H.F. Mourão Bento
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6 records found
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Aeroacoustic testing in acoustically disturbed environments
Improvements to closed test section wind tunnel experiments
The investigation of wall cavities for microphone placement was done with Computational Fluid Dynamics simulations, which in turn were validated experimentally. Cavities covered with a mesh cover were investigated, since previous literature shows that these reduce hydrodynamic noise while allowing for the transmission of waves to the microphones. The numerical simulations show that covering microphone cavities with a mesh cover results in a stagnant flow inside the cavities. As consequence, the only source of pressure fluctuations at the cavity bottoms are acoustic waves. The hydrodynamic pressure fluctuations from the wall’s boundary layer still propagate acoustically to the cavity bottoms. The findings show that increasing cavity size, by increasing the cavity opening diameter with respect to the length of the eddies in the turbulent boundary layer, leads to a lower propagation of spurious pressure fluctuations to the cavity bottoms. This in turn leads to an increased signal to noise ratio of acoustic measurements recorded at closed test section wind tunnels.
Wind tunnel wall liners have been characterized based on their viscous resistivity, inertial resistivity and roughness. Several porous liners have been tested experimentally. The aim was to analyze their impact on the aerodynamic properties of the wind tunnel boundary layer, on the generation of spurious noise, and on the absorption of acoustic reflections. The results show that the ideal choice of liner consists of a liner: with high viscous resistivity, which leads to high acoustic absorption; with low roughness, to reduce the impact on the wind tunnel wall’s boundary layer; and with low inertial resistivity, to reduce the generation of spurious noise. The best lining material tested was melamine foam.
The acoustic propagation and acoustic interference in a closed wind tunnel test section were predicted with a FEM acoustic solver. The propagation was modelled on a baseline test section, with fully reflective walls, and on test sections with lined walls. The numerical results were found to give a very accurate prediction of the acoustic experimental tests. It was possible to use the numerical results to improve the post—processing of experimental data. The Green’s function used to process experimental microphone data with beamforming was corrected, using the numerical results. Beamforming with the Green’s function corrected for the acoustically disturbed environment led to a higher beamforming spatial resolution. In addition, the estimated noise levels are more accurate when the correction is used. This improved approach was shown to work for post—processing experimental measurements of a monopole sound source placed at the center of the test section, with and without free—stream flow.
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The investigation of wall cavities for microphone placement was done with Computational Fluid Dynamics simulations, which in turn were validated experimentally. Cavities covered with a mesh cover were investigated, since previous literature shows that these reduce hydrodynamic noise while allowing for the transmission of waves to the microphones. The numerical simulations show that covering microphone cavities with a mesh cover results in a stagnant flow inside the cavities. As consequence, the only source of pressure fluctuations at the cavity bottoms are acoustic waves. The hydrodynamic pressure fluctuations from the wall’s boundary layer still propagate acoustically to the cavity bottoms. The findings show that increasing cavity size, by increasing the cavity opening diameter with respect to the length of the eddies in the turbulent boundary layer, leads to a lower propagation of spurious pressure fluctuations to the cavity bottoms. This in turn leads to an increased signal to noise ratio of acoustic measurements recorded at closed test section wind tunnels.
Wind tunnel wall liners have been characterized based on their viscous resistivity, inertial resistivity and roughness. Several porous liners have been tested experimentally. The aim was to analyze their impact on the aerodynamic properties of the wind tunnel boundary layer, on the generation of spurious noise, and on the absorption of acoustic reflections. The results show that the ideal choice of liner consists of a liner: with high viscous resistivity, which leads to high acoustic absorption; with low roughness, to reduce the impact on the wind tunnel wall’s boundary layer; and with low inertial resistivity, to reduce the generation of spurious noise. The best lining material tested was melamine foam.
The acoustic propagation and acoustic interference in a closed wind tunnel test section were predicted with a FEM acoustic solver. The propagation was modelled on a baseline test section, with fully reflective walls, and on test sections with lined walls. The numerical results were found to give a very accurate prediction of the acoustic experimental tests. It was possible to use the numerical results to improve the post—processing of experimental data. The Green’s function used to process experimental microphone data with beamforming was corrected, using the numerical results. Beamforming with the Green’s function corrected for the acoustically disturbed environment led to a higher beamforming spatial resolution. In addition, the estimated noise levels are more accurate when the correction is used. This improved approach was shown to work for post—processing experimental measurements of a monopole sound source placed at the center of the test section, with and without free—stream flow.
