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The different types of instabilities of free cylinders (diameter $D$, length$L$) have been studied in a viscous flow (velocity $U$) between parallelvertical walls of horizontal width $W$ at a distance $H$: the influence of theconfinement parameters $D/H$ and $L/W$ has been investigated. As $D/H$increases, there is a transition from stable flow to oscillations transverse tothe walls and then to a fluttering motion with oscillations of the angle of theaxis with respect to the horizontal. The two types of oscillations may besuperimposed in the transition domain. The frequency $f$ of the transverseoscillations is independent of the lateral confinement $L/W$ in the range:0.055 \le L/W \le 0.94$ for a given cylinder velocity $V_{cx}$ and increasesonly weakly with $V_{cx}$. These results are accounted for by assuming a 2Dlocal flow over the cylinder with a characteristic velocity independent of$L/W$ for a given $V_{cx}$ value. The experimental values of $f$ are alsoindependent of the transverse confinement $D/H$. The frequency $f_f$ of thefluttering motion is significantly lower than $f$: $f_f$ is also nearlyindependent of the cylinder diameter and of the flow velocity but decreasessignificantly as $L/W$ increases. The fluttering instability is then rather a3D phenomenon involving the full length of the cylinder and the clearancebetween its ends and the side walls.

Transverse and lateral confinement effects on the oscillations of a free cylinder in a viscous flow