The bifurcations and the nonlinear dynamics of the von Kármán swirling flow between exactly counter-rotating disks in a stationary cylinder are numerically and experimentally investigated. The dynamics are governed by two parameters, the radius-to-height ratio A = R / H and the Reynolds number, Re, based on disk rotation speed and cylinder height. The stability analysis performed for 2 ഛ A ഛ 20 shows that nonaxisymmetric and axisymmetric modes can be stationary or time dependent in this range. Three-dimensional modes are dominant for A ഛ 13.25 while axisymmetric modes are critical for A Ͼ 13.25. The patterns of the dominant perturbations are analyzed. In the particular case of A = 15, nonlinear computations are performed at Reynolds numbers slightly above threshold and are compared to experimental results, showing the competition between axisymmetric and three-dimensional modes.
We examine numerically the behaviour of the solutions of the axisymmetric Boussinesq equations in a tall, differentially heated, air-filled annulus. The numerical algorithm integrates the time-dependent equations in primitive variables and combines a pseudospectral Chebyshev spatial expansion with a second-order time-stepping scheme. The instability of the conduction regime is found to be unsteady cross-rolls. By assuming Hopf bifurcation, we can accurately determine the critical Rayleigh number. As the Rayleigh number increases, the solution is monoperiodic at first. Then it undergoes a period-doubling bifurcation. When the Rayleigh number is further increased, the solution reverts to a monocellular steady state through suberitical bifurcations with hysteresis. At even higher Rayleigh number, boundary-layer instability sets in, in the form of travelling waves. This instability has the characteristics of a supercritical Hopf bifurcation. We examine the space-time structure of the two types of unsteady solutions. We have presented the basic periods of the steady oscillations as functions of the Rayleigh number in the vicinity of the Hopf bifurcation points, and have also computed the Nusselt numbers for the various flow regimes.
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