含沙浑水流对固定砂床的剪切力试验研究

Experimental Study on Shear Force of Fixed Sand Bed Under Turbid Flows

海底侵淤变化计算的基础在于泥沙起动运移的准确判断,其中水流作用于底床上的剪切力是判断泥沙起动运移的关键参数。在有粘土或细粉砂组分的海底形成的高含沙浑水,运动时对底床的剪切作用应该与清水不同。本文利用自行设计的底床剪切力测量仪,对清水(牛顿流体)和含沙量为200 g/L(牛顿流体)和420 g/L(宾汉流体)的浑水,在不同流速下对固定砂床产生的剪切力进行了测量。结果表明,宾汉流体存在一个临界速度,使其对底床的剪切应力增长趋势发生变化,大于临界速度时,对底床的剪切应力高于同速度的牛顿流体且曲线变化率较大,反之则低于同速度的牛顿流体且曲线变化率较小。从剪切应力的直接测量角度,证实了宾汉流体较牛顿流体的减阻现象更明显。本文为浑水流对底床剪切力的直接测量提供了参考方法。

The basis of calculation of seafloor erosion and sedimentation changes is to accurately determine the sediment incipient motion and movement.The shearing force of fluid on the bottom bed is the key parameter to determine the sediment incipient motion and movement.In the seabed with clay or fine-silty sand components,the suspension of high sediment concentration formed on the near bed,which produce the shearing effect on the bed during movement should be different from that of fresh water.In this paper,a self-designed bed shear force measuring instrument was used to measure the shear force generated by a fixed sand bed with a sediment concentration of 0 g/L(Newtonian fluid),200 g/L(Newtonian fluid)and 420 g/L(Bingham fluid)turbid suspension at different flow velocity.The results show that there is a critical velocity of the Bingham fluid,which changes the shear stress growth tendency of the bed.When the flow velocity is greater than the critical velocity,the shear stress on the bed is higher than that of the Newtonian fluid of the same velocity and the curve change rate is larger,otherwise it is lower than the Newtonian fluid of the same velocity and the curve change rate is smaller.From the direct measurement of shear stress,it is proved that the drag reduction of bingham fluid is more obvious than that of Newtonian fluid.This paper provides a reference method for the direct measurement of bed shear force under turbid flow.