Abstract: Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° angles of attack. At 0° angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at positive angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract: [Figure not available: see fulltext.].

A hybrid Lattice-Boltzmann Model (LBM) Very Large Eddy Simulation (VLES) solver for high-speed non-isothermal subsonic flows is used to simulate the unsteady jet flow exhausting from a single axi-symmetric nozzle, as well as the associated noise spectra and directivity. The jet exit Mach number and temperature ratio are set according to the various setpoints from the NASA SMC000 experimental campaign. Both isothermal and heated core flows are considered. The far-field noise is computed through a Ffowcs-Williams and Hawkings (FW-H) analogy applied to a fluid surface encompassing the jet plume. Both time- and frequency-domain formulations are used, the latter in combination with an azimuthal Fourier transform of the linear source terms to analyze the contribution of the different azimuthal components. A resolution study is carried out for both aerodynamic and acoustic results. The near- and far-field results confirm that the underlying flow features and noise mechanisms are fully represented by the numerical solution. A physical analysis of the source mechanisms for a heated core case is performed through a wavelet decomposition applied to the turbulent flow to separate the coherent flow motion, usually attributed to the hydrodynamic fluctuations, and the chaotic perturbations, which have a more dominant acoustic character. The two separated contributions extracted from the flow on the FW-H surface are used to compute the corresponding far-field acoustic contributions. Interestingly, noise is practically generated only by the coherent flow motion. Finally, the frequency-domain FW-H formulation is used to analyze the different azimuthal flow perturbation modes. Strong convection effect along the various modes are found.

A hybrid Lattice-Boltzmann Model (LBM) Very Large Eddy Simulation (VLES) solver for high-speed non-isothermal subsonic flows is used to simulate the unsteady jet flow exhausting from a single axi-symmetric nozzle, as well as the associated noise spectra and directivity. The jet exit Mach number and temperature ratio are set according to the various setpoints from the NASA SMC000 experimental campaign. Both isothermal and heated core flows are considered. The far-field noise is computed through a Ffowcs-Williams and Hawkings (FW-H) analogy applied to a fluid surface encompassing the jet plume. Both time- and frequency-domain formulations are used, the latter in combination with an azimuthal Fourier transform of the linear source terms to analyze the contribution of the different azimuthal components. A resolution study is carried out for both aerodynamic and acoustic results. The near- and far-field results confirm that the underlying flow features and noise mechanisms are fully represented by the numerical solution. A physical analysis of the source mechanisms for a heated core case is performed through a wavelet decomposition applied to the turbulent flow to separate the coherent flow motion, usually attributed to the hydrodynamic fluctuations, and the chaotic perturbations, which have a more dominant acoustic character. The two separated contributions extracted from the flow on the FW-H surface are used to compute the corresponding far-field acoustic contributions. Interestingly, noise is practically generated only by the coherent flow motion. Finally, the frequency-domain FW-H formulation is used to analyze the different azimuthal flow perturbation modes. Strong convection effect along the various modes are found.

Trailing-edge serrations are add ons retrofitted to wind-turbine blades to mitigate turbulent boundary-layer trailing-edge noise. This manuscript studies the physical mechanisms behind the noise reduction by investigating the far-field noise and the hydrodynamic flow field. A conventional sawtooth and a combed-sawtooth trailing-edge serration are studied. Combed-sawtooth serrations are obtained by filling the empty space between the teeth with combs (i.e. solid filaments). Both serration geometries are retrofitted to a NACA 0018 aerofoil at zero degree angle of attack. Computations are carried out by solving the explicit, transient, compressible lattice Boltzmann equation, while the acoustic far field is obtained by means of the Ffowcs Williams and Hawkings analogy. The numerical results are validated against experiments. It is confirmed that the combed-sawtooth serrations reduce noise more than the conventional sawtooth ones for the low- and mid-frequency range. It is found that the presence of combs affects the intensity of the scattered noise but not the frequency range of noise reduction. For both configurations, the intensity of the surface pressure fluctuations decreases from the root to the tip, and noise sources are mainly located at the serrations root for the low- and mid-frequency range. The presence of the filaments generates a more uniform distribution of the noise sources along the edges with respect to the conventional serration. The installation of combs mitigates the interaction between the two sides of the aerofoil at the trailing edge and the generation of a turbulent wake in the empty space between teeth. As a result, the inward (i.e. from the serration edge to the centreline) and outward (i.e. from the serration centreline to the edge) flow motions, due to the presence of the teeth, are mitigated. It is found that the installation of serrations affects the surface pressure fluctuations integral parameters. Both the spanwise correlation length and convective velocity of the surface pressure fluctuations increase with respect to the baseline straight configuration. When both quantities are similar to the one obtained for the straight trailing edge, the effect of the slanted edge is negligible, thus corresponding to no noise reduction. It is concluded that the changes in sound radiation are mainly caused by destructive interference of the radiated sound waves for which a larger spanwise correlation length is beneficial. Finally, the difference between measurements and the literature is caused by an incorrect modelling of the spanwise correlation length, which shows a different decay rate with respect to the one obtained for a straight trailing edge.

