Longitudinal instability and turbulent transition observed in Taylor-Couette flow

The characterization of time varying flows within the Taylor Couette system has received some attention to date, for example with modulations of cylinder rotation, as well with impulsive starts or stops. Here we briefly present observations where the outer cylinder motion is unsteady, undergoing both positive and negative acceleration, while the inner cylinder remains at rest. Longitudinal streaks (i.e. along the rotation axis) develop during the period of deceleration. These waves become increasingly corrugated before merging with the developed turbulence emanating from both end-cap boundaries. The flow altogether then further relaxes to a wavy vortex state. Given the narrow gap width and velocimetry measurements, these longitudinal modes likely represent a boundary layer instability. Similar structures were observed in passing by Coles (1965), but they remain to be characterized in the context of Taylor Couette flow.