Abstract Wave environment equivalent to the full-scale nearshore storm was generated in a large wave flume, and the flow within wave boundary layer was measured using an acoustic Vectrino profiler. Analysis were made on various factors including the velocity profile, phase lead, boundary layer thickness, steady streaming, turbulence as well as bottom shear stress, and comparisons were made with existing studies using oscillatory flow tunnels. Results show that the phase lead in the boundary layer is less significant compared to previous tunnel experiments, generally smaller than 20°. Free surface and vertical velocity lead to a wave-induced Reynolds stress at the top of the boundary layer, resulting in a thinner boundary layer. Both wave-induced Reynolds stress and wave shape asymmetry contribute to the steady streaming, making its direction either onshore or offshore. Relationship of friction factor versus relative roughness generally agrees with the exponential expression in the literatures for rough turbulent boundary layer, however the value of friction factor is larger. It is stressed that the friction factor would be several times higher if the free stream velocity is calculated using linear wave theory, which is questionable as the measured waves in the large wave flume were shown to be highly nonlinear. To compensate the nonlinear effects which significantly contribute to the maximum bed shear stress, a velocity skewness factor based on second-order Stokes wave theory was introduced. For predicting the intra-wave bed shear stress, a time-varying friction factor was also constructed, which was proved to have acceptable precision compared with experimental data. These formulations have simple, explicit expressions that can be used in practical engineering applications, especially for the nonlinear waves under nearshore storm conditions.

中文翻译：

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暴风雨下非线性湍流边界层的流动结构和底部摩擦

摘要 在大波浪水槽中产生相当于全尺度近岸风暴的波浪环境，利用声学Vectrino剖面仪测量波浪边界层内的流动。对各种因素进行了分析，包括速度剖面、相位超前、边界层厚度、稳定流、湍流以及底部剪应力，并与使用振荡流隧道的现有研究进行了比较。结果表明，与之前的隧道实验相比，边界层中的相位超前不太明显，通常小于 20°。自由表面和垂直速度导致边界层顶部的波浪引起的雷诺应力，导致边界层更薄。波浪引起的雷诺应力和波形不对称都有助于稳定流动，向陆上或离岸方向发展。摩擦系数与相对粗糙度的关系一般符合文献中粗糙湍流边界层的指数表达式，但摩擦系数的值更大。需要强调的是，如果使用线性波理论计算自由流速度，摩擦系数会高几倍，这是有问题的，因为大波浪水槽中的测量波被证明是高度非线性的。为了补偿对最大床层剪应力有显着贡献的非线性效应，引入了基于二阶斯托克斯波理论的速度偏度因子。为了预测波床内剪应力，还构建了一个时变摩擦系数，与实验数据相比，它被证明具有可接受的精度。这些公式具有简单、明确的表达式，可用于实际工程应用，尤其是近岸风暴条件下的非线性波浪。