Axisymmetric pulse train solutions in narrow-gap spherical Couette flow

Highlights: • Pulse trains of Taylor vortices travel towards the equator from each hemisphere. • Solutions are equatorially symmetric or asymmetric, and periodic or quasi-periodic. • Separate bifurcation sequences are found for narrow and very-narrow gaps. We numerically compute the flow induced in a spherical shell by fixing the outer sphere and rotating the inner one. The aspect ratio $ϵ=(r_o-r_i)/r_i$ is set at 0.04 and 0.02, and in each case the Reynolds number measuring the inner sphere's rotation rate is increased to ∼10% beyond the first bifurcation from the basic state flow. For $ϵ=0.04$ the initial bifurcations are the same as in previous numerical work at $ϵ=0.154$, and result in steady one- and two-vortex states. Further bifurcations yield travelling wave solutions similar to previous analytic results valid in the $ϵ\to 0$ limit. For $ϵ=0.02$ the steady one-vortex state no longer exists, and the first bifurcation is directly to these travelling wave solutions, consisting of pulse trains of Taylor vortices travelling toward the equator from both hemispheres, and annihilating there in distinct phase-slip events. We explore these time-dependent solutions in detail, and find that they can be both equatorially symmetric and asymmetric, as well as periodic or quasi-periodic in time.