Nonlinear phenomena in hybrid Couette flow composed of planar and circular shear

Results are presented from an experimental investigation of a novel shear flow. Two parallel sections of planar Couette flow are connected by two semicircular sections of circular Couette flow to give a flow domain with the shape of a running track. Driving the flow with a moving inner boundary leads to centrifugal instability in the curved regions as in conventional Taylor–Couette flow. This is in contrast to the planar regions, which are linearly stable and are characterized instead by finite-amplitude instability. In the steady regime, the entire flow field is dominated by structures akin to Taylor vortices. The mechanism of exchange between a four-cell and a six-cell flow over a range of aspect ratio is qualitatively the same as for the standard Taylor–Couette problem. In the unsteady regime, the flow is characterized by various spatiotemporal modes, the selection of which is dependent on the manner of flow evolution. Quasistatic increase of the Reynolds number from zero typically results in flow with a banded spatial structure and low-dimensional dynamics, both of which are associated with instability in the semicircular regions. However, an abrupt step-like increase of Reynolds number produces a persistent flow state with strong spatial disorder and a broadband dynamical spectrum. The results of this study have implications for the conventional distinctions between the properties of open and closed flows, and suggest the possibility of intermediate flows which are worthy of investigation in their own right.