Investigation of alternative drag-reduction strategies in turbulent boundary layers by using wall forcing

Lead Research Organisation: Imperial College London
Department Name: Dept of Aeronautics


The aim of this research is to investigate, in an interactive programme involving several mutually supportive computational approaches and paradigms, the feasibility of achieving sustained and economically worthwhile frictional-drag reduction at flight Reynolds numbers using cross-flow wall forcing. While the emphasis of the programme is on the fundamental turbulence physics and the prediction of its interaction with wall drag, in general, the programme is closely associated with an important civil aviation goal, namely the reduction in emissions per passenger km by 50% by 2020. The programme will combine studies involving direct numerical simulations and highly-resolved large eddy simulations with two approaches based on linearised streak modelling, one developed by Chernyshenko (Imperial College) and the other by Lockerby (Warwick). The general strategy is to use the full-resolution schemes to gain insight into the near-wall turbulence mechanisms associated with frictional drag, to generate calibration-related input into the linearised streak modelling and to investigate the validity of this modelling for a range of actuation parameters examined with the full-resolution approaches. The proposed research is fundamental in nature and complements well EPSRC's Active Aircraft programme, which is practically-oriented. The ultimate objective is to derive a prediction procedure, based on linearised streak modelling that allows the effect of different configurations of cross-flow wall forcing on drag at flight Reynolds numbers to be quantified. The programme is financially supported by EADS.


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Description The research focused on computational and theoretical approaches to gaining insight into, and predicting, the physical processes associated with turbulent-friction-drag reduction by oscillatory wall motion. The objectives set out in the original application were fully achieved. A theoretical framework was developed, based on linear theory, to describe the near-wall-streak formation and its alteration by spanwise forcing. Alongside this theory, and in support of the same, extensive direct nume
Exploitation Route The major interest of the Aeronautics sector in this research was identified and explained in the previous section. In addition, the research is very pertinent to ship hydrodynamics, high-speed trains and (to a more limited extent) to high-speed road transport. Especially In the first two areas, surface drag contributes significally to fuel consumption,. Both can, therefore, derive suignificant benefits from the results of this research. The project focused, primarily, on generic and fundamen
Sectors Aerospace/ Defence and Marine

Description The statement provided herein conveys the perception of the PI of the impact achieved by the project. From an academic perspective, the research has made a substantial contribution to understanding the interactions between turbulence and drag reduction. This knowledge will impact on approaches taken by future researchers in respect of active flow contyrol of near-wall flows. One particularly important interaction that has been identified relates to the role of strong turbulent structures in the outer flow on the near-wall streak stability, which is of major importance to the drag level. The identification of this role will steer future research towards controlling the outer structures, not simply the small-scale near-wall streaks, in an effort to reduce the drag. Substantial friction-drag reduction is the "holy grail" of civil aviation, and this is reflected by the significant financial support offered by EADS and Airbus UK, as well as its direct technical involvement. Neither has firm ideas about the engineering realisation of the knowledge derived from the project, simply because the major engineering challenges associated with forcing by wall motion need to be addressed. The time horizon of this challenge is thus at least 20 years. However, the UK's Aeronautics industry is extremely interested in pursuing all concepts leading to drag reduction and fuel savings, so as to remain competitive (especially against Boeing). It is especially interested in the promise of wall actuation at high (flight-condition) Reynolds numbers. The project partners therefore pushed their methods to the limit in terms of the proximity of the modelled conditions to reality, and this led to some important conclusions of major practical interest. In recognition of the relevance of exploitation issues, progress made by the academic project partners was presented at 5 workshops involving industry (Airbus, BAE Systems, EADS, various members of EU FP7). In addition, during the project, 6 1-day review meetings were held with industrial partners present. Finally, a closing 1-day open workshop was held, attended by several industrial representatives from the UK and abroad, with EADS making a presentation of its own conclusions derived from the project in relation to its future industrial strategy. . Beneficiaries: The academic flow-control and turbulent-flow communities; industrial researchers in the eronautical sector Contribution Method: First and foremost, the research has set markers in respect of the long-term potential of exploiting surface movement to achieve drag reduction in transport. Beyong this practice-related impact, the research has yielded knowledge and insight into how drag can be maniputated by creative near-wall straining.
Sector Aerospace/ Defence and Marine
Impact Types Cultural