The sedimentation and fluttering (angular oscillation of the axis) of straight cylinders are studied in a viscous fluid at rest filling a vertical Hele-Shaw cell for different density contrasts ${\rho }_s-{\rho }_f$ and fluid viscosities ${\mu }_f$ and for two cylinder densities ${\rho }_s$ and diameters $D$. The influence of confinement in the cell is studied by comparing the present results to those of the literature for nonconfined fluids. While the confinement and the cylinder length $L$ both influence strongly the mean sedimentation velocity $V_s$, the characteristics of the fluttering instability are much more similar in the confined and nonconfined cases. While the drag coefficient is nearly constant in a nonconfined fluid, it is larger here and depends both on $L$ (due to flow blockage) and on the Reynolds number ${\text{Re}}_D=V_{s}D{\rho }_f/{\mu }_f$; the inertial and viscous drag components have equal magnitudes for ${\text{Re}}_D\simeq 40$. For fluttering, instead, the key parameter is the Froude number $\text{Fr}=V_s/V_g [V_g=\sqrt{({\rho }_s-{\rho }_f)gL/{\rho }_f}]$, and the fluttering oscillations vanish below $\text{Fr}\sim 0.07$ for all cylinders and fluids investigated. Above this threshold, the angular amplitude increases with $\text{Fr}$ up to a plateau value, while that of the horizontal oscillations is, at first, very large and then decreases; both amplitudes are reduced when the viscous drag is dominant, but, if inertial drag is dominant, all data points follow a common trend. For all fluids and cylinders, too, the fluttering frequency varies as $f=0.102V_g/L$. These features of fluttering are generally qualitatively similar to those reported in nonconfined fluids, but this instability is observable down to lower ${\text{Re}}_D$ values ($\simeq 24$ instead of $\sim 200$).

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• Flows in porous media
• Fluid-particle interactions
• Granular flows
• Particle-laden flows