用附加细磨粗糙度延迟分布粗糙度带引起的过渡
Delaying transition induced by a strip of distributed roughness using additional fine grit roughness
摘要：
分布粗糙度出现在空气动力学表面上，如风力/燃气轮机叶片和飞机机翼，导致早期边界层过渡，从而导致升阻比/功率产生的降低。尽管在工程场景中是一个反复出现的主题，分布粗糙度引起的边界层过渡的机制还没有得到很好的理解，因此，减轻其影响的方法是稀缺的。在这项工作中，我们提出了一种被动的方法延迟边界层过渡所造成的分布粗糙度（砂纸带）使用的组合，二次精细粗糙带直接上游和下游的分布粗糙度。热线和PIV测量用于表征流动特征和量化过渡延迟。研究表明，上游和下游放置的二级粗糙度带的组合对于延迟主要分布粗糙度引起的过渡最为有效。结果表明，上游粗糙度抬升了边界层，降低了主粗糙度的有效雷诺数，而下游粗糙度降低了主粗糙度脱落的涡的强度。二次粗糙度的长度和类型的参数研究表明，光滑的条带也可以延迟过渡，并可能有一个最佳的二次粗糙度的长度超过增加下游粗糙度的程度有边际效应过渡延迟。最后，在过渡开始后，在流动中没有对应于二次粗糙度的特定特征，这表明二次粗糙度可以延迟过渡，而基本上不改变过渡流动特征。结果指出了一个适应性强的和实用的方法，以增加分布粗糙度的气动表面的寿命周期和效率。
英文摘要：
Distributed roughness occurs on aerodynamic surfaces like wind/gas turbine blades and aircraft wings causing early boundary layer transition resulting in a reduction of the lift-to-drag ratio/power production. Despite being a recurring theme in engineering scenarios, the mechanism of boundary layer transition caused by distributed roughness is not well understood, and consequently, methods for mitigating its effects are scarce. In this work, we present a passive method for delaying boundary layer transition caused by distributed roughness (sandpaper strip) using a combination of secondary fine roughness strips placed immediately upstream and downstream of the distributed roughness. Hot-wire and PIV measurements are used to characterize the flow features and quantify transition delay. A combination of secondary roughness strips placed both upstream and downstream is shown to be most effective in delaying transition caused by the primary distributed roughness. Results suggest that the upstream roughness lifts the boundary layer reducing the effective Reynolds number of the primary roughness, while the downstream roughness reduces the strength of vortices shed from the primary roughness. A parametric study on the length and type of secondary roughness shows that smooth strips can also delay the transition and there is likely an optimal length of the secondary roughness beyond which increasing the extent of the downstream roughness has marginal effects on transition delay. Finally, after the onset of transition, there are no specific signatures in the flow corresponding to the secondary roughness which suggests that the secondary roughness can delay transition without substantially altering the transitional flow features. The results point to an adaptable and practical method for increasing the life cycle and efficiency of aerodynamic surfaces with distributed roughness.
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