浅水非线性波作用下沙纹床面底层流动特性试验研究

通过水槽试验研究浅水非线性波作用下沙纹床面底层流动特性,利用CCD图像技术观测分析非对称沙纹的形成和演化规律。利用声学多普勒测速仪(ADV)测量非对称沙纹底床上的流场,得到了不同波高、周期、水深条件下的沙纹峰顶和谷底断面的瞬时速度。试验结果分析表明,浅水非线性波作用下床面上形成非对称沙纹,其近底流速具有较强紊动特性,随着距床面距离的增大紊动强度逐渐减弱。在水流方向改变时,沙纹背部具有明显漩涡运动。沙纹背后形成的漩涡能起到维持沙纹的作用。浅水非线性波作用下,沙纹的形成原因主要是床面泥沙颗粒在非对称流动和床面近壁粘性底层中漩涡结构动力作用下,作受迫摆动、推移所致。

Experimental study on characteristics of flow field over asymmetric rippled bed under nonlinear shallow water wave

Vortex ripple is widely formed in the coastal region,and the dynamics of vortex is responsible for the sediment transport.The evolution of the initially flat sandy bed is studied in a laboratory wave tank under cnoidal wave conditions.The Acoustic Doppler Velocimeter(ADV) was used to acquire detailed velocity measurements at different positions.The nonlinear wave flow induces asymmetric rippled bed.CCD images were analyzed and the data on topography changes were obtained.The flow structure in the asymmetric rippled bed under the action of the cnoidal wave,and the complete process of the vortex formation,evolvement and disappearance were observed.The formation and the evolution of ripple were analyzed.A separation of the near-bed velocity field of the cnoidal waves over the asymmetric ripples indicates that the near-bed flow field is dominated by the turbulent motion,and the level of turbulence decreases with the increase of the distance away from the bed.The turbulence is anisotropic.The experiment result shows that the mechanism by which sand ripples maintain their shape is in the form of the dynamic equilibrium.Sediment may be lifted from the ripple face,picked up in suspension by the local velocity,and deposited over the crest and in the lee of the ripples.