Published online by Cambridge University Press: 01 October 2008
Transitions between regular oscillations and bursting oscillations that involve a bifurcational process which culminates in the creation of a relative periodic orbit of infinite period and infinite length are investigated both experimentally and numerically in a short-aspect-ratio Taylor–Couette flow. This bifurcational process is novel in that it is the accumulation point of a period-adding cascade at which the mid-height reflection symmetry is broken. It is very rich and complex, involving very-low-frequency states arising via homoclinic and heteroclinic dynamics, providing the required patching between states with very different dynamics in neighbouring regions of parameter space. The use of nonlinear dynamical systems theory together with symmetry considerations has been crucial in interpreting the laboratory experimental data as well as the results from the direct numerical simulations. The phenomenon corresponds to dynamics well beyond the first few bifurcations from the basic state and so is beyond the reach of traditional hydrodynamic stability analysis, but it is not fully developed turbulence where a statistical or asymptotic approach could be employed. It is a transitional phenomenon, where the phase dynamics of the large-scale structures (jets of angular momentum emanating from the boundary layer on the rotating inner cylinder) becomes complicated. Yet the complicated phase dynamics remains accessible to an analysis of its space–time characteristics and a comprehensive mechanical characterization emerges. The excellent agreement between the experiments and the numerical simulations demonstrates the robustness of this complex bifurcation phenomenon in a physically realized system with its inherent imperfections and noise. Movies are available with the online version of the paper.
Movie 1. An animation of isosurfaces of the axial component of the angular momentum, rv, for the symmetric modulated rotating wave MRWs at Re=830 and Γ=3.14. MRWs is a quasi-periodic solution with two frequencies, but one of these corresponds to the precession frequency of the underlying symmetric rotating wave and plays no dynamic role. The other (modulation) frequency corresponds to the period over which the tilted jets oscillate in and out of phase relative to each other, as is evident in the movie, which is shown in the precessing frame of reference. (This movie corresponds to figure 6 in the paper.)
Movie 2. The same rv isosurfaces, but for the asymmetric MRWa at the same Re=830, but larger Γ=3.17. As with MRWs, MRWa is also periodic in a frame of reference rotating at its precession frequency, but for MRWa both jets undergo modulated oscillations in the azimuthal direction, with the phase difference oscillating about a mean value. Moreover, the lower jet is thicker during the first half of the modulation period than in the second half.