Airfoil noise reductions through leading edge serrations

Abstract

This paper provides an experimental investigation into the use of leading edge serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil's leading edge (LE) and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz to 8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (∆PWL) is sensitive to the amplitude, 2h of the LE serrations but much less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude act as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

On the reduction of aerofoil–turbulence interaction noise associated with wavy leading edges

“…The current inflow turbulence setup results in L t /L c = 0.04 and u rms /u ∞ = 0.025 at the position of the aerofoil LE (calculated without the aerofoil), which was guided by a previous wind-tunnel measurement (Narayanan et al 2015). The calculated synthetic-turbulence spectra that are in good agreement with the von Kármán ones can be found in .…”

“…The relative noise reduction quantified by W (0)/W (h LE ) varying with h LE is plotted in figure 7, where W (0) indicates the SLE case. It is shown in figure 7 that the relative reduction of ATI noise in decibels increases with the amplitude of the WLE profile in an almost linear fashion, which has also recently been observed experimentally by Narayanan et al (2015) and numerically by Lau et al (2013).…”

“…The span of the aerofoil (domain length in the z−direction) is set to L z = 2λ LE as mentioned earlier. These geometric parameters are chosen as they were used in the previous wind-tunnel experiment (Narayanan et al 2015). The current computational setup was validated against the previous experiment as reported in .…”

“…This Mach number corresponds to the flow speed (u ∞ = 80 m/s) used in a previous wind-tunnel experiment by Narayanan et al (2015). Herein, L c denotes the mean chord length of the aerofoil, L z is the span of the aerofoil, h LE is the amplitude of WLE (2h LE being the Peak-to-Root amplitude) and λ LE is the spanwise wavelength of WLE.…”

“…Guided by a prior experimental measurement (Narayanan et al 2015), the authors have performed a series of numerical simulations for 12 different cases of WLE amplitude varying from h LE = 0 (SLE) to h LE = 0.2L c with an increment of L c /60. The WLE wavelength is fixed at λ LE = 2L c /15.…”