推移质动床模型粒径比尺计算式选择

Selection of formula calculating grain size scale for bed load model

描述泥沙起动的方法可分为以起动流速为参数和以起动拖曳力为参数两大类。由这两类方法推导得到的模型沙粒径比尺计算式是不一样的。实际应用时,如何选取粒径比尺计算式是必须回答的问题。首先根据两种粒径比尺计算式的推导过程,弄清了两者各自成立的物理条件。依照变态模型水流惯性力相似和阻力相似均应满足,且动床模型中阻力相似须由模型沙自身糙度来满足等条件,推导得到模型变率值与模型沙特性之间的关系式。最后指出,泥沙起动拖曳力公式所对应的粒径比尺计算式中所包含的λ_J(等于变率值倒数)是不能自由选取的。实际应用时,建议选用泥沙起动流速公式所对应的粒径比尺计算式。

Methods of describing the starting of sediment movement may be classified as two categories:one taking the incipient velocity as governing parameter,another competent drag force.The formulas of calculating size scale for model sand derived from these two kinds of methods,are different from each other.Thus,in model design,formula of calculating size scale must be carefully selected.In this paper,the respective physical conditions governing the validity of these formulas are identified,in the light of the formula derivation process.And,the relationship between the distortion ratio for distorted mod- el and properties of model sand is derived,according to that for distorted model,the similarity relationship for both inertia force and resistance drag must be met,and that for the movable bed model,the similarity relationship for resistance drag should be met by roughness of the model sand itself.Finally,it is pointed out that the λ_J,a reciprocal of the distortion ratio η,in the size scale formula derived from the competent drag force is not assumed arbitrarily.The formula derived from the incipient velocity is recommended by the author for practical application.