On the universal trends in the noise reduction due to wavy leading edges in aerofoil–vortex interaction

Abstract

Existing studies suggest that wavy leading edges (WLEs) offer substantial reduction of broadband noise generated by an aerofoil undergoing upstream vortical disturbances. In this context, there are two universal trends in the frequency spectra of the noise reduction which have been observed and reported to date: (i) no significant reduction at low frequencies followed by (ii) a rapid growth of the noise reduction that persists in the medium-to-high frequency range. These trends are known to be insensitive to the aerofoil type and flow condition used. This paper aims to provide comprehensive understandings as to how these universal trends are formed and what the major drivers are. The current work is based on very-high-resolution numerical simulations of a semi-infinite flat-plate aerofoil impinged by a prescribed divergence-free vortex in an inviscid base flow at zero incidence angle, continued from recent work by the authors (Turner & Kim, J. Fluid Mech., vol. 811, 2017, pp. 582–611). One of the most significant findings in the current work is that the noise source distribution on the aerofoil surface becomes entirely two-dimensional (highly non-uniform in the spanwise direction as well as streamwise) at high frequencies when the WLE is involved. Also, the sources downstream of the LE make crucial contributions to creating the universal trends across all frequencies. These findings contradict the conventional LE-focused one-dimensional source analysis that has widely been accepted for all frequencies. The current study suggests that the universal trends in the noise-reduction spectra can be properly understood by taking the downstream source contributions into account, in terms of both magnitude and phase variations. After including the downstream sources, it is shown in this paper that the first universal trend is due to the conservation of total (surface integrated) source energy at low frequencies. The surface-integrated source magnitude that decreases faster with the WLE correlates very well with the noise-reduction spectrum at medium frequencies. In the meantime, the high-frequency noise reduction is driven almost entirely by destructive phase interference that increases rapidly and consistently with frequency, explaining the second universal trend.

Effect of serrated trailing edges on aerofoil tonal noise

“…There exist similar phase interference effects discussed for TE broadband noise (Avallone et al. 2018) and leading edge interaction noise (Turner & Kim 2019).

Figure 12.Surface contour maps of the Fourier transform of the wall pressure jump at the tonal frequency : ( a ) the magnitude in log scale and ( b ) the phase distribution.

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“…10 -10 10 -11 10 -12 10 -13 10 -14 10 0 10 1 this paper. There exist similar phase interference effects discussed for TE broadband noise (Avallone et al 2018) and leading edge interaction noise (Turner & Kim 2019).…”

On the use of leading-edge serrations for noise control in a tandem airfoil configuration

“…In the recent years, a lot of work has been devoted to studying noise reduction achieved by single-wavelength, sinusoidal leading-edge serrations, optimization of their serrationgeometry parameters and inlet conditions to achieve the best noise reduction, and identification of the key non-dimensional parameters and their dependence on the achieved noise reduction. [16][17][18][19][20][21][22][23][24][25] More recently, the idea has been extended to using doublewavelength serrations 26 and more sophisticated serration geometries 27,28 such as chopped-peak and slitted-root, which are shown to provide enhanced noise reduction than the single-wavelength serrations.…”

“…The noise-reduction mechanism of the use of serrations has been studied extensively, analytically, 22 numerically, 18,21 and experimentally. 19,20,29 The serrated leading edges induce an unsynchronized response in span, resulting in smaller amplitudes and derivatives of surface pressure fluctuations along the span.…”

Trailing-edge noise generation from a flat-plate aerofoil interacting with a prescribed vortex

“…An aerofoil interacting with upstream vortical disturbances is considered one of the fundamental sources of aerodynamic noise and has been investigated extensively through a wide range of approaches. This includes the pioneering theoretical work of Amiet [1] for a flat-plate aerofoil subjected to a frozen turbulent stream [2][3][4], further theoretical work based on harmonic gusts [5][6][7][8][9], wind-tunnel experiments [10][11][12] and various numerical simulations [13][14][15][16][17][18][19][20][21]. The majority of the aerofoil noise due to upstream vortical disturbances emanate from the leading edge (LE) where the impinging vortices/eddies scatter into acoustic waves, which is the primary source mechanism.…”