Large Eddy Simulation (LES) and Ffowcs Williams-Hawkings acoustic analogy are performed to study the effect of trailing-edge blowing on airfoil self-noise. Simulations were conducted using a National Advisory Committee for Aeronautics 0012 airfoil at zero angle of attack and a chord-based Reynolds number of $4\times {10}^5$ . The aerodynamic and aeroacoustic characteristics of the baseline airfoil were thoroughly verified by comparison with previous numerical and experimental data. The noise reduction effects of continuous and local blowing with different blowing ratios and blowing momentum coefficients were compared. A maximum noise reduction of 20 dB was achieved via trailing-edge blowing and the noise reduction mechanisms of the two blowing methods were discussed. The LES results show a pair of recirculation bubbles in the airfoil wake which are suppressed by trailing-edge blowing. As the blowing vortices convect into the wake, they stretch and stabilize the shear flows from airfoil surfaces. Instantaneous vorticity and root mean square velocity fluctuations are also weakened. There is a decrease in the spanwise coherence and an increase in the phase difference, which contribute to noise reduction. It is concluded that the suppression of turbulence fluctuations in the near wake is the main mechanism of noise reduction for airfoil trailing-edge blowing.

Noise reduction by collars applied to rod–airfoil was studied numerically. The flow field and acoustic far-field are predicted using a large eddy simulation and the Ffowcs Williams and Hawking acoustic analogy. The present numerical method is first validated by existing experimental and numerical results for the baseline case. Then, to reduce interaction noise, a rod with collars is designed (denoted as the Col case). The main noise reduction mechanisms of the collars are investigated in detail. The numerical results show that the collars reduce the noise in the low- and medium-frequency bands of the rod, for which the tonal noise is reduced by 24.83 dB. The airfoil noise throughout the frequency band is thereby reduced as the main sound source. The upstream wake is regularized, and vortex shedding is suppressed. The surface pressure fluctuations along the rod, leading edge, and trailing edge of airfoil exhibit an obvious attenuation in the Col case compared with the baseline, which leads to a decrease in the sound source strength. It is also found that there exist spanwise decorrelation and decoherence effects along the rod with collars, which means the evolution of the turbulent vortices is regularized and the physical size of eddies is minified.

A comprehensive experimental study was carried out to investigate the effectiveness of a local blowing through a structured porous-coated cylinder (SPCC) as a novel technique for vortex-induced noise suppression in subcritical Reynolds number. The active flow control method was applied using a special design chamber with some holes distributed along two spanwise lines at angles 𝜃𝑏 = ±130.9○. The near- and far-field noise measurements were simultaneously carried out to shed more light on the physical phenomenon and underlying noise-reduction mechanism of the application of the blowing to the SPCC. A highly instrumented cylinder with several pressure taps distributed around the circumference of the cylinder at midspan was used to analyze the near-field noise using remote-sensing techniques of the fluctuating pressure fields. Although quite similar acoustic spectra were demonstrated by the SPCC with and without blowing, it was found that applying the local blowing to the SPCC causes a significant reduction of the vortex shedding tone produced by the bare cylinder, for any of the non-dimensional equivalent momentum coefficient considered in this study. A substantial reduction in energy content of the surface pressure fluctuations over the whole frequency range has also been observed for the SPCC with and without blowing especially in the post-separation region. The SPCC, with or without the local blowing resulted in higher frequency contributions that were shown to be independent of the surface pressure fluctuations. The near- to far-field coherence revealed that the increased noise observed for the SPCC with and without blowing configurations is solely due to the near-field non-propagating hydrodynamic field.

This study employs a three-dimensional wall-resolved large eddy simulation in conjunction with the Ffowcs Williams–Hawkings acoustic analogy to systematically investigate the evolution of aerodynamic and aeroacoustic characteristics of a forced rotating cylinder over a Reynolds number range of 0.5×104 to 6×104 and a rotation ratio range of α=0–2. The results indicate that forced rotation can significantly reduce aerodynamic drag by delaying flow separation, with a maximum drag reduction of 82% (Re = 6×104, α=2). Simultaneously, it enhances lift by amplifying the negative pressure peak on the upper surface of the cylinder, leading to a maximum increase in lift-to-drag ratio by up to 18 (Re = 2×104, α=2). Moreover, forced rotation effectively suppresses both tonal noise from vortex shedding and broadband noise from turbulence, achieving a maximum reduction in overall sound pressure level (OASPL) of about 20 dB in the far field (Re = 6×104, α=2). Normalized noise analysis reveals that the far-field noise peak and OASPL can be predicted by 0.4α and 0.1α scaling laws, respectively, when the Reynolds number exceeds 0.5×104. Further analysis shows that Reynolds number effects are more pronounced at low rotation ratios (α ⩽ 1) and diminish under high rotation ratios (α⩾1). Additionally, forced rotation demonstrates superior control performance in flow reconstruction and noise suppression at higher Reynolds numbers. Evaluation of control efficiency confirms the engineering feasibility and potential of the forced rotation technique across a broad range of operating conditions.