Aeroacoustic performance of cylinders with a circumferential varying porous coating

The vortex shedding tone magnitude of a cylinder in uniform flow can be suppressed by applying a porous coating. The mechanism for the vortex shedding tone suppression is not fully understood, nor is the optimal porous coating to achieve maximum tone suppression. Understanding the internal flow field of Porous Coated Cylinder (PCC) can provide a deeper understanding of the interaction of the porous media and the development of the external flow field and therefore lead to the improved designs of PCCs. This paper presents a flow visualization test conducted in a water tunnel at The University of Adelaide that reveals the internal flow field of a Structured PCC (SPCC). The results of these experiments were then used to design SPCCs that possess two different porosities along the cylinder circumference which were then tested in a small anechoic wind tunnel at Brandenburg Technical University Cottbus-Senftenber using two microphone arc arrays and a hot-wire anemometry probe in the wake and separated shear layer. Each SPCC revealed two tones and a high frequency broadband contribution, yet each tone magnitude was weaker than the equivalent bare cylinder shedding tone and presented decreased overall sound pressure levels. Applying a more densely spaced porous region on the cylinder windward side resulted in stronger higher frequency broadband contributions yet when placed on the cylinder leeward side negligible changes in the acoustic spectrum were recorded. By investigating the coherence between the flow and acoustic signals stronger evidence was gathered to suggest that the higher frequency tone is generated by localized flow behavior in the separated shear layer. Using time-averaged profiles in the wake revealed that the SPCC with uniform porosity around its circumference has the lowest drag coefficient and the cylinder with denser porous spacing on the windward side has the highest drag coefficient.