急弯河道壁面切应力及计算方法研究

The accurate calculation of boundary shear stress is very important to the deep understanding of sediment transportation and river evolution.The current research is limited to straight and mildly curved channels,while for a sharply-curved channel,the flow is complex accompanied by transverse circulation and large transverse slope which was affected by the dual effect of gravity and centrifugal force.As the boundary shear stress was affected by many factors,the applicability of differentcalculation methods requires further study.The distribution of three-dimensional flow velocity and dynamic-staticpressure difference were monitored by ADV as well as Preston tube in a 180°;sharply-curved flume under the subcritical flow condition.The typical features of transverse flow circulation,redistribution of longitudinal flow velocity and turbulence kinetic energy were analyzed.Based on the analyses,four empirical formulae andk-εmodel were selected to calculate the boundary shear stress on the control section of the flume.The computational results of turbulence kinetic energy method and Preston tube method agree well with thek-εnumerical simulation not only in distribution pattern but also in magnitude.Therefore,the above three methods are feasible for the calculation of boundary shear stress in a sharply-curved channel.The boundary shear stress in bed and bank slope of the whole flume was calculated by numerical simulation.The results show that the value of boundary shear stress is small and evenly distributed in thestraight section following the flow entrance,then gradually increases and exhibits non-uniformity in the bending section. Finally it reaches the maximum near the bank slope in the straight section in front of the outlet.On the transverse section,the boundary shear stress is well distributed along the bed while strongly fluctuated near the toe of slope in which the flow is complex with large circulation.Under the effect of the bend,the maximum boundary shear stress in the transverse section gradually shifted from convex bank to concave bank,in order to keep up with the mainstream.The maximal boundary shear stress of the flume is located in the convex bank of 110°; cross-section of the bend and the concave bank of the cross-section 0.5 m downstream off the bend outlet.By changing the rate of flow but keeping the downstream water depth constent,the overall distribution of the boundary shear stress is similar and shows the characteristic of strong water flow along straight line while weak water flow along curve line.These researches provided support for flow shear transport mechanism,forecast of river evolution and safety management in the sharply-curved channel.