This paper presents an experimental investigation into the aeroacoustic and aerodynamic impact of various flow-permeable fairings having different levels of airflow resistivity, including wire meshes, perforated plates, and 3D-printed materials based on the repetition of diamond-lattice unit cells. The fairings are installed upstream of a scaled LAGOON landing gear model, which incorporates a torque link and brake-like protuberances to replicate realistic noise sources. Acoustic-imaging measurements carried out on the baseline model reveal that these additional components contribute significantly to far-field acoustic radiation, altering both the location and strength of dominant noise sources. The flow-permeable fairings decrease the model loading and turbulence kinetic energy in its wake compared to a fully solid configuration due to less abrupt flow deflection, with a positive impact on undesired noise possibly arising from interactions with downstream, uncovered gear components. Furthermore, fairings characterized by high airflow resistivity offer comparable or superior sound reductions to the solid fairing within a frequency range where the self-noise produced by the airflow through material pores does not dominate. Beyond generating an extensive dataset to support the validation of numerical simulations, this study provides valuable insight into the development of innovative and more efficient passive sound-control solutions for landing gear systems.

A solid fairing and a wire-mesh fairing consisting of very fine wires and pores are numerically and experimentally investigated for the mitigation of landing gear noise. A slightly modified LAGOON landing gear and two configurations, one equipped with a solid fairing and the other with a wire-mesh fairing, are numerically simulated using the Improved Delayed Detached-Eddy Simulation (IDDES) in combination with the Ffowcs Williams and Hawkings (FW-H) analogy. Instead of resolving the detailed flow features through the wire mesh, a recently proposed numerical model is used to represent the effect of the wire-mesh fairing. The simulated flow fields and the far-field noise spectra are validated against the experiments conducted in an anechoic wind tunnel. The superiority of the recently proposed wire-mesh model over a classical wire-mesh model in modelling both the aerodynamic and aeroacoustic effects of the wire mesh is demonstrated. Results also show that the dense wire-mesh fairing functions very similarly to the solid fairing and that significant noise can be reduced through the installation of a solid fairing or a wire-mesh fairing upstream of the landing gears. For the baseline landing gear, the torque link and the brakes are identified noise sources. With the aerodynamic penalty of a 50% increase in drag, both fairings mitigate the pressure fluctuation on the torque link and brakes, resulting in the reduction of surface noise sources. The noise directivity shows that a solid fairing or a dense wire-mesh fairing contributes to a noise reduction of 4-6 dB in all radial directions. The findings in this study pave the way for the low-noise design of aircraft landing gears.

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.

Vortex shedding in the wake of a cylinder in uniform flow can be suppressed via the application of a porous coating; however, the suppression mechanism is not fully understood. The internal flow field of a porous coated cylinder (PCC) can provide a deeper understanding of how the flow within the porous medium affects the wake development. A structured PCC (SPCC) was three-dimensionally printed using a transparent material and tested in water tunnel facilities using flow visualisation and tomographic particle image velocimetry at outer-diameter Reynolds numbers of and, respectively. The internal and near-wall flow fields are analysed at the windward and mid-circumference regions. Flow stagnation is observed in the porous layer on the windward side and its boundary is shown to fluctuate with time in the outermost porous layer. This stagnation region generates a quasi-aerodynamic body that influences boundary layer development on the SPCC inner diameter, that separates into a shear layer within the porous medium. For the first time via experiment, spectral content within the separated shear layer reveals vortex shedding processes emanating through single pores at the outer diameter, providing strong evidence that SPCC vortex shedding originates from the inner diameter. Velocity fluctuations linked to this vortex shedding propagate through the porous layers into the external flow field at a velocity less than that of the free stream. The Strouhal number linked to this velocity accurately predicts the SPCC vortex shedding frequency.

This manuscript presents a numerical investigation of an open-cell 3D-printed perme-able/porous insert used to reduce turbulent 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 2.8 × 10 ⁵ . 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.

