Investigation On Bed Shear Stress In Smooth Open Channels

Based on the experiments date measured with a Preston tube,the bed shear stresses in open channel in different hydraulic conditions are compared for non-uniform and uniform flow.The formulas of center-line bed shear stress,mean bed shear stress and bed shear stress distribution are discussed in this two different fluid types.There is difference on the characteristics of shear stress in non-uniform and uniform flows.At first,the center-line bed shear stress is discussed.For uniform flows,the center-line bed shear stress increases with the increasing of discharge at the same slope and increases with the increasing of slope at the same discharge.The relative depth and relative center-line bed shear stress are almost same as that of the non-uniform flows.The relationship between them in flat slop and steep slope can be expressed by a formula presented in this dissertation. The formula of the relationship between Froude number and center-line bed shear stress in M₁ non-uniform and S₁ non-uniform flows are also given in this dissertation.Then, the formulas of center-line bed shear stress,mean bed shear stress and bed shear stress distribution are discussed in uniform and non-uniform flows.It indicates that the uniform flow equation and other formulas based on it is not applicable to non-uniform flows in open channels.The cross distribution of bed shear stress is also analyzed in this dissertation.The bed shear stress is symmetrical on the two sides of centre-line.The bed shear stress in the center is smaller than the bed shear stress near the sides,this indicates that the sides have some influence on the bed shear stress.Finally, different methods are discussed for the calculation of friction velocity.The results from the calculation are nearly same for uniform flows but different for non-uniform flows.