Oscillations and translation of a free cylinder in a confined flow

TL;DR

This study investigates the oscillatory behavior of a freely moving and rotating cylinder in a confined flow, revealing viscosity-driven oscillations with unique characteristics distinct from vortex shedding.

Contribution

It provides experimental insights into the oscillations of a free cylinder in confined flow, highlighting the influence of viscosity and blockage ratio on its oscillatory dynamics.

Findings

Oscillations occur at a frequency independent of mean flow velocity.

Strouhal number scales as 1/Re, indicating viscosity's strong influence.

Oscillation period matches viscous diffusion time over the cylinder diameter.

Abstract

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.