Leading-Edge Separation Initiation of Large Amplitude Oscillating Airfoils using a Discrete Vortex Model

Leading edge separation of an oscillating airfoil operating in the energy harvesting regime is predicted in this study. This prediction can be implemented in discrete vortex models as a criteria to shed point vortices from the leading edge.  This criteria uses the transient local wall stress distribution determined from computational fluid dynamics (CFD) simulations, which indicates the time and location of zero wall shear stress on the foil surface. The occurrence of separation is found to collapse during the cycle when scaled using the leading edge shear layer velocity which is determined from the airfoil kinematics. The advantage of the proposed separation criteria is that it can be fully determined from the motion kinematics and then applied to a wide range of low order models for design purposes. Results are obtained for a thin flat airfoil undergoing sinusoidal heaving and pitching motions for a range of reduced frequencies k=fc/U_∞ = 0.06 – 0.16 where f is the heaving frequency of the foil, c is the chord length and U_∞ is the freestream velocity. The heaving and pitching amplitudes are h₀ = 0.5c and q₀ = 70°, respectively, and the airfoil pitches about the mid-chord. This study uses a panel method with the proposed leading-edge vortex (LEV) shedding criteria which is applicable to a wide range of foil geometries an empirical trailing-edge separation correction is also applied to the transient force results. Lastly, the effects of a wide range of Reynolds numbers on the leading-edge separation is shown for the given range of reduced frequencies.  Lastly, the low order model results of transient lift force are calculated and compared with the CFD simulations.