Novel passive techniques for reducing skin-friction drag

空中客车公司的目标是到2020年将每名乘客公里的燃油消耗量减少至少50%，如果不将表面摩擦阻力减少30%至50%，这一目标将难以实现。因此，我们建议，研究新的，实用的，有效的流动控制技术，以实现这一目标。在湍流边界层的表面摩擦阻力是由流动物理非常接近表面的流场区域称为粘性子层。壁摩擦的产生也被认为是准循环的。这个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。由此产生的条纹和爆炸性增长的演变密切相关的壁摩擦，从而drag.大多数研究人员专注于这些子层条纹，因为他们是非常接近的壁，并服从基于壁的驱动和传感。然而，我们估计在空中客车A340-300的机身上在巡航期间的任何时刻都有O（109）亚层条纹。其他人也做出了类似的估计。这个巨大的数字使得单独实施针对条纹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破坏这种循环是可行的。小脊（微小的波峰和波谷在流动方向上运行，横流间距约为人类头发宽度的1/3）通过破坏条纹生长来实现这一点，实际上是通过规则化和部分稳定它们。但是，传统的肋条在飞行试验中只能提供不到1.5%的减阻，尽管在理想的实验室实验中可以达到6%。除非这种糟糕的性能可以得到极大的改善，否则riblet几乎没有什么实际意义。展向振荡最近已被研究，并表明在减少表面摩擦阻力方面比沟槽有效得多。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。但是这种技术需要大量的功率输入。鉴于上述循环过程，另一种选择是用随机扰动来破坏波和条纹的相互作用。Sirovich等人尝试了这种方法，他们在随机表面粗糙度元件的实验流动研究中获得了12%的阻力减少。这种方法还没有真正得到进一步的研究，虽然破坏波条纹相互作用的随机扰动可能是更有效的比riblets.我们建议调查：（i）使用随机分布的小规模亥姆霍兹谐振器，创建强大的微射流没有任何功率输入，因此可能是更有效的粗糙元素或riblets;（ii）常规的肋使条纹局部化，因此将它们与谐振器组合可比单独的肋有效得多;（iii）用非常规的肋提高效率;例如，模仿展向振荡和其他3D图案的波状肋。我们的研究将基于我们的简化理论模型的子层条纹，可以使用在飞行雷诺数。亥姆霍兹谐振器作为无源控制装置具有很大的前景，因为：（i）所产生的控制干扰比粗糙元件（包括脊）成比例地大得多;（ii）它们不需要功率输入;以及（iii）仅由具有颈状出口孔的腔组成，它们在MEMS（微）尺度上直接制造。

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.