This work examines the influence of body porosity on the wake past nominally two-dimensional rectangular plates of fixed width $D$ in the moderate range of Reynolds numbers $Re = UD/\nu$ (with $U$ the incoming velocity and $\nu$ the kinematic viscosity) between 15 000 and 70 000. With porosity $\beta$ defined as the ratio of open to total area of the plate, it is well established that as porosity increases, the wake shifts from the periodic von Kármán shedding behaviour to a regime where this vortex shedding is absent. This change impacts the fluid forces acting on the plate, especially the drag, which is significantly lower for a wake without vortex shedding. We analyse experimentally the transition between these two regimes using hot-wire anemometry, particle-image velocimetry and force measurements. Coherence and phase measurements are used to determine the existence of regular, periodic vortex shedding based on the velocity fluctuations in the two main shear layers on either side of the wake. Results show that, independent of $Re$ , the wake exhibits the classical Kármán vortex shedding pattern for $\beta <0.2$ but this is absent for $\beta >0.3$ . In the intermediate range, $0.2<\beta <0.3$ , there is a transitional regime that has not previously been identified; it is characterised by intermittent shedding. The flow alternates randomly between a vortex shedding and a non-shedding pattern and the total proportion of time during which vortex shedding is observed (the intermittency) decreases with increasing porosity.

中文翻译：

---

与流动方向垂直的多孔平板后面的涡流脱落

这项工作研究了体孔隙率对经过名义上固定宽度的二维矩形板的尾流的影响 $D$ 在雷诺数的中等范围内 $Re = UD/\nu$ （和 $U$ 传入速度和 $\nu$ 运动粘度）在 15 000 至 70 000 之间。具有孔隙率 $\测试版$ 定义为板的开放面积与总面积的比率，众所周知，随着孔隙率的增加，尾流从周期性冯卡门脱落行为转变为不存在涡旋脱落的状态。这种变化会影响作用在板上的流体力，尤其是阻力，对于没有涡流脱落的尾流来说，阻力要低得多。我们使用热线风速测量、粒子图像测速和力测量来通过实验分析这两种状态之间的转变。相干和相位测量用于根据尾流两侧两个主剪切层的速度波动来确定是否存在规则的周期性涡旋脱落。结果表明，独立于 $再$ ，尾迹表现出经典的卡门涡旋脱落模式 $\beta <0.2$ 但这不存在 $\beta >0.3$ 。在中间范围内， $0.2<\beta <0.3$ ，存在以前未确定的过渡制度；它的特点是间歇性脱落。流动在涡流脱落和非脱落模式之间随机交替，并且观察到涡流脱落的总时间比例（间歇性）随着孔隙率的增加而减少。