Spacing effect on the vortex-induced vibrations of near-wall flexible cylinders in the tandem arrangement

Abstract

The vibration responses and flow dynamics in the vortex-induced vibrations of two near-wall flexible cylinders in the tandem arrangement are investigated through three-dimensional direct numerical simulations with the spacing ratio s/D =1.5–6 (D = diameter of the cylinder), gap ratio G/D = 0.8, cylinder length of 25D, and Reynolds number of 500. The in-line (IL) and crossflow (CF) vibrations are predominated by the first-order mode along the span. The upstream cylinder oscillates at a higher CF amplitude than the downstream one, and the maximum IL and CF vibration amplitudes of the tandem cylinders are both smaller than those of the single cylinder. The dominant frequencies of IL and CF oscillation are identical for the tandem cylinders, and they are larger than that of the single cylinder. The smaller mean drag and larger rms drag occur on the downstream cylinder than in the upstream counterpart. The difference between the spanwise rms lift of the two cylinders reduces as the s/D increases. Different flow types are observed along the flexible cylinders: at s/D = 1.5–2, an “extended-body regime” and a “reattachment regime” are excited near the two-ends and the middle regions along the span, respectively; at s/D = 3, a reattachment regime and a “co-shedding regime” appear; at s/D = 4–6, the co-shedding regime is observed but with different vorticities related to the vibration amplitudes. At s/D = 3, the wall proximity induces multi-frequencies in both the IL and CF oscillations, compared to single-frequency oscillations in wall-free conditions. The lower IL and higher CF vibration amplitudes are excited in the near-wall conditions. Weak “2S” and the typical 2S vortex shedding patterns are observed in the near-wall and wall-free conditions, respectively.