Vortex dynamics of tandem bare and spiral finned cylinders in cross-flow and their susceptibility to acoustic resonance excitation

Abstract

The vortex dynamics of tandem bare and spiral finned cylinders in the cross flow are experimentally investigated at a spacing ratio of L/Deq=2. Three different fin pitch-to-root diameter ratios (p/Dr) are considered. For high p/Dr, the fins on the downstream cylinder inhibit the shear layer growth and flapping in the cylinders' gap. Shear layer flapping is only observed for finned cylinders with the lowest p/Dr. Moreover, the gap shear layers enclose the downstream cylinder, enabling the formation of large vortex cores. The flow characteristics of finned cylinders are found to be independent of the Reynolds number with higher values of the Strouhal number due to an enlarged vortex formation length. For tandem bare cylinders, acoustic resonance excitation is triggered by two sources: (1) shear layer flapping in the cylinders' gap and (2) vortex shedding in the wake of the cylinders. All the tandem finned cylinders were able to excite acoustic resonance by the latter source. However, only tandem finned cylinders with the lowest p/Dr were capable of exciting resonance by the former source. During this excitation, weaker shear layer flapping resulted in the formation of smaller vortex cores in the finned cylinders' gap. During the second resonance, single vortex pairs formed per cycle in the wake of the downstream finned cylinder, whereas two vortex pairs formed per cycle in the wake of the downstream bare cylinder. This work shows that the addition of fins changes the impinging flow mechanism and the flow topology, which cannot be captured using the equivalent diameter approach.