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The aerodynamic noise radiated by the flow past a cylinder in the subcritical regime can be modeled by a quadrupolar sound source placed at the onset position of the vortex-shedding instability that is scattered by the surface with a dipolar directivity. When the cylinder is coated with a porous material, the intensity of the shed vortices is greatly reduced, determining a downstream shift of the instability-outbreak location. Consequently, sound diffraction is less efficient, and noise is mitigated. In this paper, an innovative design approach for a flow-permeable coating based on a further enhancement of such an effect is proposed. The results of phased-microphone-array measurements show that, once the leeward part of the cover is integrated with components that make the flow within the porous medium more streamlined, the quadrupolar source associated with the vortex-shedding onset is displaced more downstream, yielding additional noise attenuation of up to 10 dB with respect to a uniform coating. Furthermore, the same noise-control mechanism based on the weakening of the sound scattering can be exploited when these components are connected to the bare cylinder without the porous cover. In this case, the mitigation of overall sound pressure levels is comparable to that induced by the coated configurations due to the lack of noise increase produced by the inner flow interacting within the pores of the material. Remarkable sound reductions of up to 10 dB and a potential drag-force decrease are achieved with this approach, which paves the way for disruptive and more optimized noise-attenuation solutions. [ABSTRACT FROM AUTHOR]

The link between jet-installation noise and the near-field flow features of the corresponding isolated jet is studied by means of lattice-Boltzmann numerical simulations. The computational set-up consists of a flat plate placed in proximity to a jet, replicating the interaction benchmark study carried out at NASA Glenn. Installation effects cause low-frequency noise increase with respect to the isolated case, mainly occurring in the direction normal to the plate and upstream of the jet's exit plane. It is shown that the Helmholtz number, based on the wavelength of eddies in the mixing layer and their distance to the plate trailing edge, predicts the frequency range where installation noise occurs. Based on the isolated jet near field, scaling laws are also found for the far-field noise produced by different plate geometries. The linear hydrodynamic field of the isolated jet shows an exponential decay of pressure fluctuations in the radial direction; it is shown that the far-field spectrum follows the same trend when moving the plate in this direction. In the axial direction, spectral proper orthogonal decomposition is applied to filter out jet acoustic waves. The resultant hydrodynamic pressure fluctuations display a wavepacket behaviour, which can be fitted with a Gaussian envelope. It is found that installation noise for different plate lengths is proportional to the amplitude of the Gaussian curve at the position of the plate trailing edge. These analyses show that trends of jet-installation noise can be predicted by analysing the near field of the isolated case, reducing the need for extensive parametric investigations. [ABSTRACT FROM AUTHOR]

To analyze the downstream effects of bypass transition strips on a laminar incoming flow, a direct numerical simulation of the fully transient, explicit and compressible Lattice Boltzmann equations is performed. Near wake analysis of a staggered grid of cubic blocks as transition device is compared with a more conventional zigzag strip, to ensure transition to a fully developed conical boundary layer. The staggered grid of blocks is more efficient in stopping the flow and creating large, coherent, flow structures of the size of the blocks, which results in a stronger transition. However, the downstream merging of spanwise created structures is relatively long resulting in higher correlated boundary layers. If the spanwise variation of the zigzag strip is small, the streamwise vortices created merge quicker, resulting in an earlier uncorrelated boundary layer. The self-noise created by zigzag strip is also significantly less than the noise from the staggered grid of blocks and becomes only dominant at high frequency compared to the predicted trailing edge noise [ABSTRACT FROM AUTHOR]