Experimental study on flow/noise of a circular cylinder with concentric/eccentric microperforated ring fairings

The effects of microperforated ring fairings on the flow and noise of a circular cylinder were investigated experimentally at Reynolds number based on the cylinder diameter (D) ranging between 1.3 and 2.6 × 104. The fairings were installed either concentrically or eccentrically, and the parameters investigated are the perforation rate σ (11.8%–34.6%), the clearance ratio δ/D (0.1–0.3), and the deflection angle θ (0°–180°). The noise was measured using far-field microphones, and flow characteristics were tested by the particle image velocimetry (PIV) and a dynamic balance. The acoustic results showed that the aerodynamic noise of the cylinder generally decreases with the increase in the perforation rate σ and the clearance δ. The maximum noise reduction at the fundamental vortex shedding frequency can reach 25 dB when arranged concentrically at δ/D = 0.3 and σ = 34.6%. Under the same perforation and clearance, the eccentric arrangement at θ = 60°–120° significantly improves the noise control performance. Flow visualization by PIV test demonstrated that the perforated fairing effectively controls the unsteady flow downstream of the cylinder and attenuates the large-scale vortex shedding, resulting in the noise reduction. The force measurement results showed that unsteady lift coefficient is significantly reduced, but the mean drag coefficient of the cylinder together with the fairings is generally higher than the bare cylinder. Nevertheless, the increment in drag coefficient can be lessened by eccentrically arrangement of fairings at deflection angle between θ = 60°–120°.