On the Reductions of Airfoil Broadband Noise through Sinusoidal Trailing-Edge Serrations

Abstract

The present study investigates the efficacy of sinusoidal trailing edge (TE) serrations as a passive means for the reductions of airfoil broadband noise, theoretically and experimentally.Comprehensive parametric studies are conducted to determine the effect of serration amplitudes and wavelengths on the noise reduction performance of a NACA airfoil. Initially, the present paper shows the use of the Trailing-Edge Noise Model (TNO) for the accurate predictions of the surface pressure spectrum near the TE and hence the far-field noise using the Wiener-Hopf method. The predicted spectra and the noise reduction levels showed good agreement with the measurements for a wide range of frequencies. The present study reveals that the local maxima of the overall noise reductions occur when the transverse turbulence integral length scale is either 1.2 or 0.2 times the serration wavelength, which corresponds to λ/Λt = 0.833 or 5, where λ and Λt are the serration wavelength and integral length scale. One of the key findings of the paper is that the serration wavelength at which the highest noise reductions occur when the acoustic emissions vary inversely with the modified Strouhal number Sthm (i.e., wsste (ω)/wbl (ω) ∝ 1/Sthm) for narrow (i.e., small wavelengths) and wider serrations (i.e., large wavelengths), where wsste and wbl are the acoustic emissions radiated from the serrated and baseline airfoils. Further, the TE serrations are also observed to reduce leading-edge (LE) noise along with the self-noise, which indicates the efficacy of TE serrations in reducing the total far-field noise.

Effect of sawtooth trailing edge serrations on the reductions of airfoil broadband noise

“…The serrated airfoil with λ = 25 mm, h = 25 mm, 2hꞌ = 30 mm shows much-reduced noise emissions in the interaction noise dominating zone, while the one with λ = 30 mm, h = 25 mm, 2hꞌ = 20 mm indicates lower noise emissions in the self-noise dominating zone. The minimal decrease in interaction noise dominating zone in the first case is because of the destructive interference between the upstream propagating serration tip acoustic radiations with the leading-edge radiations thus creating a feedback loop, thereby reducing the leading edge interaction noise along with the self-noise, as highlighted in Singh et al 24 The lower noise emissions in the self-noise dominating zone in the second case is possibly due to the presence of higher degree of spanwise de-coherence with reduced wall surface pressure fluctuation near the trailing edge which reduces the scattering intensity and reduces the radiation to the acoustic far fields. For all the serrations studied, it is observed that the noise mitigation level found to be minimal with an increase in jet velocity from 20 to 40 m/s.…”

“…The turbulence intensity of the tunnel was measured to be 3% . 24 A detailed analysis of calibration was done by Sushil et al 25 Figure 3(a) and 3(b) show the schematic and picture of the anechoic jet facility. In order to achieve 2-dimensional flow and to avoid tip effects, both baseline and serrated airfoils are placed between two side plates.…”

Predicting aerodynamic characteristics of airfoils using artificial neural network

“…Analysis of Ćow past airfoils has many practical applications in aerodynamics (Ismail et al 2015;Singh et al 2022;Gao et al 2017) and hydrodynamics (Guo et al 2019;Karim et al 2014;Sener and Aksu 2022), such as the design of air vehicles, wind turbines, fans, rudders, and aircraft (Lin et al 2013). The airfoil shape is responsible for producing lift and drag for 1…”

Comparative Investigation on Improved Aerodynamic and Acoustic Performance of Abnormal Rotors by Bionic Edge Design and Rational Material Selection

“…It was proved by numerical simulation that bionic structures can cause changes of streamwise vortices. Besides, the influence of bionic trailing edge fringes on the noise and thrust performance of rotors was also investigated and discussed [8,28,29]. Candeloro et al [30] experimentally confirmed that a sensible noise reduction with strong directivity can be achieved.…”