A secondary modulation mechanism for aerofoil tonal self-noise generation

Abstract

Acoustic emission of a NACA 0012 aerofoil is investigated over a range of free-stream velocities. Acoustic spectra show a dominant tone and two sets of weaker side tones characterised by different frequency intervals. The frequency of the dominant tones in the acoustic spectra varies with velocity in a ladder-type structure. With increasing Reynolds number, the spectrum becomes progressively more broadband in nature. Through synchronised particle image velocimetry and acoustic measurements, the aeroacoustic noise generation mechanisms, resulting in different spectral characteristics and modulation types, are further investigated. A separation bubble and related significant velocity fluctuations are observed on the pressure side. Pressure side velocity spectra show characteristics similar to the acoustic ones, whereas velocity spectra on the suction side feature broadband characteristics. These findings confirm that noise emission is dominated by pressure side events for the Reynolds number range of this study, i.e. $2 \times 10^{5}$ – $7 \times 10^{5}$ . As the acoustic emission is defined by coherent flow structures, the proper orthogonal decomposition method is adopted to facilitate the understanding of the relation between the complex flow field and acoustic emission. Side tones in the acoustic spectra are attributed to two different modulation mechanisms in the aeroacoustic source region near the trailing edge. By aligning the sound pressure time history and the time coefficients of the dominant modes, the primary modulation of the dominant tone is found to be related to the amplitude modulation of the high-frequency velocity fluctuations associated with the acoustic feedback loop. A secondary modulation is attributed to periodic variation of the separation bubble and, therefore, variation in the roll-up of the shear layer, which results in a modulation of the amplitude of the velocity fluctuations associated with the convecting vortices at the trailing edge.

The switching process of trailing edge dominant tones on the different ladders

“…Through Proper Orthogonal Decomposition (POD) analysis from the flow field, Yang (2021) identified different modes from flow field structures, which contributed to the main tone and the secondary tone. A later PIV experiment by Yang (2022) further discoveried two modulations within the flow field velocity fluctuations in accordance with the acoustic feedback loop and LSB variations. Besides PIV results, a transient analysis by microphone array also reveals a similar modes of vortices and sound sources assoicated with the main tone's amplitude modulations (Yu et al 2022).…”

“…Although there have been five decades since the first thoroughly investigation of the trailing edge tonal noise by Paterson (1973), new theories and explanations are still generated continuously based on the state-of-the-art experiments and numerical simulations. Simultaneous measurement by microphones and time-resolved particle image velocimetry (PIV) helps to resolve the coherent flow structure related to the tonal noise (Yang 2021;Yu 2022;Yang 2022), while large eddy simulations (LES) and direct numerical simulations offer insights into the role of laminar separation bubble (Wu et al 2021) and global instabilities related to the secondary tones . With the development of experiment techniques and advanced frequency-time postprocessing methods, the transient and spatial characteristics of trailing edge tonal noise are attracting widespread interests.…”

“…It was the simultanous measurement of flow field and far field noise that made it possibile to detect the flow structure involved with the trailing edge noise sources, especially about the side tones. Since then, a series of PIV experiments had been conducted to investigate the physical meaning of this secondary modulation (Pröbsting 2015a;Yang 2021;Yang 2022). Through Proper Orthogonal Decomposition (POD) analysis from the flow field, Yang (2021) identified different modes from flow field structures, which contributed to the main tone and the secondary tone.…”

Experimental Investigation of Airfoil Instability Tonal Noise Reduction Using Structured Porous Trailing Edges

“…The reduction in an airfoil's self-noise level is one of the practical advantages of reducing the community noise impact of airports that are close to densely populated residential areas. Tonal noise from instabilities is an important airfoil self-noise mechanism at the trailing edge [1][2][3], especially on airfoils operating at low-to-moderate Reynolds numbers and angles of attack, e.g., small, unmanned air vehicles. To reduce the tonal noise from instabilities, a number of bio-inspired aerodynamic noise control techniques have been investigated in the past few decades [4], which can be classified into trailing edge serrations, brush-type extensions, and porous edges.…”

The Influence of Low-Frequency Oscillations on Trailing-Edge Tonal Noise with Symmetry Spanwise Source Regions

“…The dominant tone is related to the steady periodic symmetric shedding vortical structures along the spanwise direction, while the secondary tones are attributed to the intermittent unsymmetric changes in the frequency, time and spatial domains [23]. As a result, interesting transient patterns related to secondary tones have been found, such as switching tones [19,24], side secondary tones [21] and the periodic spanwise movement of sources [22]. Meanwhile, the difference between experiments and simulations [19] has attracted researchers to analyze the occurrence of transient patterns in spatial distributions.…”