Hysteresis effect on airfoil stall noise and flow field

The effect of hysteresis on the lift coefficient of an airfoil is well-known, i.e., the lift coefficient is higher during the upstroke than during the downstroke for the same angle of attack (AoA). In contrast, the effect of hysteresis on the noise emission has not been reported before. In order to investigate this effect, a NACA 0012 airfoil is investigated during upstroke and downstroke between zero and post-stall AoA at a constant Reynolds number of 5.1×105. A map of the noise emission vs AoA is presented revealing tonal noise emission at low AoA, and broadband noise beyond the linear range of lift coefficient. The trend of overall sound pressure level vs AoA shows two local peaks: one at 4.4° with high tonal noise and a second one at 12.6° where the maximum lift coefficient is observed. Flow measurements near the leading edge and trailing edge by particle image velocimetry show that the airfoil noise trend vs AoA is dominated by the trailing-edge flow features. The magnitude of noise emission during the upstroke is higher than that during the downstroke in the hysteresis loop, although the velocity fluctuation magnitude is smaller. This implies that the vortex coherence during upstroke is higher than downstroke, which is evidenced by a more coherent vortex during the upstroke.