Roughness Coefficient And Turbulent Characteristics For Non-uniform Flow In Open Channels

Based on the classification of types of flow for open channel flows, the characteristics of roughness coefficient and turbulent flow in open channels were studied both for uniform flow and non-uniform flow, and the characters of roughness coefficients also be discussed and interpreted with turbulent characteristics in this dissertation. Due to studied non-uniform flow on roughness coefficient and turbulent characteristics, this dissertation extends investigation of open channel flow.This study indicates that Manning's roughness coefficients do not be constant for an open channel. In the case of uniform flow, the roughness coefficients decrease with increasing of either water depth or mean velocity of cross section; in the case of non-uniform flow, the roughness coefficient varies with relative depth and relative energy slope. The relationship of them can be expressed by a formula presented in this dissertation. Also, the method named two-steps method could be applied to modify the value of roughness coefficients in hydraulic calculation for MI non-uniform flow.For smooth open channels, non-uniform flow has similar regions and turbulence intensity distribution with uniform flow, but the constants A and B of the log law equation are different in non-uniform flow and in uniform flow. The Karman's constant in non-uniform flow is smaller than that in uniform flow, which indicates that the classic log law equation obtained from uniform flow cannot be universally applied to non-uniform flow. Using mean velocity of velocity profile rather than friction velocity as a parameter, a new log law equation could describe the velocity profile well both for uniform flow and non-uniform flow. The study for shear stress indicates that the uniform flow equation is not suitable to non-uniform flow in open channels.For rough bed open channels, the velocity profile could be described by a log formula in uniform flow, but it cannot be described by single log formula in non-uniform flow. The flow could be divided into two regions and the velocity profile could be described by different log formulas respectively. There are evident differences between smooth open channel flow and rough bed open channel flow on velocity profiles. The velocity distribution in smooth open channel flow is much uniform than in rough bed open channel flow. Therefore, one needs carefully to apply velocity profile into practice because this study shows that open channel flow has complex characters.The study for relationship between roughness coefficient and turbulent characteristics indicates that roughness coefficient does not have evident physical senses, it only is an adjustable coefficient in Manning's formula.