Impact of vortex interference on airfoil tonal noise emission

Vortex interference with lifting surfaces has been observed in wide applications, e.g., rotors, propellers, fans, and wind turbines, which influences the available lift and causes unsteady loading on the lifting surfaces. While previous research focused on the aerodynamic impact of vortex interference, the influence of vortex on the aeroacoustic noise and associated flow physics has not yet been investigated. In order to generate a vortex interacting with an airfoil, a tripped airfoil (vortex generator, VG) is placed upstream of a clean airfoil. The noise generated by the clean airfoil and VG is recorded using a microphone placed in the far field. The clean airfoil shows tonal noise emission due to the acoustic feedback loop typically observed at low-to-moderate Reynolds number. The flow field near the tonal noise source region of the airfoil is quantified by particle image velocimetry (PIV) measurements at a sweep of span-normal planes. It is observed that the three-dimensional vortex structure has a spanwise shift with the direction dependent on the suction or pressure side interference. In addition, PIV in the spanwise and wall-normal plane is conducted to analyze the airfoil wake. The overall sound pressure level vs vortex position shows a minimum value where the vortex is near the leading edge, which is featured by a significantly disrupted wake shape and a minimum spanwise correlation length.