The energy extraction potential from pitch-heave coupled flutter

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An experimental investigation of a pitch-heave fully-passive system undergoing large amplitude coupled flutter induced limit cycle oscillations (LCO) is conducted. The main objective of this paper is to report and document the large aerodynamic efficiencies that can be experimentally obtained from coupled flutter. This is in contrast to a number of investigations in the energy harvesting literature which i. report relatively very low efficiencies for coupled flutter, and ii. emphasise the case of stall flutter and the related phenomenon of dynamic stall. The elastic axis of the set-up is located at 27% chord. The main parameters varied are the airspeed (Reynolds number) and stiffness coefficient in heave, as expressed by the frequency ratio ranging from below to above one. For a frequency ratio ω¯=ωhωθ=1.22, amplitudes up to 80 deg in pitch and 0.6 heave-to-chord ratio are observed at airspeeds close to the flutter speeds. In conjunction with favourable phase differences, these large LCO amplitudes occurring at low speeds lead to aerodynamic efficiency values hovering around 40%. It is also observed that maximum efficiency calculated solely from the heave motion is 50%. This is a consequence of the negative work being done on the pitch by the aerodynamic moment, which can be related back to the inertial coupling and positive phase difference between pitch and heave. For the vast majority of our observations, the aerodynamic moment does negative work, whereas the lift does positive work.