Flow-Induced In-Line Oscillation of Two Circular Cylinders in Tandem Arrangement

Abstract

Flow-induced in-line oscillation of two tandem circular cylinders has been experimentally studied by free-oscillation tests in a wind tunnel. Only one of the two cylinders was elastically supported easily to move in the in-line direction for reduced mass-damping parameter Cn ≈ 1; the other of the tandem cylinders was fixed to the tunnel sidewalls. The gap between upstream and downstream cylinders was changed from s = 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. As a result of the upstream cylinder, there was a wide excitation region of reduced velocity, Vr = 1.5 to 2.5 in all ranges of gap ratios of s = 0.3 to 3. This excited oscillation was mainly induced by symmetric vortex shedding; the other excitation of Vr = 3 to 3.5 at wide gap ratios of s = 2 to 3, was induced by alternate Karman vortex shedding, as with 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 of s = 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 regions of gap distances of s = 0.75 to 2. Furthermore, the downstream cylinder oscillated as a buffeting phenomenon influenced by wake-fluctuation of the upstream cylinder, when gap distance was greater than s = 2.5.

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“…Due to its convenient and lowcost merits, flow over multiple cylinders has attracted the interest of a number of investigators. Uniform flow past two identical circular cylinders in tandem [35,36], side-by-side [37] or generally staggered arrangements [38] have been studied extensively using either numerical or experimental methods. Reynolds number and the gap between the cylinders are the two main parameters determining the fluid forces and flow regimes.…”