  An oscillatory instability has been observed experimentally on an horizontal
cylinder free to move and rotate between two parallel vertical walls of
distance H; its characteristics differ both from vortex shedding driven
oscillations and from those of tethered cylinders in the same geometry. The
vertical motion of the cylinder, its rotation about its axis and its transverse
motion across the gap have been investigated as a function of its diameter D,
its density s, of the mean vertical velocity U of the fluid and of its
viscosity. For a blockage ratio D/H above 0.5 and a Reynolds number Re larger
then 14, oscillations of the rolling angle of the cylinder about its axis and
of its transverse coordinate in the gap are observed together with periodic
variations of the vertical velocity. Their frequency f is the same for the
sedimentation of the cylinder in a static fluid (U = 0) and for a non-zero mean
flow (U 6= 0). The Strouhal number St associated to the oscillation varies as
1/Re with : St.Re = 3 $\pm$ 0.15. The corresponding period 1/f is then
independent of U and corresponds to a characteristic viscous diffusion time
over a distance ~ D, implying a strong influence of the viscosity. These
characteristics differ from those of vortex shedding and tethered cylinders for
which St is instead roughly constant with Re and higher than here.
