Flow-induced vibration of tandem flexible cylinders with a larger-diameter upstream cylinder

A systematic experimental study is conducted for the flow-induced vibration (FIV) of tandem flexible cylinder pairs with diameter ratios (D,u/D,d) ranging from 1 to 2. The diameter of the downstream cylinder (DC), D,d, is fixed at 16 mm, while that of the upstream cylinder (UC), D,u, evenly varies from 16 mm to 32 mm. Cylinders with a total length of 2.7 m and an immersion depth of 1.2 m are allowed to freely oscillate in the cross-flow and in-line directions. Results are examined for Re = 1600-11,200 (based on D,d) and a fixed streamwise separation between the cylinder centers T =6D,d. It is shown that the FIV of the DC (DC-FIV) is strongly sensitive to D,u/D,d, and that increasing D,u/D,d significantly weakens the high-mode and high-frequency response of the DC. A deeper analysis of the dual-cylinder interaction reveals the perturbation mechanism of the UC on the DC-FIV, i.e., the UC forces the DC to oscillate following its dominant frequency and dominant mode through "vibration control". A map containing eight response patterns is further provided in the D,u/D,d-Vr plane. The Strouhal number rule between DC-FIV and its reduced velocity, which is broken by the UC, is reconstructed with a variation factor.