Sound propagation in closed test section wind tunnels suffers from reflections and diffraction, which compromise acoustic measurements. In this article, it is proved possible to improve the post-processing of phased-array microphone measurements by using an approach based on the combination of numerical acoustic simulations and beamforming. A Finite Element Method solver for the Helmholtz equation is used to model the acoustic response of the experimental facility. The simulations are compared with acoustic experiments performed at TU Delft's Low Turbulence Tunnel, using both fully reflective (baseline) and lined test sections. The solver accurately predicts the acoustic propagation from a monopole sound source at the centre of the test section to the microphones in the phased-array, for frequencies in the range 500Hz<f<2000Hz. It is shown that a (lower fidelity) geometric modelling method is unable to precisely predict the acoustic response of the Low Turbulence Tunnel at these frequencies, due to strong acoustic diffraction. The numerical results are used to implement corrections to the post-processing of experimental data. A corrected version of the Source Power Integration method is able to increase the accuracy of the source's noise levels calculation, based on a single numerical simulation with the source at the same location as in the experiment. A Green's function correction increases the beamforming resolution and the source's noise levels estimation accuracy from the beamforming maps, without a priori knowledge of the source's location. Both corrections perform well at processing flow-on acoustic measurements, and the Green's function correction shows an additional benefit. The improvement in beamforming spatial resolution leads to an increase of the signal to noise ratio.
Sound absorbing porous materials are used to line a wind tunnel wall, in order to reduce reflections. However, the lining can have a detrimental effect on the acoustic measurements due to an increase in the noise radiated from the walls. In addition, the aerodynamic fidelity of the tunnel can be affected. In the present study, the influence of the porous materials on the boundary layer aerodynamic characteristics is assessed. The consequent aerodynamic noise scattering is also studied, and compared against the acoustic benefit from absorbing reflections in the test section. Geometric modelling is used to understand the influence of varying absorbing materials in reducing the acoustic interference caused by the reflections. The aerodynamic and acoustic results are related to the roughness, and to the viscous and inertial resistivities of the three porous materials studied. The material with highest roughness (polyester wool) is found to result in the strongest turbulent fluctuations in the boundary layer. However, it is the material with the thickest fibre diameter (PU foam), and consequent highest inertial resistivity, which generates the strongest surface noise scattering. Materials with high viscous resistivity, together with low inertial resistivity, are found to provide good sound absorbing capabilities. The results therefore indicate that the best choice of sound absorbing wall treatment for wind tunnel applications results from minimizing roughness and inertial resistivity, while maximizing viscous resistivity.
Microphone measurements in a closed test section wind tunnel are affected by turbulent boundary layer (TBL) pressure fluctuations. These fluctuations are mitigated by placing the microphones at the bottom of cavities, usually covered with a thin, acoustically transparent material. Prior experiments showed that the cavity geometry affects the propagation of TBL pressure fluctuations toward the bottom. However, the relationship between the cavity geometry and the flowfield within the cavity is not well understood. Therefore, a very large-eddy simulation was performed using the lattice Boltzmann method. A cylindrical, a countersunk and a conical cavity are simulated with and without a fine wire-cloth cover, which is modeled as a porous medium governed by Darcy's law. Adding a countersink to an uncovered cylindrical cavity is found to mitigate the transport of turbulent structures across the bottom by shifting the recirculation pattern away from the cavity bottom. Covering the cavities nearly eliminates this source of hydrodynamic pressure fluctuations. The eddies within the boundary layer, which convect over the cover, generate a primarily acoustic pressure field inside the cavities and thus suggesting that the pressure fluctuations within covered cavities can be modeled acoustically. As the cavity diameter increases compared to the eddies' integral length scale, the amount of energy in the cut-off modes increases with respect to the cut-on modes. Since cut-off modes decay as they propagate into the cavity, more attenuation is seen. The results are in agreement with experimental evidence.