Colloquia

Summary

Airfoils at low-to-moderate Reynolds numbers cause tonal noise. The Kelvin-Helmholtz instability of a laminar separation bubble causes vortices to shed, leading to tonal noise. However, airfoils with blunt trailing edges also shed vortices from the blunt edge, which also cause tonal noise.

In this experimental study, a NACA 0012 airfoil is investigated in a closed-section non-anechoic wind tunnel at low-to-moderate Reynolds numbers and angles of -4 to 4 degrees. A sharp trailing edge with a trailing edge radius of 0.2% and two flatback blunt trailing edges of 2.1 and 4.7% thickness, relative to the geometric chord length, are examined. The data is post-processed with in-house conventional beamforming codes.

Results show that lock-in of the shear layer vortex shedding to the blunt trailing edge vortex shedding frequency occurs for all Reynolds numbers with the 4.7% thickness trailing edge. As a result, the tonal noise is concentrated around a trailing-edge thickness-based Strouhal number of 0.2.

For the 2.1% thickness trailing edge, the results suggest that lock-in occurs only at high Reynolds numbers and high angles of attack. This shows that blunt trailing edges can influence the tonal noise generation of the NACA 0012 airfoil.

Additionally, cylindrical roughness elements are employed to reduce tonal noise by generating streaks that stabilise Kelvin-Helmholtz waves and promote the break-up of the Kelvin-Helmholtz vortices. Roughness elements are shown to suppress and attenuate tonal noise at Reynolds numbers 100,000 to 300,000. For the 2.1% trailing edge, the best results are achieved with roughness elements on both sides of the airfoil. For the 4.7% trailing edge, roughness elements on the suction side of the airfoil give the greatest noise reduction.

Finally, extreme tonal peaks are identified in the data. These peaks occur over a range of angles when the angle is increased in small increments. But the cause of the extreme peaks remains unknown.