Effect of serrated trailing edge on boundary layer instability noise

Wings operating at low and moderate Reynolds number such as the ones of UAVs or the blades of small wind turbines and of compressor fans, can be the source of an aeroacoustics phenomenon called laminar boundary layer instability noise. This paper presents an investigation with time-resolved PIV and far field microphones of the dynamical flow phenomena and the acoustic emissions of a NACA 0018 airfoil, with and without serrated trailing edges, operating in a transitional regime (Reynolds number ≈ 3.3 – 4.7 × 105). The measurements are done in an open jet wind tunnel facility with low noise and turbulence intensity. The planar PIV measurements are done on the pressure side of the airfoil in two different planes, parallel and orthogonal to the wing span at the trailing edge region, to capture the flow evolution along the airfoil chord as well as its spanwise arrangement. The results presented here show the effectiveness of the serrated geometries to reduce the noise emitted by this type of flows. Recently published literature outcomes concerning the streamwise evolution of the flow with and without serrations are confirmed. Moreover new insights related to the spanwise coherence of the traveling vortical structures are presented leading to a better understanding of the hydrodynamic and acoustic mechanisms involved in this aeroacoustics phenomenon.