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

Flow-induced streamwise oscillation of two tandem circular cylinders has been studied by means of free-oscillation testing in a wind tunnel. One cylinder was elastically supported so as to allow it to move in the streamwise direction; the other was fixed to the tunnel sidewalls. Small values of the reduced mass-damping parameter (Cn ⩽ 1) have been considered. When the upstream cylinder is free to oscillate, there are two excitation regions: the first for reduced velocity, Vr, in the range 1.4 ⩽ Vr ⩽ 2.5 and cylinder gap distance to diameter ratio, s, between 0.3 and 3, is due to movement-induced excitation accompanied by symmetrical vortex shedding, while the second, for 2.7 ⩽ Vr ⩽ 3.7 and 1.75 ⩽ s ⩽ 3, is due to vortex excitation by alternate Karman vortex shedding. On the other hand, the response characteristics when the downstream cylinder is free to oscillate, have an excitation region due to Karman vortex shedding from the two cylinders, connected by the dead water region between them for 2 ⩽ Vr ⩽ 4 and 0.3 ⩽ s ⩽ 0.75, and it is followed by a second excitation region, due to symmetrical vortex shedding, that is limited to 0.75 ⩽ s ⩽ 2. Furthermore an unstable limited cycle oscillation with large amplitude appears for 1 ⩽ s ⩽ 1.25, by giving an initial oscillating displacement to the cylinder. When s is greater than 2.5, the downstream cylinder experiences buffeting.