Abstract
Lattice Boltzmann simulations were carried out to investigate the noise mitigation mechanisms of a 3-D printed porous trailing-edge insert, elucidating the link between noise reduction and material permeability. The porous insert is based on a unit cell resembling a lattice of diamond atoms. It replaces the last 20 % chord of a NACA 0018 at zero angle-of-attack. A partially blocked insert is considered by adding a solid partition between 84 % and 96 % of the aerofoil chord. The regular porous insert achieves a substantial noise reduction at low frequencies, although a slight noise increase is found at high frequencies. The partially blocked porous insert exhibits a lower noise reduction level, but the noise emission at mid-to-high frequency is slightly affected. The segment of the porous insert near the tip plays a dominant role in promoting noise mitigation, whereas the solid-porous junction contributes, in addition to the rough surface, towards the high-frequency excess noise. The current study demonstrates the existence of an entrance length associated with the porous material geometry, which is linked to the pressure release process that is responsible for promoting noise mitigation. This process is characterised by the aerodynamic interaction between pressure fluctuations across the porous medium, which is found at locations where the porous insert thickness is less than twice the entrance length. Present results also suggest that the noise attenuation level is related to both the chordwise extent of the porous insert and the streamwise turbulent length scale. The porous inserts also cause a slight drag increase compared to their solid counterpart.
Highlights
Turbulent boundary-layer trailing-edge (TBL-TE) noise (Brooks, Pope & Marcolini 1989) is one of the main noise generation mechanisms of various aerodynamic bodies, including 926 A17-1C
Unlike serrations, which are often manufactured as TE add-ons, porous materials are usually employed as inserts that replace the aft section of aerofoils
Numerical simulations using the transient, explicit and compressible lattice Boltzmann (LB) solver, SIMULIA PowerFLOW, have been carried out to investigate the TBL-TE noise generated by a NACA 0018 aerofoil
Summary
Turbulent boundary-layer trailing-edge (TBL-TE) noise (Brooks, Pope & Marcolini 1989) is one of the main noise generation mechanisms of various aerodynamic bodies, including. The use of porous medium models introduces another layer of uncertainty into the investigation, and it remains an open question whether the noise reduction mechanisms discussed in Teruna et al (2020) are still appropriate when permeability is realised using a porous geometry instead of an equivalent fluid region. For this purpose, a synthetic unit-cell geometry has been designed and utilised to construct a porous TE insert, which is later manufactured and tested in experiments, allowing for direct comparison with simulation results.
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