Use of a fluid membrane in the observation of a shear instability caused by rotational motion

An experiment to study the shear layer disturbances formed around a cylinder rotating in a fluid by means of a fluid film is presented. A cylinder with a large radius relative to its height was covered with a soapy film over the top, while motor driven disks with variable speed and radius were inserted from the bottom. Traces of HCl and NH3 were added to the top of the disk to form a smoke which was entrained on the inner side of the soapy film as the disk below turned and turbulent shear zones formed at certain critical speeds of the inner disks. A speed was eventually found with total oscillation of the vortexes. Replacement of the soap film with glass required much greater thickness of the transparency and higher speeds of the rotating disks, giving rise to unstable vortex formation. The method is effective in quantitative visualization of vortex formation and mode transitions.