动床水流卡门常数变化规律的研究

Karman Coefficient of Flow with Movable Bed

本文初步分析了动床水流速分布与卡门常数x值的关系,指出在主流区仍遵循对数定律.但x值常常偏离0.4.动床水流流速分布是含沙量及其分布、推移质运动强度、沙波发育、消长、床面糙度、紊流涡体尺寸、强度及其分布等诸因素综合作用的结果.在相同的U值下.可以有不同形状的流速分布.x值可以远大于0.4,也可以远小于0.4.x=0.4只代表一种平均分布. 卡门常数x值实质上是反映流速分布变化的一个参数.流速分布越均匀,du/dy就越小,x值大于0.4越多.流速分布越不均匀,du/dy就越大,x值小于0.4越多.平均x值将因采用的理论参考平面不同而有不同的变化.不能简单地取K=D_65和x=0.4.初步分布认为,x值与水流相对强度参数F_rU/W_50有关.根据不同的情况可用式(12)～(16)或式(17)～(18)来描述K值和x值的变化规律.

The relation of the velocity distribution of flow with movable bed to Karman coefficient is analysed. It is found that the velocity distribution in the main flow range still fits the logarithmic law, but, the value usually deviates from o. 4. The velocity distribution of the flow with movable bed is the result of synthetic effects of the sediment-charge (concentration) and its distribution, the intensity of bed load, bed roughness, growing or fading of sand waves, the intensity and distribution of the turbulence and large scale vortices, and etc. Even U* the same, the velocity distribution may presents different profile. The value may be larger than 0.4 and may be less than 0.4. The value 0.4 presents merely for an average profile. In fact, Karman Coefficient x is a parameter showing the variation of the velocity distribution. The more uniform the velocity distribution is,the less du/dy is and the value x is larger than 0.4. Conversely, themore nonuniform the velocity dislribution is, the larger du/dy is and the value x is less than 0.4. With different theoretical reference datum, the average value x will take different value. Preliminarily, it is found that x varies with the parameter of the relative strength of flow,FrU/w50, It is suggested thet equ. (14)-(16) or equ. (17)-(18) presented inthe paper may be used to describe the variations of the value x.The form of the theoretical resistance epu.( 9 ) presented by G.H.Keu-legan is fundamentally correct. However, it does no mean to let Ks to be D65 and to be 0.4 simply, it is necessary to correct Ks and x according to different conditions. The values Ks and are given by the equ. (12)-(16) or the equ. (17)-(18) on the basis of the various conditions.The method suggested in the paper is applicable to the alluvial channels only.