Novel passive techniques for reducing skin-friction drag

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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.