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Wind tunnel experiments on the optimization of distributed suction for laminar flow control

Abstract It is well known that surface suction can delay boundary layer transition from laminar to turbulent flow, thus reducing drag on the surface in question. In order for laminar flow control by means of suction to be a paying proposition on aircraft, however, it must reduce total energy consumption as well as net drag. The flow control system that will have the best chance of doing this will use the minimum amount of suction pump energy to achieve a given transition position. Previous work has shown how a steepest- descent constrained optimization algorithm in conjunction with a radial basis function gradient estimator is capable of doing this online. In this paper the control strategy is applied to a large (2 m chord, 1.6 m span) aerofoil model in a low turbulence tunnel, with variable incidence and direct force measurement. The effect of pressure gradient over the suction section on the efficacy of the suction can therefore be observed, as can the relationship between transition and drag. The system is shown to converge reliably as long as the desired transition is within its range, and to be capable of maintaining control after a change in aerodynamic conditions without needing to re-identify the gradient estimation coefficients.