Stiffness effects on laminar separation flutter

Abstract

This work explores self-sustained pitching oscillations of a NACA0012 airfoil operating at low-to-moderate Reynolds numbers in which the aerodynamic flow is in a transitional regime. One-degree of freedom (1-DOF) pitching oscillations are explored for chord-based Reynolds numbers of Rec=77,000 and 110,000, which fall on the lower end of the flutter regime, over a large range of torsional spring constants. Laminar separation flutter (LSF) is shown to persist over several orders of magnitude of structural rigidity. At the Reynolds numbers tested, the effect of changing structural rigidity at a fixed Reynolds number exhibits remarkable similarity to the effect of varying Reynolds number at a fixed stiffness. Although the two effects are not perfectly analogous, both parameters influence the timing of transition events relative to the limit-cycle. Raising stiffness values at either Reynolds number causes the structural response to outpace boundary layer transition which in turn yields an increasingly nonlinear aeroelastic response. Oscillations are no longer sustained when the structural frequency merges with the frequency of the aeroelastic response. At this point, the slow relaxation time of boundary layer transition fails to keep pace with the structural frequency and the processes that sustain LSF are inhibited. Oscillations terminate at a lower stiffness in the lower Reynolds number case—an apparent effect of slower boundary layer transition. Interestingly, hysteresis patterns and flow topologies coalesce into nearly identical topologies for both Reynolds numbers when stiffness is very low.