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Flow Control Over an Airfoil : Physical Analysis of Transient Phenomena to Improve Performance and Maneuverability

Abstract : The flow around a NACA 0015 airfoil is experimentally controlled using pulsed vortex generator jets (PVGJs) at a chord-based Reynolds number of 4.6 × 105. A special focus is given to the transient effects occurring after the actuation onset. The main objective is the improvement of the performance and maneuverability of the airfoil and more generally of aircrafts. In order to take into account all the phases of the flight envelope, the control has been tested on "baseline" configurations where the flow is attached to (e.g. cruise) or partially separated (e.g. take-off, landing) from the airfoil surface. In all these phases, unsteady loads fluctuations might arise from continuously changing flow conditions such as gusts or rapid maneuvers. The fluidic actuation might help to alleviate these unsteady loads, but to increase the control effectiveness we need to optimize the variation rate of the forces and moment coefficients induced by actuation. For this reason, a detailed study of the transient phenomena occurring after the actuation onset in response to a single-pulse actuation, of duration smaller than the convective time over the airfoil, is carried out. The results of the single-pulse actuation are compared with those obtained from a steady actuation, both operated with the same maximum jet exit velocity.It is observed that a fine-tuned single-pulse actuation, over a partially separated airfoil, can improve the initial rate of variation of the aerodynamic loads by up to 50 % compared to the case of the steady blowing.The analysis of unsteady pressure fields on the airfoil surface and of the velocity fields, obtained using particle image velocimetry (PIV), around the airfoil, showed the preponderant role of the duration of the actuation on the gains obtained. The use of Lagrangian tools (finite-time Lyapunov exponent -FTLE) for the detection of unsteady flow detachments/reattachments showed that the optimal duration of actuation (in terms of loads variation) is associated with transient reattachment/detachment of the flow over the suction surface of the airfoil. Therefore, the transient effects on naturally attached configurations are much less important and the evolution of the loads is not strongly affected by the duration of the actuation. Parametric studies have been carried out to investigate the influence of the location of the actuation over the suction surface of the airfoil compared to the mean baseline separation point. We show that the rates of variation obtained are considerably decreased if the actuation is operated from inside the naturally separated zone. The optimal pulse duration is therefore obtained by a fine-tuned control of the formation and evolution of an unsteady separation region over the suction surface of the airfoil. The understanding of the transient physical mechanisms induced by single-pulse control has finally allowed us to develop energy-efficient control strategies (in terms of the trade-off between the aerodynamic gain and energy expenditure). In particular, we found that a fine-tuned control strategy,consisting of periodic repetition of the single-pulse, can improve the energy efficiency of the control compared to steady blowing. This periodic control strategy is, also, able to increase the initial rate of loads variation compared to the steady control because it exploits the beneficial effects associated with the formation of a separation bubble between successive pulses. The transient improvement of the rate of loads variation obtained in this work might be effective in situations where a fast time response is needed to compensate unsteady aerodynamic effects, such as in gusting flows or during rapid maneuvers.
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Submitted on : Friday, September 24, 2021 - 1:37:10 PM
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Armando Carusone. Flow Control Over an Airfoil : Physical Analysis of Transient Phenomena to Improve Performance and Maneuverability. Other. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2021. English. ⟨NNT : 2021ESMA0007⟩. ⟨tel-03353832⟩

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