Nonlinear phenomena in hybrid Couette flow composed of planar and circularshear

Results are presented from an experimental investigation of a novel shear f low. Two parallel sections of planar Couette flow are connected by two semi circular sections of circular Couette flow to give a flow domain with the s hape of a running track. Driving the flow with a moving inner boundary lead s to centrifugal instability in the curved regions as in conventional Taylo r-Couette flow. This is in contrast to the planar regions, which are linear ly stable and are characterized instead by finite-amplitude instability. In the steady regime, the entire flow field is dominated by structures akin t o 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 chara cterized by various spatiotemporal modes, the selection of which is depende nt on the manner of flow evolution. Quasistatic increase of the Reynolds nu mber from zero typically results in flow with a banded spatial structure an d low-dimensional dynamics, both of which are associated with instability i n the semicircular regions. However, an abrupt step-like increase of Reynol ds number produces a persistent flow state with strong spatial disorder and a broadband dynamical spectrum. The results of this study have implication s for the conventional distinctions between the properties of open and clos ed flows, and suggest the possibility of intermediate flows which are worth y of investigation in their own right. (C) 2001 American Institute of Physi cs.