Analysis of flow transition and separation on oscillating airfoil by pressure signature

To have a better understanding of the unsteady aerodynamic characteristics of the airfoil which play important roles in wind turbine blade design, we investigated the boundary layer transition and separation on oscillating airfoil S809 using pressure signature captured in wind tunnel testing. The developed data processing technique of “sliding window” was applied to get useful transition and separation information. Meanwhile, the hysteresis effects of oscillation frequency on transition and separation were studied. It is found that (1) the root mean square (RMS) of pressure signature can indicate the transition and separation with the dimensionless window width of m¯ = 0.0015 (2) the transitional attack of angle in up stroke is larger than that in down stroke at the state of the relative chord length of x/c ≥ 0.14, while the situation is opposite at the state of the relative chord length of x/c ≤ 0.14; (3) the flow separation is advanced and the reattachment is delayed with the increase of the oscillation frequency, which results in a greater hysteresis effect. The sliding window technique, whose parameters were determined in this paper, is effective for detecting boundary layer transition and separation from pressure signature.