基于ADV的底床切应力测量精度提升实验探究

Experimental study on the measurement accuracy improvement of bed shear stress by ADV

泥沙冲刷实验常利用三维流速仪的测速数据剥离近底紊动动能参数,推算底床切应力。然而不同研究中流速测点高度不同,推算的底床切应力往往差异较大,影响了该方法的推广应用。基于管式冲刷水槽实验,分析了定床和动床下近底水流结构,探讨了提高底床切应力推算精度的方法。研究结果表明,管流距底0.3~1.3 cm处流速符合对数分布,在该区间内,紊动动能垂线剖面向上先增大后减小,动床的梯度更为显著,造成距底不同高度的紊动动能相对偏差达50%,这是单点法推断底床切应力误差的主要原因。在此基础上,提出使用紊动动能垂线平均值反算底床切应力,其计算结果优于传统的流速剖面法以及单点法。

In sediment scouring experiments,the bed shear stress is often deduced by extracting the turbulent parameters(e.g.,turbulent kinetic energy,TKE)from the 3D velocity measurements by Acoustic Doppler Velocimeter(ADV).However,due to the differences of the velocity measuring elevation,the deduced bed shear stress is quite different in different studies,which restricts the further application of this method.Based on the experiment in a tubular flume,this study focuses on the flow structure near bottom under fixed bed and movable bed,and the method to improve the accuracy on deducing bed shear stress by TKE method.The results show that there is a logarithmic velocity distribution at 0.3~1.3 cm from the bottom of the pipe flow.In this area,the TKE vertical profile increases first and then decreases upward,and the gradient under the movable bed is more significant,which resulting in the TKE relative deviation of 50%at different height from the bottom.Therefore,that is the principle cause of the error of the bed shear stress inferred by the single point method.Subsequently,the vertical-averaged TKE is used to deduce the bed shear stress.The results are better than the traditional velocity profile method(LP)and single-point method,which can be further verified,extended and applied in flumes of different scales.