An acoustic investigation of non-uniformly structured porous coated cylinders in uniform flow

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.