Regimes of tonal noise on an airfoil at moderate Reynolds number

Abstract

Tonal noise generated by airfoils at low to moderate Reynolds number is relevant for applications in, for example, small-scale wind turbines, fans and unmanned aerial vehicles. Coherent and convected vortical structures scattering at the trailing edge from the pressure or suction sides of the airfoil have been identified to be responsible for such tonal noise generation. Controversy remains on the respective significance of pressure-and suction-side events, along with their interaction for tonal noise generation. The present study surveys the regimes of tonal noise generation for low to moderate chord-based Reynolds number between Re c = 0.3 × 10 5 and 2.3 × 10 5 and effective angle of attack between 0 • and 6.3 • for the NACA 0012 airfoil profile. Extensive acoustic measurements with smooth surface and with transition to turbulence forced by boundary layer tripping are presented. Results show that, at non-zero angle of attack, tonal noise generation is dominated by suction-side events at low Reynolds number and by pressure-side events at high Reynolds number. At smaller angle of attack, interaction between events on the two sides becomes increasingly important. Particle image velocimetry measurements complete the information on the flow field structure in the source region around the trailing edge. The influences of both angle of attack and Reynolds number on tonal noise generation are explained by changes in the mean flow topology, namely the presence and location of reverse flow regions on the two sides. Data gathered from experimental and numerical studies in the literature are reviewed and interpreted in view of the different regimes.

Laminar separation bubble development on an airfoil emitting tonal noise

“…The suction side dominated regime (figure 13a) is expected to occur at relatively low angles of attack, as it requires the formation of a separation bubble close to the trailing edge, which is known to move upstream with increasing angle of attack (Boutilier & Yarusevych 2012b). This is supported by recent acoustic measurements conducted in an anechoic facility by Pröbsting et al (2015). Finally, the development of a separation bubble on the suction side, which was of particular interest in the present investigation, has been shown to be influenced significantly by tonal emissions.…”

“…From their onset at 10 m s −1 , shear layer vortices on the pressure side are the dominant contributors. These trends agree with those reported for acoustic measurements in an anechoic wind tunnel on a half-scale NACA 0012 model at the same angle of attack(Pröbsting, Scarano & Morris 2015).…”

“…Under such conditions, boundary layer separation on the pressure side occurs closer to the trailing edge compared to that on the suction side. As a result, shear layer vortices forming on the pressure side maintain higher strength and correlation across the span as they pass over the trailing edge (Pröbsting et al 2015), a condition for which classical edge noise theory predicts stronger noise emission (Amiet 1976). At low velocity, when shear layer separation on the pressure side does not occur upstream of the trailing edge, weaker tonal emissions can also be produced by roll-up vortices forming on the suction side.…”

Numerical Prediction of the Tonal Airborne Noise for a NACA 0012 Aerofoil at Moderate Reynolds Number Using a Transitional URANS Approach

“…Future developments of the present work can include new testing conditions, as per recent contributions from (Arcoundoulis et al, 2013) and (Pröbsting et al, 2015).…”

“…(1). Similarly, Pröbsting et al (2015) identified tonal noise generation on a NACA 0012 aerofoil in a range of Reynolds number from 30,000 to 230,000 and AoA from 0 to 6.3…”