In this paper, the performance of four acoustic imaging methods: conventional frequency domain beamforming (CFDBF), functional beamforming (FUNBF), enhanced high resolution CLEAN–SC (EHR–CLEAN–SC) and generalized inverse beamforming (GIBF), is investigated in terms of accuracy and variability. Three experimental test cases are considered: 1) a single speaker emitting synthetic broadband noise, 2) two speakers emitting incoherent synthetic broadband noise, and 3) trailing–edge noise generated by a tripped NACA 0018 airfoil. All the measurements were performed in the anechoic wind tunnel of Delft University of Technology. Overall, GIBF and EHR–CLEAN–SC offer the most accurate results when point sources (speakers) are present. They even achieve super–resolution by separating sound sources beyond the Rayleigh resolution limit. Repeating the measurements indicates a standard deviation in the results of less than 1 dB. When analyzing distributed sound sources, such as trailing–edge noise, CFDBF and FUNBF provide the best performance. This indicates that the acoustic imaging method needs to be selected based on the expected sound source configuration.

The newly refurbished vertical tunnel (V-tunnel) at Delft University of Technology has been redesigned as a state-of-the-art facility for research in aeroacoustics (A-tunnel), as well as fundamental studies in laminar-turbulent transition and flow control. This manuscript focuses on the design and refurbishment aspects of the facility, including a description of the main modifications in the supporting structures and the drive system of the fan, with details of the flow conditioning and anechoic performance. A rigorous aeroacoustic and aerodynamic characterization of the facility is also presented, benchmarking the flow quality and acoustic performance of the new wind tunnel with respect to other aeroacoustic facilities across the world.

An experimental study on broadband noise scattered by permeable trailing edges with different pore arrangements is performed. A NACA 0018 airfoil with chord c = 0.2 m is investigated at chord-based Reynolds numbers ranging from 1.4 × 10⁵ to 3.8 × 10⁵ and angles of attack of 0.2 and 5.4 degrees. Noise emission from five 3D-printed perforated trailing-edge inserts, with channels normal to the chord, is measured with a microphone antenna. For comparison, inserts manufactured with metallic foams, with comparable flow permeability K but more tortuous pore paths, are also analysed. All the inserts have a permeable extension s equal to 20% of the chord (s/c = 0.2). It is shown that noise mitigation ΔL_p, computed as the difference between far-field noise scattering from solid and permeable edges, collapse when nondimensionalizing frequency as Strouhal number based on the chord. From the collapsed data, it is observed that the maximum noise attenuation reported for each insert ΔL_p,max reaches an asymptotic value of 9.3 dB for increasing K. To parameterize such asymptotic behaviour, noise reduction levels are fitted to a newly proposed relation ΔL_p,max=γ₁tanh(γ₂K) (where γ₁ and γ₂ are fitting coefficients, that depend on the type of insert and angle of attack). Following this analysis, limit permeability values for perforated and metal foam inserts of K = 3.5 × 10⁻⁹ and 1 × 10⁻⁹ m² are found, respectively; above these thresholds, less than 1 dB additional noise mitigation is reported; below, a difference in ΔL_p,max of up to 4 dB for a given K is measured depending on the pore organization. Consequently, the tortuosity of the permeable structure is identified as an additional parameter (to K) controlling noise attenuation. It is also observed that the acoustic performance of lower-permeability edges is less sensitive to changes in the angle of attack. Tests for permeable lengths equal to s/c = 0.05 and 0.1 are performed: the change of ΔL_p,max with increasing s/c is also properly described with a hyperbolic tangent, evidencing equally good performance in noise reduction for all measured extents. Finally, for the most permeable insert with periodic pore arrangement, an extremely loud tonal noise caused by vortex shedding (+30 dB higher than broadband levels) from the blunt solid-permeable junction at s/c=0.8 is reported. Since applying a longer permeable surface, or increasing the permeability at the trailing edge decreases the aerodynamic performance of the blade, a permeable trailing edge with s/c = 0.05, K = 1 × 10⁻⁹ m² and tortuosity of 1.15 is recommended to optimize broadband noise abatement and avoid shedding-related tones for the conditions explored in the current study.

