Higher frequencies in stall flutter moment development

Abstract

An experimental study of stall flutter kinematic, moment, and fluid dynamic development is presented. This study focuses upon understanding the higher frequency content observed in stall flutter limit cycle oscillations and how that content relates to specific fluid dynamic structures and states. This is accomplished using angular position, pitching moment, and particle image velocimetry measurements of a cyber–physically mounted, NACA 0018 rigid finite span wing undergoing stable limit cycle oscillations. The wing model is mounted with a single degree of freedom in pitch. A conceptual argument is introduced to predict the model’s stall flutter susceptibility from the static moment response using two static equilibrium conditions. These conditions lead to a bound on the range of stiffnesses and reduced frequencies where stall flutter is expected to occur. These bounds, which are related to the attached and separated flow states, account for the fundamental frequency of oscillation. It is then shown that stable limit cycle oscillations contain multiple harmonic frequencies and it is argued that these higher harmonics represent important physics. In particular, it is found that the fifth harmonic is correlated with the dynamic stall vortex shedding frequency. Finally it is noted that pitching moment overshoot linearly scales with normalized pitch rate with a correlation coefficient R=0.85.