Experimental characterization of the turbulent boundary layer over a porous trailing edge for noise abatement

The hydrodynamic and acoustic fields for a NACA 0018 with solid and porous trailing edge inserts are investigated. The porous inserts, covering 20% of the chord, are manufactured with metal foams with cell diameters of 450 and 800 μm and permeability values of 6 × 10−10 and 2.7 × 10−9 m2. The experiments are performed at a chord-based Reynolds number of 2.63 × 105 and an angle of attack of 0°. The porous trailing edge with higher permeability provides up to 11 dB noise attenuation with respect to the solid case for frequencies below a cross-over Strouhal number St = 0.26. Lower noise abatement (up to 7 dB) takes place below St = 0.3 for the insert with lower permeability. Conversely, noise increase with respect to the solid case is measured above the previously defined St value. A decrease in turbulence intensity is reported (up to 3% of the free-stream velocity), with lower intensity being measured for the insert with lower permeability. It is also observed that the permeability of the insert is linked to the increase of the anisotropy of highly energetic turbulent motions, being stretched in the streamwise direction, and the reduction of the eddy convection velocity (up to 20% with respect to the baseline case). In view of the results, the reduction of the velocity fluctuations is proposed as one of the mechanisms for low frequency noise abatement, being more relevant for the metal foam insert with lower permeability.