The present work analyses broadband noise scattering from permeable trailing edges with identical micro-structure but under a change of temperature. Experiments are performed in an anechoic wind tunnel using a NACA0018 airfoil at chord-based Reynolds numbers between 1.88 × 10⁵ and 3.14 × 10⁵ and no incidence. A microphone array is used to determine far-field sound pressure level changes upon trailing edge heating. It is found that broadband noise emission can be actively controlled by varying the temperature of the porous trailing edge inserts. Specifically, the electrically heated inserts yield a noise level variation of up to 2.5 dB with the heated one being noisier compared to a baseline, unheated material with similar micro-structure. Resistivity measurements of permeable samples with varying temperature show that flow resistivity increases with the fluid temperature which is in agreement with the reported trailing edge noise increase.

The turbulent flow over a porous trailing edge of a NACA 0018 airfoil is experimentally investigated to study the link between the hydrodynamic flow field and the acoustic scattering. Four porous trailing edges, obtained from open-cell metal foams, are tested to analyze the effects on far-field noise of the permeability of the material and of the hydrodynamic communication between the two sides of the airfoil. The latter is assessed by filling the symmetry plane of two of the porous trailing edges with a thin layer of adhesive that acts as a solid membrane. Experiments are performed at a zero degree angle of attack. Far-field noise measurements show that the most permeable metal foam reduces noise (up to 10 dB) with respect to the solid trailing edge for Strouhal numbers based on the chord below 16. At higher nondimensional frequencies, a noise increase is measured. The porous inserts with an adhesive layer show no noise abatement in the low frequency range, but only a noise increase at higher frequency. The latter is, therefore, attributed to surface-roughness noise. Flow field measurements, carried out with time-resolved planar particle image velocimetry, reveal correlation of near-wall velocity fluctuations between the two sides of the permeable trailing edges only within the frequency range where noise abatement is reported. This flow communication suggests that permeable treatments abate noise by distributing the impedance jump across the foam in the streamwise direction, promoting noise scattering from different chordwise locations along the inserts. This is further confirmed by noise source maps obtained from acoustic beamforming. For the frequency range where noise reduction is measured, the streamwise position of the main noise emission depends on the permeability of the insert. At higher frequencies, noise is scattered from upstream the trailing edge independently of the test case, in agreement with the roughness-generated noise assumption.

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.

Turbulent boundary layer trailing-edge noise scattered by a NACA0018 airfoil equipped with 3D printed perforated trailing-edge inserts, i.e. with straight cylindrical channels connecting the two sides of the airfoil, is investigated. The inserts have different permeability in order to assess the effect of this property on broadband noise generation. Far-field noise is measured with a phased microphone array. The experiments are performed at free-stream velocities of 26 and 41 m/s, corresponding to chord-based Reynolds numbers of 3.4×10 ⁵ and 5.4×10 ⁵, and at angles of attack of 0 and 4.8 ^◦. The inserts, with permeability values of 1.5×10 ⁻⁹ and 5.4×10 ⁻⁹ m ², attenuate respectively up to 5 and 9.5 dB at 0 ^◦ and up to 4 and 7.5 dB at 4.8 ^◦ incidence. The noise abatement of inserts with straight passages is compared with that of inserts manufactured using metallic foams with a random pore distribution but similar permeability. It is found that to achieve similar overall noise attenuation levels, the perforated inserts require at least 3 times higher permeability than the metal foam inserts. From this we conclude that in order to maximize the noise attenuation potential of permeable inserts, the inner structure of the permeable trailing-edge insert must be considered.