Novel passive techniques for reducing skin-friction drag

空中客车到2020年将每名乘客公里的燃油消耗量减少至少50%的目标，如果不减少30%到50%的表面摩擦阻力(由气流的直接作用在飞机表面产生的摩擦产生)，就很难实现。因此，我们建议研究新的、实用的、有效的流动控制技术来实现这一目标。湍流边界层中的表面摩擦阻力是由非常接近表面的流动物理控制的，该区域被称为粘性亚层。壁面摩擦的产生也是已知的准循环的。这一循环和近壁流动物理的一个基本特征是低速和高速流动的条纹及其与波浪状扰动的强烈相互作用。由此产生的条纹的演化和它们的爆炸性增长与壁面摩擦的产生密切相关，因此大多数研究人员关注这些亚层条纹，因为它们非常接近壁面，并且易于基于壁面的激励和传感。然而，我们估计，在巡航过程中的任何时刻，空客A340-300的机身上都有O(109)亚层条纹。其他人也做出了类似的估计。如此巨大的数字使得实施针对个别连胜的主动控制策略是完全不切实际的。但以一种全球性的、无针对性的方式扰乱这一循环是可行的。肋纹(沿着流动方向的微小波峰和波谷，横流间距约为人类头发宽度的三分之一)通过破坏条纹生长来实现这一点，实际上是通过使它们规律化和部分稳定来实现这一点。但传统的肋片在飞行试验中只能减少不到1.5%的阻力，尽管在理想的实验室实验中可以达到6%的减阻效果。除非这种糟糕的表现能够得到极大的改善，否则肋骨几乎没有实际意义。最近的研究表明，跨向振动比肋板更能有效地减少壁面摩擦阻力。同样，这些似乎是通过迫使条纹进入更稳定的方向而起作用的。但这项技术需要大量的电力输入。给出上述循环过程，另一种选择是用随机扰动来中断波和条纹的相互作用。这是由Sirovich等人尝试的。世界卫生组织在使用随机表面粗糙度元素的实验流动研究中获得了12%的减阻效果。这一方法还没有真正得到进一步的研究，尽管用随机扰动破坏波纹相互作用可能比肋条更有效。我们建议研究：(I)使用随机分布的小规模Helmholtz谐振器，在没有任何功率输入的情况下产生强大的微射流；因此可能比粗糙元件或肋条更有效；(Ii)传统肋条将条纹局部化，因此与谐振器结合可能比单独肋条更有效；(Iii)使用非传统肋条来提高效率；例如，模仿跨向振荡和其他3D图案的波纹肋。我们的研究将基于我们简化的亚层条纹理论模型，该模型可以在飞行雷诺数时使用。亥姆霍兹谐振器作为被动控制器件很有希望，因为：(I)产生的控制干扰按比例比粗糙元件(包括凸缘)大得多；(Ii)它们不需要电源输入；(Iii)它们只由一个带有颈口的腔体组成，可以直接在MEMS(微型)规模上制造。

Airbus's aim to reduce fuel burn per passenger km by at least 50% by 2020 will be difficult to achieve without a 30 to 50% reduction in skin-friction drag / the drag arising from the friction generated on the aircraft's surface by the direct action of the air flow. We propose, therefore, to investigate novel, practical, effective flow-control techniques for achieving this aim.Skin-friction drag in turbulent boundary layers is governed by the flow physics very close to the surface in a region of the flow field known as the viscous sublayer. The generation of wall friction is also known to be quasi-cyclic. An essential characteristic of this cycle and the near-wall flow physics are streaks of low- and high-speed flow and their strong interaction with wave-like disturbances. The resulting evolution of the streaks and their explosive growth are intimately connected with the generation of wall friction and thereby drag.Most researchers focus on these sublayer streaks because they are very closest to the wall and amenable to wall-based actuation and sensing. We estimate, however, that there are O(109) sublayer streaks over the fuselage of an Airbus A340-300 at any instant during cruise. Others have made similar estimates. This enormous number makes it utterly impractical to implement an active control strategy targeting streaks individually. But disrupting the cycle in a global untargeted way is feasible. Riblets (minute peaks and troughs running in the flow direction with crossflow spacing of about 1/3 of a human hair width) do this by disrupting streak growth, in effect by regularizing and partially stabilizing them. But conventional riblets only deliver less than 1.5% drag reduction in flight tests, although 6% is achieved in idealized laboratory experiments. Unless this poor performance can be greatly improved, riblets are of little practical interest. Spanwise oscillations have been studied recently and shown to be much more effective than riblets at reducing skin-friction drag. Again these appear to work by forcing the streaks into more stable orientations. But this technique requires substantial power input. Given the cyclic process described above, another option is to disrupt the interaction of the waves and streaks with randomized perturbations. This was tried by Sirovich et al. who obtained 12% drag reduction in experimental flow studies with randomized surface roughness elements. This approach has not really been further investigated, although disrupting the wave-streak interaction with randomized perturbations is likely to be much more effective than riblets.We propose to investigate: (i) the use of randomized distributions of small-scale Helmholtz resonators that create strong microjets without any power input; thus are likely to be more effective than roughness elements or riblets; (ii) conventional riblets localize the streaks, thus combining them with resonators could be much more effective than riblets alone; (iii) improving effectiveness with unconventional riblets; e.g., wavy riblets mimicking spanwise oscillations and other 3D patterns. Our study will be based on our simplified theoretical model of the sublayer streaks which can be used at flight Reynolds number. Helmholtz resonators hold great promise as passive control devices because: (i) the control disturbance produced is proportionately much greater than for roughness elements, including riblets; (ii) they require no power input; and (iii) consisting simply of a cavity with a necked exit orifice, they are straightforward to manufacture at MEMS (micro) scale.