• Energy Research

boundary layer-trailing edge (TBL-TE) noise. The matrix topology of the insert resembles the lattice of diamond atoms, and thus, it is also referred to as the diamond trailing edge (TE). The porous insert replaces the last 20% of the chord of a NACA 0018 airfoil. The airfoil is set to zero angle of attack and the chord-based Reynolds number equals to 280000. The geometrical details of the 3D-printed insert are replicated in the simulation to allow comparison with the corresponding experimental measurements. The diamond TE is found to reduce noise by up to 10 dB in the low frequency range. At higher frequencies however, the diamond TE causes a slight noise increase. Using a wake survey method, the porous insert is found to cause a minor drag increase compared to its solid counterpart. It is found that the diamond TE produces stronger surface pressure fluctuations, which would have resulted in higher noise intensity according to analytical models. However, by using a source localization method based on the vortex sound theory, it is observed that the increase in pressure fluctuations is primarily due to the exposed pores at the surface of the porous material, which is responsible for the high-frequency excess noise. These analyses also support the argument that a permeable TE produces different acoustic scattering characteristics with respect to the solid TE.

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.

This manuscript presents a numerical investigation of the turbulent boundary layer-trailing edge (TBL-TE) noise reduction with an open-cell porous material. The implementation of the porous media is verified by emulating a facility for characterizing the flow resistivity of the porous material. Subsequently, the porous media is applied on the trailing edge of a NACA 0018 airfoil to examine its capability to mitigate TBL-TE noise. The airfoil is set at zero angle of attack and the chord-based Reynolds number is 2.8 × 10 5. Boundary layer profiles and integral boundary layer quantities have been compared with reference experimental data. The noise reduction obtained with the porous trailing edge at low to mid frequency ranges has been found to be in good agreement with the experiment. However, the simulation is unable to predict the noise increase at high frequency, which is considered due to the neglected surface roughness effects in the adopted porous media model. Conventional beamforming is also used to locate the dominant sound sources. In contrast with the solid trailing edge case, it has been found that the solid-porous interface is the location of the dominant sound source for the porous trailing edge case.

An experimental campaign to study the impact of a distinct type of vortex generator — rod type (RVG), on the flow characteristics and the acoustic far-field pressure of a wind turbine airfoil, is conducted. Airfoils exhibit decreased aerodynamic performance at high inflow angles due to turbulent boundary layer flow separation. RVGs are applied to mitigate the flow separation. However, this benefit is accompanied by an acoustic penalty. An assessment of the impact of RVGs on the far-field noise emission is conducted for the DU96-W-180 airfoil. The evolution of the boundary layer impacted by the rods is analyzed through Particle Image Velocimetry (PIV) measurements. The resulting reduction in the separation zone is observed through oil flow visualization. Analysis of the sound spectrum for airfoils with/without RVGs is conducted for a range of frequencies (300 Hz to 4000 Hz). Results show a reduction of the noise level at relatively low frequencies, at the expense of an increased noise level in the mid-high frequency ranges. While the former is caused by the reduction of the flow separation, the latter is determined by the combined contribution of the noise scattered by the RVG and by the change in boundary layer characteristics at the airfoil trailing edge.