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The very soft and flow-permeable plumage is among the special adaptations of the owl that the silent flight is attributed to. Using a specially designed apparatus that provides a low-speed volume flow of air through a small sample of porous material, measurements of the air flow permeability were performed in accordance to ISO 9053 on a total of 39 prepared wing specimen from six different bird species, including three species of silently flying owls and three non-silently flying bird species. The resulting data set described in the present paper contains the static airflow resistance measured at different positions on the wing., (© 2023 The Author(s).)

The application of a porous coating to a smooth cylinder placed in uniform flow can reduce its vortex shedding tone and overall sound pressure level. The responsible noise generation mechanisms are not fully understood nor has an optimal porous coating type been presented. Structured Porous Coated Cylinders (SPCCs) have been recently investigated as an alternative to randomized porous coated cylinders that use metal foam or polyurethane. SPCCs possess similar noise reduction characteristics to randomized porous coated cylinders, yet their porosity can be modified in circumferential and spanwise directions. An acoustic investigation was conducted in a small anechoic wind tunnel using SPCCs constructed in four spanwise segments, yielding varying spanwise and circumferential porosities. Acoustic signals were recorded using two microphone arc arrays symmetric about the flow axis. Each SPCC revealed a reduced vortex shedding tone, a primary harmonic, and a high frequency broadband contribution. For each SPCC, the vortex shedding tones were weaker than the equivalent bare cylinder shedding tone and presented decreased overall sound pressure level. Variations in circumferential and spanwise porosity were shown to have little effect on tonal noise reduction yet influence high frequency contributions.

This paper is concerned with the influence of camber on the noise of a wall-mounted finite airfoil with natural boundary layer transition. Tonal noise measurements taken in an aeroacoustic wind tunnel are presented for airfoils with aspect ratio of 2, NACAxx12 profile and camber between 0 and 6% at 40% chord. The results show camber is an important parameter that determines the operating conditions for which acoustic tone generation occurs and the number and intensity of the tones produced. Airfoils with 0%-2% camber have an acoustic signature that is dominated by a high amplitude primary tone, whereas the spectra of airfoils with higher camber of 4%-6% feature a more pronounced side tone structure. Tonal noise production does not collapse with lift coefficient, demonstrating that the local flow conditions influence the noise source. Tonal noise production is explained in terms of changes to mean flow topology, namely the location of flow separation, which is linked to tonal noise generation. Scaling of airfoil tonal noise is found to vary with angle of attack and pressure gradient. Empirical scaling laws for the primary tone frequency dependence on velocity are also derived for the cambered airfoils.