The scope of this paper is to perform a detailed analysis of the unsteady flow properties in proximity of the trailing-edge of a lifting free transition NACA 64-618 extruded airfoil. The natural transition cases 6 and 7 of the AIAA workshop on Benchmark Problems for Airframe Noise Computations (BANC-V Category 1) are considered as references. The numerical flow solution is carried out by using the fully explicit, transient and compressible lattice-Boltzmann equation implemented in the CFD/CAA solver Exa PowerFLOWR. The acoustic far-field is obtained by using the Ffowcs-Williams and Hawking integral solution applied to the wing surface. In addition, the validity of Roger and Moreau's trailing- edge model, fed with Schlinker-Amiet's and Rozenberg's wall-pressure spectrum model, is checked by comparison with both experimental and numerical results. The influence of the computational grid on the turbulent boundary layer statistics at the trailing-edge is documented as well. As conclusive effort, the effectiveness of sawtooth serrations on noise reduction is investigated by considering two different flap angles at a fixed airfoil incidence.

Far-field noise and flow field over a novel curved trailing-edge serration (named as iron-shaped serration) are investigated. Spectra of the far-field broadband noise, directivity plots and the flow-field over the iron-shaped serration are obtained from numerical computations performed using a compressible Lattice-Boltzmann solver. The new design is compared to a conventional trailing-edge serration with a triangular geometry. Both serration geometries were retrofitted to a NACA 0018 airfoil at zero degree angle of attack. The iron-shaped geometry is found to reduce far-field broadband noise of approximately 2 dB more than the conventional sawtooth serration for chord-based Strouhal numbers St_c<15. At higher frequencies, the far-field broadband noise for the two serration geometries has comparable intensity. Near-wall velocity distribution and surface pressure fluctuations show that their intensity and spectra are independent on the serration geometry, but a function of the streamwise location. It is found that the larger noise reduction achieved by the iron-shaped trailing-edge serration is due to the mitigation of the scattered noise at the root. This effect is obtained by mitigating the interaction between the two sides of the serration, by delaying toward the tip both the outward (i.e., the tendency of the flow to deviate from the centerline to the edge of the serration) and the downward (i.e., the tendency of the flow to merge between the upper and bottom side of the serration) flow motions present at the root of the sawtooth.

The current study analyzes the flow topology and acoustic emission around straight, sawtooth serrated and slitted-sawtooth serrated trailing edges using a NACA0018 airfoil at zero degree angle of attack. By using this specific setup, pressure differences between both airfoil sides are avoided so that the focus will purely lie on the mean and fluctuating velocity. The flow field is analyzed by evaluating the fully explicit, transient, compressible Lattice Boltzmann equation. Acoustic perturbations are directly obtained from the computational measurement domain, as well as by means of a Ffowcs Williams and Hawking and Curle integral solution. Noise reductions up to 6 dB are achieved for the sawtooth serrated edge in comparison with the straight trailing edge case, while the slitted-sawtooth edge reaches a maximum of only 5 dB. At low frequencies, the solid-sawtooths outperform the slitted-sawtooths. The general trend in term of velocity results shows favorable streamwise, wall-normal and spanwise quantities for the slitted-sawtooth design, while the solid-sawtooth design shows less fluctuations in both streamwise, wall-normal and spanwise direction. Especially the increase of local, wall-normal fluctuations over the slitted-sawtooth serrated edge, enchances the generation of acoustic pressure waves, making it a less effective trailing edge noise-suppression add-on.