Spacing ratio effect on the vortex-induced vibration performance of a double-layer rectangular cross section

Vortex-induced vibration (VIV) represents a critical wind-induced large-amplitude oscillation phenomenon. In the field of civil engineering, the prevalent arrangement of multiple bluff bodies in close proximity introduces complex aerodynamic interference that fundamentally alters vortex shedding dynamics. This study conducts comprehensive investigations of flow around single- and double-layer models of a 6:1 rectangular cylinder throughout the complete VIV processes. Wind tunnel investigations reveal distinct behavioral regimes in VIV performance between single- and double-layer cylinders. Then, the spacing effects on aerodynamic characteristics and flow mechanisms are systematically examined via pressure coefficients, time-averaged/fluctuating flow fields, and reduced-order analyses, elucidating distinct vortex-shedding modes governing dual lock-in ranges. Results show that the first range of VIV occurs when motion-induced vortices generated at the leading edge propagate downstream and merge with the vortices shed at the trailing edge, exhibiting reduced spacing sensitivity due to dominant energy extraction from structural inertia rather than fluid–structure momentum transfer. All spacing configurations within the second range ultimately converge to a classical Karman vortex street with sustained amplitude growth driven by fluidic energy influx, and the performance is enhanced with increasing spacing. Comparative analysis under equivalent mass-damping parameters confirms superior VIV performance in the single-layer model relative to closely spaced double-layer systems. The spacing effects on the development of the separation bubble and the variation of the pressure coefficient are particularly pronounced in the first range relative to the second range. Proper orthogonal decomposition analysis confirms the dominance of alternating vortex shedding across both lock-in ranges, with symmetrical shedding modes constituting secondary components.