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Reducing drag over wings

Final Report Summary - REDWING (Reducing drag over wings)

The initial aim of the project was that of making progress in understanding how a variety of rough / hairy / compliant natural surfaces act on the fluid flowing over and around them, modifying some global characteristics of the motion, such as the drag or the lift coefficients. The project was cut short after ten months, which was just enough time to develop and validate the numerical tool on a preliminary configuration. Nonetheless, the configuration studied proved to be so interesting as to deserve a journal publication.

The set up studied consists simply in the uniform flow past a bluff body (a cylinder) with a flexible filament attached to its rear side. Despite its simplicity, this configuration represents an initial prototype for the flow past a body covered with hairs; the focus of the project is on the interaction between the filament(s) and the wake of the body. Preliminary studies have shown that a well-chosen coating behind a up-down-symmetric bluff body may reduce drag, maintaining a zero mean lift. The initial work with the single filament, initially placed exactly parallel to the horizontal line through the cylinder parallel to the upstream flow, has confirmed the expected drag reduction, via a redistribution of the momentum flux, but has also shown another unexpected feature: for certain rigidity parameters and filament lengths, the filament oscillates asymmetrically, on either the top portion of the wake or the bottom portion, with a consequent lift force on the body. This is unexpected since both the wake and the filament, taken alone, maintain symmetric behaviour; their coupling results in the breaking of symmetry, associated to a resonance between the frequency of shedding of the vortices behind the body and the frequency of the free bending vibrations of the filament.

The study has been conducted via a numerical code, written for the purpose by the fellow, based on the Navier-Stokes equations expressed in immersed-boundary form. The code has been extensively validated and will remain available for future collaborators of both the fellow and the PI. The results so far have mostly a theoretical interest, although the possibility of enhancing lift and reducing drag via aeroelastic flaps seems very promising in the aeronautical context.

During his stay as IEF fellow in Genova, Dr Bagheri has interacted with all the members of the local research team, more particularly with Prof. A. Mazzino, a colleague of the PI expert on aeroelasticity who will go to Sweden in June 2012 to collabourate with the fellow, and has extensively collabourated with both an undergraduate student of the PI, Mr A. Tripodi, and one doctoral student, Mr D. Natali. The latter will spend a period in Sweden in the third year of his thesis period, to work with Dr Bagheri on certain numerical aspects of fluid flows in the presence of interactions with the structures. Dr Bagheri has also lectured to graduate students of the 'Advanced fluid mechanics' course in Genova on model reduction, and has been invited to deliver a few lectures on flow control in September 2011 at the University of Toulouse (see http://www.math.univ-toulouse.fr/Control-of-NSE/verang/index.html for details). Furthermore, he has benefitted from an extended visit (one month) to the Hydrodynamics Laboratory of the Ecole Polytéchnique in Paris, where he has started a joint research activity with Prof. Peter Schmid.

Finally, the work of the IEF fellow has resulted in an invitation to conduct experiments in Bordeaux (France) in the laboratory of Prof. Hamid Kellay, to reproduce and extend the numerical results obtained, and a graduate student from the University of Genova, Mr A. Orchini, is at present being hosted in Bordeaux.