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.

中文翻译：

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均匀流动中非均匀结构多孔涂层圆柱体的声学研究

将多孔涂层应用于放置在均匀流动中的光滑圆柱体可以降低其涡流脱落音调和整体声压级。负责任的噪音产生机制尚未完全了解，也没有提出最佳的多孔涂层类型。结构化多孔涂层圆柱体 (SPCC) 最近被研究作为使用金属泡沫或聚氨酯的随机多孔涂层圆柱体的替代品。SPCC 具有与随机多孔涂层圆柱体相似的降噪特性，但它们的孔隙率可以在圆周和展向方向上进行修改。在一个小型消声风洞中进行了声学研究，该风洞使用在四个展向段上建造的 SPCC，产生不同的展向和周向孔隙率。使用关于流动轴对称的两个麦克风弧阵列记录声学信号。每个 SPCC 都显示出减少的涡旋脱落音调、一次谐波和高频宽带贡献。对于每个 SPCC，涡旋脱落音比等效的裸圆柱脱落音弱，并且整体声压级降低。周向和展向孔隙率的变化对降低音调噪声几乎没有影响，但会影响高频贡献。