摘要
用以描述无粘性均匀沙起动现象的 Shields 函数曲线,存有较大的局限性.无量纲起动拖曳力θ_c 是沙粒雷诺数 Re_*和重率系数 a 的函数(亦可表达为 Re_*和 u_*/ω的函数)。基于大量的实测资料导出的无粘性均匀沙起动拖曳力的计算式,其计算结果与实测值的相对误差为9%。
It can not be accepted as of general validity that Shields diagram is used to describe initiation of motion of cohesionless uniform sized grains under unidirectional flow and indicates that nondimensional crit- ical tractive force,θ_c=τ_o/(γ_s-γ)d,varies only with shear velocity Reynolds number,Re_*=u_*d/ν.In fact,the nondimensional critical trac- tive force depends on the shear velocity Reynolds number and also on specific weight coefficient,a=(γ_s-γ)/γ,of a grain in the liquid.The di- mensionless relationship can be expressed in the form,θ_c=f((gd)^(1/2)d/v,a), also,θ_c=θ_o(Re_*,u_*/ω).The latter means that the nondimensional critical tractive force is a function of the shear velocity Reynolds num- ber and the ratio of shear velocity and the terminal fall velocity of a grain in the liquid,it can be found from experimental results. The empirical formula(17)computing the critical tractive force is established and compared with experimental data.The relative error of the formula is 9% within the range of shear velocity Reynolds number from 0.034 to 2028.
