We investigate experimentally the effects of micro-grooves on the development of a zero pressure gradient turbulent boundary layer at two different values of the friction Reynolds number. We consider both the well-known streamwise aligned riblets as well as wavy riblets, characterized by a sinusoidal pattern in the mean flow direction. Previous investigations by the authors showed that sinusoidal riblets yield larger values of drag reduction with respect to the streamwise aligned ones. We perform new particle image velocimetry experiments on wall-parallel planes to get insights into the effect of the sinusoidal shape on the near-wall organisation of the boundary layer and the structures responsible for the friction drag reduction and the turbulence generation. Conditional averages, aimed at identifying the topology of the low-speed streaks in the turbulent boundary layer, reveal that the flow is highly susceptible to wall manipulation. This is particularly evident in the cases that are associated with greater values of drag reduction. The results suggest a fragmentation and/or weakening of the streaks in the sinusoidal cases, that is triggered by the larger values of the wall-normal vorticity found at the streaks’ edges. The results are also confirmed by applying the variable interval spatial averaging events eduction technique. The turbulent kinetic energy budget also shows that the sinusoidal geometry significantly attenuates the turbulence production, hence supporting the idea of the manipulation of the turbulence regeneration cycle.

中文翻译：

---

使用正弦肋条操纵湍流边界层

我们通过实验研究了微槽在两个不同摩擦雷诺数值下对零压力梯度湍流边界层发展的影响。我们考虑众所周知的流向排列的肋条以及波状肋条，其特征在于平均流动方向上的正弦模式。作者之前的研究表明，正弦形肋条相对于流向排列的肋条能产生更大的减阻值。我们在壁平行平面上进行了新的粒子图像测速实验，以深入了解正弦曲线形状对边界层近壁组织以及负责减少摩擦阻力和湍流产生的结构的影响。条件平均值旨在识别湍流边界层中低速条纹的拓扑结构，表明流动非常容易受到壁面操纵的影响。这在与较大减阻值相关的情况下尤其明显。结果表明，在正弦曲线情况下，条纹会破碎和/或减弱，这是由在条纹边缘发现的壁法向涡度的较大值触发的。应用可变间隔空间平均事件导出技术也证实了该结果。湍流动能预算还表明，正弦几何形状显着减弱了湍流的产生，因此支持了操纵湍流再生循环的想法。