沿岸流中混合系数的实验研究

Experimental study on the depth-averaged mixing coefficient in longshore currents

进行了在规则波作用下破波带内水平混合系数测量的物理模型实验。通过在沿岸流流场中投放墨水点源和采用CCD摄像机摄像,测量了点源扩散过程。利用水深平均二维扩散方程近似解析解得到了由实验结果计算混合系数的方法。实验结果表明:扩散系数仅依赖于当地水深,与波浪参数(周期和波高)无关。沿岸流沿水深变化产生的离散作用导致顺流方向(纵向)混合系数远大于横流方向(横向)混合系数。横流方向混合系数中由波浪产生的扩散系数占总扩散系数约40%,其余为波浪破碎引起的湍流产生的扩散系数。

A laboratory experiment of measuring horizontal mixing coefficients was performed in the surf zone on two plane beaches with slopes 1 ： 40 and 1 ： 100 for regular waves. The transport and diffusion of a point ink in longshore currents were recorded by CCD cameras over the beach. Based on an approximate analytical solution to the depth-averaged two-dimensional convection-diffusion equation, a method of calculating mixing coefficients using experiments data was developed. The result for mixing coefficients show that it only depends on the local water depth and not on the wave parameters such as periods and heights. The dispersion caused by the vertical variation of longshore currents leads to the longshore mixing coeffi- cients much greater than the cross-shore mixing coefficients. The contribution of waves to cross-shore mixing coefficients accounts for 40 %, and the rest is due to the turbulence produced by wave breaking.