Parametric instability in a rotating cylinder of gas subject to sinusoidal axial compression. Part 2. Weakly nonlinear theory

A weakly nonlinear analysis is presented of parametric instability in a rotating cylinder subject to periodic axial compression by small sinusoidal oscillations of one of its ends (‘the piston’). Amplitude equations are derived for the pair of parametrically resonant (primary) inertial modes which were found to arise from linear instability in Part 1. These equations introduce an infinity of geostrophic mode amplitudes, representing a nonlinear modification of the mean flow, for which evolution equations are also derived. Consequences of the total system of equations are investigated for axisymmetric modes. Different possible outcomes are found at large times: (a) a fixed point, representing a saturated state in which the oscillatory toroidal vortices of the primary mode are phase-locked to the piston motion with half its frequency; (b) a limit cycle or chaotic attractor, corresponding to slow-time oscillations of the primary mode; or (c) exponential divergence of the amplitudes to infinity. The latter outcome, a necessary condition for which is derived in the form of a threshold piston amplitude for divergence, invalidates the theory, inducing a gross change in the character of the flow and providing a route out of the weakly nonlinear regime. Non-zero fixed-point branches arise via bifurcations from both sides of the linear neutral curve, where the basic flow changes local stability. The loweramplitude branch is shown to be unstable, while the upper one may lose local stability, resulting in a Hopf bifurcation to a limit cycle, which can subsequently become aperiodic via a series of further bifurcations. Typically, during the resulting oscillations, whether periodic or not, the perturbation first grows from small amplitude owing to basic-flow instability, then nonlinear detuning of the parametric resonance causes decay back to small amplitude in the second half of the cycle, which then restarts.