Boundary Layer Turbulence over Surface Waves in a Strongly Forced Condition: LES and Observation
Boundary Layer Turbulence over Surface Waves in a Strongly Forced Condition: LES and Observation
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DOI：
10.1175/jpo-d-19-0070.1
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发表时间：
2019-08-01
影响因子：
3.5
通讯作者：
Sullivan, Peter P.
中科院分区：
文献类型：
--
作者：
Husain, Nyla T.;Hara, Tetsu;Sullivan, Peter P.
海况对海气动量通量（或风应力）的影响是风浪相互作用的一个组成部分，尤其是在强风条件下，人们对此知之甚少。海洋上的风应力和平均风廓线受到表面波边界层湍流特征的影响，而表面波边界层湍流受到瞬态气流分离事件的强烈调制；然而，控制其发生和强度的特征尚不清楚。采用正弦波列风的大涡模拟 (LES) 来重现强强迫条件下波浪上相位平均气流的实验室观察结果。 LES 和观测都使用波浪跟随坐标系，将风速分解为平均、波浪相干和湍流脉动分量。平均风廓线以及波浪诱发和湍流应力分量的结构的 LES 结果与观测结果相当吻合。 LES 和观测均显示波峰高度处的湍流应力和平均风切变增强，这表明间歇性气流分离事件的影响。差异尤其存在于波峰附近，表明气流分离和遮挡受到实验室波的非线性和不稳定性的影响。我们的结果还表明，气流分离的强度对波峰附近的波陡度和表面粗糙度参数化最敏感。这些结果阐明了有限振幅波的特性如何控制气流动力学，这可能会对平均风廓线、等效表面粗糙度和阻力系数产生重大影响。
The impact of sea state on air-sea momentum flux (or wind stress) is a poorly understood component of wind-wave interactions, particularly in high wind conditions. The wind stress and mean wind profile over the ocean are influenced by the characteristics of boundary layer turbulence over surface waves, which are strongly modulated by transient airflow separation events; however, the features controlling their occurrence and intensity are not well known. A large-eddy simulation (LES) for wind over a sinusoidal wave train is employed to reproduce laboratory observations of phase-averaged airflow over waves in strongly forced conditions. The LES and observation both use a wave-following coordinate system with a decomposition of wind velocity into mean, wave-coherent, and turbulent fluctuation components. The LES results of the mean wind profile and structure of wave-induced and turbulent stress components agree reasonably well with observations. Both LES and observation show enhanced turbulent stress and mean wind shear at the height of the wave crest, signifying the impact of intermittent airflow separation events. Disparities exist particularly near the crest, suggesting that airflow separation and sheltering are affected by the nonlinearity and unsteadiness of laboratory waves. Our results also suggest that the intensity of airflow separation is most sensitive to wave steepness and the surface roughness parameterization near the crest. These results clarify how the characteristics of finite-amplitude waves can control the airflow dynamics, which may substantially influence the mean wind profile, equivalent surface roughness, and drag coefficient.