Aerofoil noise consists of five mechanisms: interaction of turbulent boundary layer and trailing edge (TE); interaction of laminar boundary layer and TE; separation stall; vortex shedding caused by a blunt TE; tip vortex. The turbulent-boundary-layer trailing edge (TBL-TE) noise and laminar-boundary-layer trailing edge (LBL-TE) noise are two major parts of the aerofoil self-noise, which feature as broadband and tonal noise spectra, respectively. The noise reduction treatments are generally inspired by features of the owl’s feathers (e.g., comb, flexible TE fringe, and velvet) due to their silent flight. For instance, the serrated and porous TEs are generally applied to mitigate noise. Recently, flexible TE serrations and flexible flaplets have also demonstrated potential for noise reduction. The theoretical part of this study takes into account both the flexibility and porosity, which are two distinctive features of the owl’s feathers. An aerofoil is simplified as a finite rigid plate. A theoretical model of wave scattering by a finite plate with a poroelastic extension using the unified transform method is constructed. Parametric studies for elasticity, porosity, and extension length are also conducted over a wide frequency range to explore the combined effects of these variables. Results show that at a low frequency (k
0 = 0.1, where k
0 is the dimensionless frequency), porosity can achieve more noise reductions than elasticity. Elasticity generally enhances noise reductions, especially when combined with low porosities. At a high frequency (k
0 = 10), the maximum noise reduction can be achieved through either high elasticity or large porosity, which is, however, still smaller than that at low frequencies. The extension length is more effective in noise reduction when applied with large porosity at a low frequency or high elasticity at a medium or high frequency.
Moreover, so far, there exist only limited experimental investigations on flexible TE. In addition, the flexible TEs are always combined with shape designs, i.e., flexible serrations or flaplets. However, the effect of a single flexible TE on TBL-TE noise and LBL-TE noise remains elusive. Herein, a single flexible TE attached to a symmetric NACA 0012 aerofoil is considered. On the one hand, the effect of the flexible TE on TBL-TE noise is studied. The acoustic measurement is conducted in an anechoic wind tunnel and the chord-based Reynolds number is between 1.93 × 10
5 and 1.16 × 10
6. The aerofoil is at a zero angle of attack, and the boundary layer is tripped near the leading edge. Flexible strips are inserted into the slit along the chord line and extend out of the TE of the aerofoil to form a flexible TE. Flexible strips of different material types, lengths, and thicknesses are also studied. The deformation characteristics of the flexible TE are investigated using two high-speed cameras. The Digital Image Correlation algorithm in the LaVision DAVIS 10.2 software is applied to obtain the spatial distribution of the out-of-plane deformation. Compared to the aerofoil without the flexible TE, the aerofoil equipped with short strips reduces noise by approximately 1 − 3 dB at low to medium frequencies around 400 Hz to 850 Hz for low free-stream velocities of 15 m/s, and at around 400 Hz to 1500 Hz for 20 m/s. Long strips made of PET, PP, and PVC are effective in reducing noise at both low and high free-stream velocities. The upper limit of the noise reduction frequency f increases with velocity U and follows a scaling law of f ∼ U
1.5. However, long strips all exhibit peaks at frequencies close to their natural frequencies at both low and high free-stream velocities, which are caused by the bending deformation at low velocities, and both bending and torsional deformations at high velocities. Thin flexible strips are more susceptible to flutter as the velocity increases. During flutter, notable vibrations on the upper section of the strip observed at several harmonics contribute to the significant harmonic noise.
On the other hand, the effect of the flexible TE on aerofoil tonal noise is studied. Flexible strips of varying material types, lengths, and thicknesses are also investigated. The chord-based Reynolds numbers are ranging from 1.93 × 10
5 to 5.02 × 10
5. The aerofoil is set at an angle of attack of 1.5º. Deformation measurement is conducted, along with planar Particle Image Velocimetry measurements in the near field and wake of the TE. The aerofoil with short PP strips generally leads to a noticeable decrease in noise levels without shifting the frequency of the dominant tones compared to the aerofoil without flexible TE. Long strips exhibit significant noise decreases at dominant tones and harmonics at lower free-stream velocities, followed by a peak-locking effect at higher free-stream frequencies and velocities, and almost complete suppression of broad humps at even higher free-stream velocities. Thin strips show jumps of dominant tones at different frequencies as the free-stream velocity increases. The deformation results show that a high vibration energy can be observed at the frequency close to the tonal noise frequency of the baseline configuration at a low velocity, while at high velocity, the deformation of the strip is dominated by the deformation near the natural frequency. The flow results in the near field of TE indicate that the proper orthogonal decomposition mode shapes for vertical velocity fluctuations near the TE are primarily characterized by coherent positive and negative velocity fluctuations on both the pressure and suction sides, with frequencies close to those observed in acoustic and deformation results. In addition, the flexible strip enhances the vertical velocity fluctuations on the suction side of the aerofoil but reduces them in the wake at a low free-stream velocity, resulting in lower tonal noise overall. At a high free-stream velocity, the flexible strip is sufficiently excited and shows significant deformation at its natural frequency. This deformation locks the vortex shedding frequency in the flow, resulting in the peak-locking effect observed in the acoustic results.
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