force coefficient and lift force coefficient in the study of the incipient velocity formula, and it can be found that the formula in this paper is in good agreement with the data measured in natural rivers than the other formulas.(3)Based on the study of relative exposure degree, the average value of relative exposure degree of sediment particles on slopes was received by the method of theoretical derivation. Then the formula for incipient motion velocity of sediment particles under different incipient probability was deduced, using a rolling model. The unified formula for incipient velocity was established to fit for non-cohesive uniform sediment on different slopes and under different incipient probability by introducing a correction factor. It can be found from the comparison with the measured data and the presented formulas of sediment incipient motion velocity on slopes that the formula deduced in this paper not only meets well with the measured data, but also has a higher calculation accuracy than the presented formulas.(4)summary of concave bank incipient motion of the two major problems, namely the force analysis and the discussion of incipient motion direction. On this basis, build a new mechanical model, combining with the results of exposure, based on concave bank even starting at the bottom velocity formula of sediment, and the generality and rationality are expounded and verified, the results showed that the formula in this paper, the structure is reasonable, the calculation accuracy is higher.