A numerical investigation of acceleration-skewed oscillatory flows
A numerical investigation of acceleration-skewed oscillatory flows
复制标题
加速度偏态振荡流的数值研究
作者：
P. Scandura;C. Faraci;E. Foti
本文对有壁面加速度偏态振荡流进行了数值模拟。这种类型的边界层的相关性与沿海流体动力学和沉积物输运有关，因为它是在浅水中的海浪底部产生的。由于加速度的偏态，在陆上半周期的床面剪应力大于在海上半周期。床面剪应力的不对称性随着加速度偏度的增加而增加，而层流状态下雷诺数的增加会导致不对称性首先减小，然后增大。在每个半周期开始后不久，在取决于流动参数的阶段，在流向方向上伸长的流体的低速和高速条纹出现在壁附近。这种流动结构在加速阶段的第一部分期间加强，而不会引起流向壁面剪应力与层流值的显著偏差。在自由流速度峰值出现之前，低速条纹分解成小的湍流结构，导致壁面剪应力大幅增加。均方根（r.m.s.）在整个流动循环的相对较宽的间隔内，波动相对于壁面剪切应力的平均值（相对强度）的比值约为0.4，该流动循环开始于在整个流体域中已经发生条纹破裂时。加速度偏度和雷诺数决定了该时间间隔开始的相位。当存在细长条纹时，流向壁面剪应力的偏斜度和平坦度系数都很大，而在破裂发生后，其值分别约为1.1和5.4。展向壁面剪应力的相对强度和平坦度的变化趋势与流向壁面剪应力的变化趋势定性相似。由于加速度的偏态，周期平均雷诺应力并不为零。因此，产生了离岸定向定常流，该定常流持续到无旋区域。
Numerical simulations of wall-bounded acceleration-skewed oscillatory flows are here presented. The relevance of this type of boundary layer arises in connection with coastal hydrodynamics and sediment transport, as it is generated at the bottom of sea waves in shallow water. Because of the acceleration skewness, the bed shear stress during the onshore half-cycle is larger than in the offshore half-cycle. The asymmetry in the bed shear stress increases with increasing acceleration skewness, while an increase of the Reynolds number from the laminar regime causes the asymmetry first to decrease and then increase. Low- and high-speed streaks of fluid elongated in the streamwise direction emerge near the wall, shortly after the beginning of each half-cycle, at a phase that depends on the flow parameters. Such flow structures strengthen during the first part of the accelerating phase, without causing a significant deviation of the streamwise wall shear stress from the laminar values. Before the occurrence of the peak of the free stream velocity, the low-speed streaks break down into small turbulent structures causing a large increase in wall shear stress. The ratio of the root-mean-square (r.m.s.) of the fluctuations to the mean value (relative intensity) of the wall shear stress is approximately 0.4 throughout a relatively wide interval of the flow cycle that begins when breaking down of the streaks has occurred in the entire fluid domain. The acceleration skewness and the Reynolds number determine the phase at which this time interval begins. Both the skewness and the flatness coefficients of the streamwise wall shear stress are large when elongated streaks are present, while values of approximately 1.1 and 5.4 respectively occur just after breaking has occurred. The trend of both the relative intensity and the flatness of the spanwise wall shear stress are qualitatively similar to those of the wall shear in the streamwise direction. As a result of the acceleration skewness, the period-averaged Reynolds stress does not vanish. Consequently, an offshore directed steady streaming is generated which persists into the irrotational region.