Flow-induced in-line oscillation of two circular cylinders in tandem arrangement

Flow-induced in-line oscillation of two tandem circular cylinders has been experimentally studied by free-oscillation tests in a wind tunnel. Only one cylinder composed of two tandem cylinders was elastically supported easily to move in the in-line direction for reduced mass-damping parameter Cn ≈ 1 and the other of the tandem cylinders was fixed to the tunnel sidewalls. The gap distance ratio between upstream and downstream cylinders was changed from 0.3 to 3. We measured the response amplitudes of the oscillatory cylinder in the in-line direction and the vortex-shedding frequency in the wake. The flow around the tandem cylinders was visualized by the smoke-wire method. The results of in-line oscillation of an upstream cylinder are as follows ; there was a wide excitation region of reduced velocity, Vr = 1.5 to 2.5 in all ranges of gap ratio of 0.3 to 3. This excited oscillation was mainly induced by symmetric vortex shedding ; the other excitation of Vr = 2.8 to 3.5 at wide gap distance ratios of 1.75 to 3, was induced by alternate Karman vortex shedding, as in the case of the in-line oscillation of a single cylinder. The downstream cylinder had an excitation region in a wide range of Vr=2 to 4 for the narrow gap distance ratios of 0.3 to 0.75, which seems to be induced by alternate Karman vortex shedding. The other excitation regions due to symmetrical vortices were limited to the region of gap distance ratio of 0.75 to 2. Furthermore, the downstream cylinder oscillated as a buffeting phenomenon influenced by wake-fluctuation of the upstream cylinder, when gap distance ratio was greater than 2.5.