Critical Tractive Force of Very Large Gravels on Mountainous River-Bed

在本研究中，首先通过实验研究了流体作用在三种砾石模型上的阻力和升力的平均时间和脉动值的特性。其次，从理论上考虑了漏出现象的过程，并建立了任意凸出高度的孤立砾石从沉积层凸出的临界牵引力方程。本研究结果总结如下：(1)砾石顶部高度速度最适合作为流体力的代表速度。(2)升力与阻力时间平均值之比不受相对深度的影响，但随突出率的变化变化较大。(3)定量地阐明了球形和短柱（圆形截面和椭圆截面）阻力系数和升力系数的特征。(4)阻力和升力波动的概率密度可以很好地近似于高斯正态分布函数。(5)定量地阐明了流体力波动的标准差与突出率的关系。(6)在理论考虑中，建立了球形砾石在紊流作用下的滚坡行为，并利用流体力的一些假设对其进行了求解。(7)推导出的临界牵引力方程将吸出率作为参数之一，因此该方程所给出的数值的物理意义是不同的。(8)通过实验验证了方程的有效性。

In this study, at first, the characteristic of fluid force acting on three types of gravel model was investigated experimentally with respect to both the time averaged and flactuating values of the drag and lift. Secondly, the process of the escaping phenomena was considered theoretically and an equation for the critical tractive force of an isolated gravel protoruding from sediment bed with an arbitrary protrusion hight.The Summary of the Results of this Research is as Follows.(1) The velocity at the hight of the top of gravel is most pertinent as the representative velocity of the fluid force.(2) The rate of the time-averaged value of lift to that of drag is not affected by relative depth but greatly varies with the change in protrusion rate.(3) The characteristics of drag and lift coefficients of sphere and short columns (circler and elliptical cross sections) were quantitatively clarified.(4) The probability density of the fluctuation of drag and lift is well approximated by the Gauss's normal distribution function.(5) The relation between the standard deviation of the fluid force fluctuation and the protrusion rate was also clarified quantitatively.(6) In the theoretical consideration, the behavior of a spherical gravel which rolls up a slope under the effect of turbulent flow was formulated and solved by the use of some assumptions for fluid force.(7) The deduced equation for the critical tractive force includes the pick-up rate as one of the para meters, and therefore the physical meaning of the value given by the equation is distinct.(8) The Validity of the Equation was Confirmed Experimantally.