Experimental Investigation of Tip Vortex Formation Noise Produced by Wall-Mounted Finite Airfoils

This paper presents an experimental investigation of tip vortex formation noise produced by wall-mounted finite airfoils. Acoustic measurements were taken in the open jet anechoic wind tunnel at Brandenburg University of Technology with a planar 47-microphone array. A parameter space of eight airfoil models with variations in airfoil profile shape (thickness and camber) and tip geometry were tested at a range of angles of attack and Reynolds numbers. The acoustic survey shows that tip noise is a strong contributor to the overall airfoil noise signature at mid-to-high frequencies above 2–3 kHz. A critical frequency was defined to distinguish the effect of the thickness of the airfoil on the noise spectra, which increases with Reynolds number and appears constant with angle of attack. Above the critical frequency, a thicker airfoil is shown to produce a lower amplitude tip noise peak, especially at mid-to-high frequencies above 5 kHz. An increase in camber does not affect the frequency of the tip noise peak but increases the noise level, especially for $f\ge 3\mathrm{kHz}$ and high angles of attack. Further, the presence of a rounded tip is found to be effective in decreasing the noise levels by up to 5.6 dB over the frequency range of 5–15 kHz. To explore the tip flowfield, velocity data were also obtained using planar particle image velocimetry in the anechoic wind tunnel at the University of New South Wales for a few targeted cases. The results show that the size of the vortex and its turbulence intensity are influenced by the tip geometry. Specifically, the vortex size for the cambered airfoils decreases along the streamwise direction, while for symmetric airfoils it increases from $x/c=1.2$ to 1.8. Collectively, these databases can be used to aid tip noise prediction models and validate computational fluid dynamic and computational aeroacoustic simulations.