Generation of initial wavelets by instability of a coupled shear flow and their evolution to wind waves
Generation of initial wavelets by instability of a coupled shear flow and their evolution to wind waves
复制标题
耦合剪切流不稳定性产生初始小波及其演化为风波
DOI：
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发表时间：
1979
影响因子：
3.7
通讯作者：
S. Kawai
中科院分区：
文献类型：
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作者：
S. Kawai
用系统的实验和理论分析相结合的方法，研究了静止水面突然起风后风浪产生和发展过程的初始阶段出现的初始子波的产生。用直径为50μm的电阻式波计测量的初始子波的能量在恒定频率下随时间呈指数增长。频率和增长率与吸力无关，但与空气的摩擦速度u*a有关。用阴影摄影技术测量的初始子波的相速度几乎是恒定的，与u*a无关。研究了空气和水中的一个耦合剪切流模型，用不稳定机制解释了观测到的初始子波的特征。理论分析表明，对于实验中观察到的每一种剪切流型，都存在增长速率最大的波动。这些临界波的频率、增长速率和相速度与观测到的初始子波的性质基本一致。结果表明，风浪的产生是由于流动中不可避免的微小扰动通过不稳定机制的选择性放大而产生的，其初始阶段称为初始子波。文中还讨论了线性不稳定理论在风浪进一步发展过程中的局限性。根据详细的观测所确认的一些事实，可以推断，由线性机制控制的现象只持续了大约10个S，并演化成具有固有非线性特征的风浪。
The generation of initial wavelets, which appear at the initial stage of the generation and growth processes of wind waves after the abrupt start of the wind on a still water surface, has been investigated by systematic experiments together with theoretical analyses. The energy of the initial wavelets, measured by a resistance-type wave gauge of 50 μm diameter, grew exponentially with time at a constant frequency. The frequency and the growth rate were independent of the fetch but did depend on the friction velocity u*a of the air. The phase velocity of the initial wavelets, measured by a shadowgraph-photography technique, was nearly constant, independent of u*a. A coupled shear flow model in the air and water was examined, to explain the observed characteristics of the initial wavelets in terms of the instability mechanism. The theoretical analysis showed that, for each shear flow pattern observed in the experiments, there exist waves whose growth rate is maximum. The frequency, the growth rate and the phase velocity of these critical waves were virtually coincident with those properties of the observed initial wavelets. It is concluded that the generation of wind waves, whose initial stage is called the initial wavelets, is caused by the selective amplification, by the instability mechanism, of the small perturbations which inevitably occur in the flow. The limitation of the linear instability theory as applied to the process of further growth of the wind waves is also discussed. From some facts recognized through detailed observations, it is inferred that the phenomena controlled by the linear mechanism last for only about 10 s, and that these evolve to the wind waves which are characterized by inherent nonlinearity.