New flows in a circular Couette system with co-rotating cylinders

Abstract

Five previously unobserved and unpredicted flows are found to occur in a fluid contained between concentric cylinders with the inner cylinder rotating faster than, but in the same direction as, the outer cylinder. Flow visualization and spectral studies of these flows yield the following descriptions: (1) Twisted Taylor vortices—each Taylor vortex contains from 14 to 30 apparent twists. (2) Wavy inflow boundaries—traveling azimuthal waves appear on the inflow boundary between Tayor vortices. (3) Wavy outflow boundaries—traveling azimuthal waves appear on the outflow boundary between Taylor vortices. (4) Wavelets—small‐amplitude traveling azimuthal waves of high wavenumber appear on both inflow and outflow boundaries of the vortices; this flow is distinct from the wavy vortex flows observed with only the inner cylinder rotating. (5) Braided vortices—the vortices appear to wrap around one another in an irregular fashion. The first four flow patterns, produced through quasistatic changes in the inner‐cylinder speed with the outer‐cylinder speed fixed, can be periodic in the azimuthal and axial directions; in contrast, the braided‐vortex pattern, produced through a rapid increase of the inner‐cylinder speed with the outer‐cylinder speed fixed, has neither axial nor azimuthal periodicity. The power spectra of all five flows contain sharp components. The ranges of cylinder speeds for the occurrence of these flows, and for some cases, the wave speeds and wavenumbers are determined.