Flutter Behaviour of Laminar Supercritical Airfoil - A Numerical Investigation into the Influence of Boundary Layer Transition

Kurzfassung

A laminar wing has a high potential in terms of fuel savings. It should however not lead to a reduction in the capabilities of aircraft. Since the flutter boundary is one of the most important boundaries of the flight envelop of aircraft it is important to investigate the effect of laminar flow on this boundary. In this thesis a first step has been taken by investigating this for a laminar supercritical airfoil, the CAST-10 airfoil. In order to do so steady and unsteady (U)RANS simulations have been performed with both free and fixed (at the leading edge) boundary layer transition. Two CFD codes have been used; the DLR TAU code and the ANSYS CFX code, which both use another model in order to predict boundary layer transition (the e^N-method and the γ-Re model). The CAST-10 airfoil was assigned two degrees of freedom; pitch and plunge. The flutter behaviour was determined using the k-method, where the results from the unsteady CFD simulations were used as input. By plotting the quasi-steady lift-curves slope versus the Mach number, a prediction of the transonic dip in the flutter boundary could already be made. Large differences were observed between the fixed and free boundary layer transition case. The flutter boundary itself also showed large differences, especially in the depth of the transonic dip, which was much larger in case of free transition, but also in the position of the dip (it occurred at a slightly lower Mach number when transition was free). Upon comparing the flutter boundary with the quasi-steady lift-curve slope graph it was found that the location of the transonic dip in the flutter boundary can be predicted already from the maximum in the lift-curve slope graph.