Tonal noise of a controlled-diffusion airfoil at low angle of attack and Reynolds number

Abstract

The acoustic signature of a controlled-diffusion airfoil immersed in a flow is experimentally characterized. Acoustic measurements have been carried out in an anechoic open-jet-wind-tunnel for low Reynolds numbers (from 5 × 104 to 4.3 × 105) and several angles of attack. As with the NACA0012, the acoustic spectrum is dominated by discrete tones. These tonal behaviors are divided into three different regimes. The first one is characterized by a dominant primary tone which is steady over time, surrounded by secondary peaks. The second consists of two unsteady primary tones associated with secondary peaks and the third consists of a hump dominated by several small peaks. A wavelet study allows one to identify an amplitude modulation of the acoustic signal mainly for the unsteady tonal regime. This amplitude modulation is equal to the frequency interval between two successive tones. Finally, a bispectral analysis explains the presence of tones at higher frequencies.

Effect of serrated trailing edges on aerofoil tonal noise

“…An additional study on controlled-diffusion aerofoil was investigated by Padois et al. (2016) and Sanjose et al. (2017).…”

“…An additional study on controlled-diffusion aerofoil was investigated by Padois et al (2016) and Sanjose et al (2017). The latter emphasised quiet and intense time strands where the quiet period is driven by attached boundary layers and the intense period involves an unstable separation bubble.…”

Modal analysis of the laminar boundary layer instability and tonal noise of an airfoil at Reynolds number 150,000

“…[6][7][8][9][10] In addition to this broadband behavior, sharp tones that rise above the broadband noise spectrum have also been experimentally observed for the same airfoil for certain flow configurations. 1,5 Such a tonal noise has been observed within the context of several different applications, including UAVs and low-speed fans. It has been intensively studied on the symmetric NACA0012 airfoil at low to transitional Reynolds numbers since the 1970s 11 and has received renewed interest with improved measurement techniques [12][13][14][15] and with direct numerical simulations (DNS).…”

“…This process typically produces a broadband acoustic signature, as seen, for instance, in the turbulent flow regime ( 8° geometrical angle-of-attack) over a cambered thin controlled-diffusion (CD) airfoil, characteristic of modern low-speed and high-speed fan blade design. Such a flow regime has been extensively studied both experimentally 1–5 and numerically. 6–10 In addition to this broadband behavior, sharp tones that rise above the broadband noise spectrum have also been experimentally observed for the same airfoil for certain flow configurations.…”

Beamforming studies on basic models of low-speed axial fan blade sections

“…As suggested by Roger and Moreau (2010), detailed acoustic measurement data obtained for basic two-dimensional (2D), rectilinear models of blade sections can be adapted to rotating blades, by means of a spanwise segment splitting. Detailed acoustics measurements on a rectilinear airfoil, using multiple microphones, are reported by Padois et al (2016). Beamforming, relying on the phased array microphone (PAM) technique, is a recently emerging methodology for obtaining spatially simultaneously resolved acoustic data on models of blade or wing sections.…”