Vortex shedding noise and flow mode analysis of cylinder with full/partial porous coating

Abstract In this paper, the characteristics of vortex shedding and noise of the circular cylinder with full and partial porous coating are studied numerically to learn the optimal structure design of coating structure. Mode analysis methods including Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are employed to identify the potential mechanism of flow modulation by porous coating. The present results show: 1) When the thickness of full porous coating is within a reasonable range (the relative thickness is 0.2 to 0.8), the vortex shedding and noise is impacted significantly. For the partial porous coating, the noise reduction can be achieved up to 7.1 dB when 1/4 of the cylinder surface is coated. 2) Change of circumferential positions/range of coating have different effects. The frequency of the vortex shedding can be reduced obviously when the position before separation point (θ = ±45°∼±90°) is coated. And coating around the separation point (θ = ±90°∼±135°) has effect on the attenuating the amplitude of vortex shedding. 3) The POD mode analysis reveals that porous coating increases the dominant modes energy associated with lift and decreases the modes energy ratio associated with drag, which is the main reason for the reduction of cylinder drag. 4) The DMD analysis indicates that porous coating can improve the stability of the flow field which consequently reduces the flow-induced noise.