沿岸流非线性不稳定特征数值研究

Numerical study of nonlinear shear instability of longshore currents

当不稳定剪切波的波幅达到有限幅值之后,需要用非线性不稳定模型来分析沿岸流不稳定运动特征。本文通过建立并验证沿岸流非线性不稳定数学模型,在讨论该模型参数对沿岸流不稳定影响的基础上,率定实验中的模型参数并利用该数学模型模拟了实验中沿岸流不稳定的发展过程及其特征,主要结论有:(1)底摩擦系数越小,侧混系数越小,不稳定发生的时间越早,不稳定的波动幅值越大;(2)沿岸流非线性不稳定演化可能出现的5个阶段(线性阶段、倍周期阶段、大周期阶段、波群阶段以及不规则阶段),实验中出现的沿岸流通常处于线性阶段或非线性倍周期阶段,倍周期阶段的涡旋是由线性阶段的涡旋发生追赶、配对直至合并产生的。涡旋配对以碰撞的形式发生,大部分能量转移至追踪波,然后以较慢的涡旋速度沿岸传播。

It is necessary to analyze the characteristics of longshore current instability by using the nonlinear instability model when wave height reaches a finite amplitude.Building and verifying the nonlinear instability mathematical model of longshore current,this paper discusses the influences of model parameters and obtains the value of model parameters corresponding to the experimental result.The development process and its characteristics of longshore current instability in experiment are simulated and illustrated by the mathematical model.Key innovations include：（1）the smaller the bottom friction coefficient and lateral mixing coefficient,the sooner the instability occurrence,the bigger the fluctuation amplitude is;（2）the evolution processes of longshore current include five stages（linear,double period,larger period,wave group and irregular stages）.The longshore current instability in the experiment occurs in stages of linear or double period.The vortex in double period stage forms by the vortexes in the linear stage which catch,match and merge one another.The match of vortex occurs in collision,the major energy transfers to trace wave and spreads along the coast at a slower rate of vortex.