On the role of the flow permeability of metal foams on trailing edge noise reduction

The turbulent flow over a NACA 0018 airfoil with porous trailing edge inserts and the resulting scattered turbulent-boundary-layer trailing edge noise are studied to investigate the effect of the cross-flow through the material. The experiments are performed at a chord-based Reynolds number of 2.63×105 and an angle of attack of 0°. Two different porous inserts, covering 20% of the chord, are manufactured with the same metal foam (cell diameter of 800 µm and permeability of 27×10-10 m2). In order to assess the effect of the flow permeability on the far-field noise open and closed inserts are used. In the first ones the cellular structure is kept open, in the second ones the symmetry plane is filled with adherent material to impede flow communication between the two sides. The fully permeable trailing edge insert reduces noise intensity up to 11 dB for Strouhal number based on the chord lower than 16 while a noise increase is measured at higher frequencies. The non-permeable insert, on the other hand, does not show noise reduction with respect to the solid trailing edge below that Strouhal number but only noise increase at higher frequencies, confirming that flow permeability through the insert is necessary to achieve noise attenuation. The intensity of the noise increase is similar to that of the fully permeable insert thus suggesting that it is generated by the rough surface. The analysis of the mean flow field shows that only minor differences are present between the open and closed porous inserts. However, evidence of cross-flow through the material is found in the analysis of turbulent statistics.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aircraft Noise and Climate EffectsWind Energ