Applicability of Nonuniformly Varying the Fin Density of Tandem Finned Cylinders as a Viable Vortex and Noise Suppression Technique

This study investigates the use of nonuniform finned tubes in tandem arrangements with different spacing-to-average equivalent diameter ratios (⁠$L/Deq(avg)$⁠) to suppress flow-induced acoustic resonance. Particle image velocimetry (PIV) was used to visualize vortex development in the wake before and during acoustic resonance, while acoustic pressure measurements characterized the aeroacoustic response. Nonuniform finned tubes were found to weaken the vortex shedding process and reduce the sound pressure level (SPL) by 68% and 50% during flow-induced acoustic resonance at $L/Deq(avg)=2.0$ and 2.5, respectively. However, nonuniform finned tubes do not inhibit the instability of the shear layers within the gap, making them susceptible to acoustic resonance where the shear layer instability is the source of excitation. During acoustic resonance associated with the shear layer instability, highly discrete and well-organized vortex cores form in the gap and wake of both uniform and nonuniform finned tubes, producing similar SPLs to those generated by uniform finned tubes. The findings of this study indicate that varying the fin density along the cylinders can potentially be used to suppress acoustic resonance excitation in normal triangle and rotated square tube arrays where vortex shedding is the main source of excitation at $L/Deq(avg)≤2.5$⁠.