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MIE Seminar: "Low speed flutter of airfoils: post-critical behaviors, unsteady aerodynamics and control"

Date/Time: 

Thursday, November 7, 2019 - 10:00am

Presenter: 

Xavier Amandolese, Structural Mechanics and Coupled Systems Laboratory, Cnam, Paris, France

Location: 

Kellogg Room, ELab II

Details: 

Abstract:
Observed since the early days of flight, the coupled-mode flutter of airplane wing is a well-known dynamic instability for which linear unsteady aerodynamic formulations has been successfully developed to predict the critical conditions. A greater focus on the post-critical behaviors is however necessary in order to improve the characterization, physical understanding and modeling of the large amplitude self-sustained vibrations resulting from these instability.

In a first part, we will talk about the post-critical dynamics of two-degree-of-freedom airfoil sections undergoing “low speed” flutter. Experiments were performed in wind tunnel at moderate Reynolds number (Re ~ 40 000), on both flat-plate and Naca0018 section models, using an aeroelastic set-up that enables high amplitude pitch-plunge motion without nonlinear structural limitations. Dynamical behaviors resulting from laminar separation flutter, coupled-mode flutter, symmetric stall flutter will be discussed, highlighting the flutter boundaries and the evolution of stable limit-cycle oscillation regimes with the wind velocity. Physical mechanisms responsible for the observed nonlinear dynamics (subcritical branch, bifurcations and LCOs) will be discussed with the help of a nonlinear aeroelastic model based on a semi-empirical dynamic stall formulation.

In a second part, we will talk about flutter control using nonlinear tuned magnetic vibration absorbers. Two post-critical scenarios will be examined: a coupled-mode flutter scenario exhibiting limit cycle oscillations characterized by moderate amplitude in plunge and low amplitude in pitch, and a more complex flutter scenario exhibiting a second post-critical bifurcation from coupled-mode flutter to high amplitude symmetric stall flutter. Magnetic vibration absorbers have been used in plunge, in pitch or in both, and tuned as nonlinear vibration absorbers with cubic stiffness. Their efficiency will be examined in the light of wind tunnel tests and numerical results obtained with a nonlinear aeroelastic model coupled with the vibration absorbers.