Flow-induced vibration of a circular cylinder placed in the wake of an upstream cylinder, forced to oscillate in the crossflow direction

Flow-induced vibration (FIV) of a flexibly-mounted cylinder, placed in a tandem arrangement with an upstream cylinder, is studied experimentally. The upstream cylinder was forced to oscillate with a peak-to-peak amplitude of 0.5 times the cylinder’s diameter, with frequencies in the range of 0.5 to 3 times the downstream cylinder’s natural frequency. Amplitudes and frequencies of oscillation, as well as flow forces were studied in the reduced velocity range of U* = 2.9 – 18.1, corresponding to a Reynolds number range of Re = 565 – 3,500. Additionally, qualitative and quantitative flow field measurements were conducted using hydrogen bubble imaging and a volumetric Particle Tracking Velocimetry (PTV) technique, respectively.

The FIV response of the downstream cylinder varied at each forcing frequency ratio due to the different wake dynamics created at each case. For excitation frequency ratios of 0.5 and 1, the lock-in ranges were extended, where for frequency ratio of 1, continuous large-amplitude oscillations persisted to the highest reduced velocity tested. At frequency ratios of 2 and 3, the onset of lock-in was delayed to higher reduced velocities. Contributions of sub- and super-harmonics were observed in the frequency contents of oscillations.