Effects of spacing ratio on vortex-induced vibration of twin tandem diamond cylinders in a steady flow

Vortex-induced vibration of twin tandem square cylinders at an inclined angle of 45° to the fluid, i.e., twin diamond cylinders of mass ratio m* = 3, is numerically investigated at Reynolds number Re = 100 and reduced velocity Ur = 3–18. This paper focuses on the effects of cylinders' spacing ratio L* (=L/B, where L is cylinders' center-to-center spacing and B is the characteristic length) ranging from 2 to 6 on the oscillation responses of two-degree-of-freedom cylinders. The results indicate that the wake structure experiences two gap flow patterns, the reattachment and co-shedding regimes, and eight different wake modes. At a small spacing (L* = 2–3), the reattachment regime occurs for the lower or higher Ur with the approximate range of 3 and 16–18. Meanwhile, the reattachment regime mainly occurs for other ranges of Ur at L* = 2–6. The more significant oscillation of each spacing appears in the cross-flow direction, and the maximum cross-flow amplitude of the upstream cylinder is smaller than that of the downstream cylinder. Additionally, although significant cross-flow oscillations occur at small spacings (L* = 2–3) with the Ur ≈ 5–9 and 12–14, the intrinsic mechanisms are entirely different. For the cross-flow oscillation characteristics of larger spacings (L* = 4–6), they are virtually similar.