床面均匀沙拖曳力及上举力系数的确定

Determination of drag and lift coefficients of uniform sediment on the bed surface

床面泥沙颗粒由于其相对暴露度及外形的不同,即使在同样水流条件下,颗粒附近的水流结构也不相同,进而影响到颗粒的受力不同,主要表现在颗粒拖曳力系数和上举力系数的不同。在白玉川提出的双向相对暴露度概念及拖曳力系数、上举力系数与纵向相对暴露度之间关系的基础上,分析发现,拖曳力系数介于1.00~1.22区间的概率大于92%,平均值为1.11;拖曳力系数与上举力系数比值平均值随沙粒雷诺数变化而变化,但在颗粒雷诺数大于25 000时,拖曳力系数与上举力系数比值平均值趋于常数1.28。将拖曳力系数和上举力系数引用到泥沙起动底流速公式中,得到了散粒体泥沙起动流速公式。实测资料验证表明,起动流速公式与实则资料吻合良好,间接地验证了提出的推移力系数值及上举力系数值是正确的。

Due to the difference of relative exposure degree and shape of sediment particles on the river bed, different flow structures are induced in the vicinity of the particles, even in the same flow conditions. As a result, variation of the acting forces on the particles by water flow will take place, mainly reflecting in the variations of drag force coefficients and lift force coefficients. Based on the conception of two-way relative exposure degree and the relationship between the drag force coefficient,lift force coefficient and the longitudinal relative exposure degree proposed by Bai Yu-chuan, it can be found that the probability of drag force coefficient value ranging from 1.00 to 1.22 is greater than 92% , with a mean value of 1. 11 ; The average ratio （ CD/CL ） of drag force coefficient and lift force coefficient change with the sand Reynolds number Re. , but it will tend to be constant 1.28 when the particle Reynolds number is greater than 25 000. 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, and it can be found that the formula is in agreement with the data measured in natural rivers.