Incompressible flow computations have been performed on a cavity with a length to depth ratio of 2. Depending on the Reynolds number, three qualitatively different unsteady flow regimes have been identified. In the first (regular) regime the second Rossiter mode dominates the spectrum, in the second (transitional) regime two incommensurable frequencies compete with each other via mode switching, and finally, in the third (suppressed) regime a lower frequency dominates. The various forms of appearance of these regimes in the flow field are identified and discussed. Among others, the relative importance of the upstream and downstream vortices, the spatial concentration of certain frequency components, and the amplitude of the instability waves all change radically with the change of regime. In search of the hydrodynamic causes for the qualitative changes in the flow field it has become necessary to identify vortices more precisely than usual. We critically evaluate the conditions of applicability of the Truesdell-Okubo-Weiss vortex detection criterion. We propose a new vortex identification scheme by including terms describing both the local and the convective acceleration. This scheme is more accurate than that of Hua and Klein by being truly first order. The results justify the additional effort; vortical structures are more clearly identifiable with the new criterion.
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