Non-linear Phenomena of Wing Flutter and the Effect of Laminar-Turbulent Transition

A Navier-Stokes Computational Fluid Dynamics (CFD) code is coupled with a Computa-tional Structural Dynamics (CSD) code to study the flutter boundary of the NACA64A010airfoil using Isogai’s structural model in transonic conditions. This model simulates aeroelas-tic conditions on a sweptback wing. A well-known feature, only present in the inviscid flutterboundary of this airfoil, is the existence of multiple flutter points for a fixed freestream Machnumber. The fully-turbulent flutter boundary has not been studied by many researchers us-ing a Reynolds-Averaged Navier-Stokes approach. In the present study, the fully-turbulentflutter boundary reveals the existence of multiple equilibrium positions for a narrow rangeof flight conditions. The system moves away from the initial equilibrium position, finding anew set of equilibrium points and oscillating around it. This new set of equilibrium pointsreveals as stable or unstable for different structural properties of the wing.We then proceed to study the effect of turbulent transition on flutter boundary. A laminar-to-turbulent transition model is implemented in the CFD code and validated. The effect ofusing a free-transition CFD code vs. a fully-turbulent approach is evaluated on three airfoilswith different characteristics for subsonic and transonic conditions. While free-transitiondoes not affect the pressure distribution at subsonic conditions, the transonic simulationsreveal a change in the shock-wave position when laminar-turbulent effects are included. Theeffect of transition on the flutter boundary of the NACA64A010 airfoil at transonic conditionsis then investigated. A comparison between the free-transition, inviscid and fully-turbulentflutter boundaries reveals similarities between the inviscid and free-transition elastic re-sponses. Those similarities are due to the shift in the fully-turbulent shock-wave position,when accounting for free-transition effects, moving closer to the inviscid shock location