Flow-induced vibration analysis of two unequal-diameter flexible cylinders in staggered configuration from a hydrodynamic force perspective

Abstract

This paper presents an experimental study on the hydrodynamic features of two unequal-diameter flexible cylinders in staggered arrangement subject to flow-induced vibration (FIV). Five position angles (α = 15°, 30°, 45°, 60°, and 75°) are defined between the tandem (α = 0°) and side-by-side (α = 90°) cases with the centre-to-centre spacing being fixed at P = 6.0d, where d is the diameter of the small cylinder. The time-varying strains obtained from the experimental tests are converted into displacements, and then the hydrodynamic force coefficients of the two cylinders are derived using an inverse analysis method. It is found that the variation trends of the cross-flow (CF) hydrodynamic coefficients with the reduced velocity do not demonstrate qualitative differences from those of their isolated counterparts except in the mode transition regions. The downstream cylinder experiences stronger influence from the upstream cylinder on its CF hydrodynamic characteristics especially at small α. The more frequent mode transitions in the in-line (IL) direction lead to the multi-frequency responses associated with changes in the IL hydrodynamic coefficients. The typical wake-induced flutter (WIF) phenomenon is observed when Vrs = 5.01, 8.77 and 16.28–25.05 for the downstream small cylinder in Case I and α = 75°. The influence of the characteristics of the WIF phenomenon on the hydrodynamic force is evident because of the nonexistence of the high-frequency components in the oscillations.