Weak branch and multimodal convection in rapidly rotating spheres at low Prandtl number

TL;DR

This study investigates weakly nonlinear convective flows in rapidly rotating spheres at low Prandtl numbers, revealing bifurcation structures, quasiperiodic flows with multimodal resonances, and large transient behaviors through combined bifurcation analysis and direct numerical simulations.

Contribution

It provides the first detailed bifurcation diagrams and stability analysis of weak branch flows in low Prandtl number rotating spheres, including the characterization of multimodal quasiperiodic flows.

Findings

Identification of weak branch bifurcations leading to quasiperiodic flows.

Discovery of multimodal resonant triads in quasiperiodic flows.

Observation of large transient times in nonlinear saturation processes.

Abstract

The focus of this study is to investigate primary and secondary bifurcations to weakly nonlinear flows (weak branch) in convective rotating spheres in a regime where only strongly nonlinear oscillatory sub- and super-critical flows (strong branch) were previously found in [E. J. Kaplan, N. Schaeffer, J. Vidal, and P. Cardin, Phys. Rev. Lett. 119, 094501 (2017)]. The relevant regime corresponds to low Prandtl and Ekman numbers, indicating a predominance of Coriolis forces and thermal diffusion in the system. We provide the bifurcation diagrams for rotating waves (RWs) computed by means of continuation methods and the corresponding stability analysis of these periodic flows to detect secondary bifurcations giving rise to quasiperiodic modulated rotating waves (MRWs). Additional direct numerical simulations (DNS) are performed for the analysis of these quasiperiodic flows for which Poincar\'e sections and kinetic energy spectra are presented. The diffusion time scales are investigated as well. Our study reveals very large initial transients (more than 30 diffusion time units) for the nonlinear saturation of solutions on the weak branch, either RWs or MRWs, when DNS are employed. In addition, we demonstrate that MRWs have multimodal nature involving resonant triads. The modes can be located in the bulk of the fluid or attached to the outer sphere and exhibit multicellular structures. The different resonant modes forming the nonlinear quasiperiodic flows can be predicted with the stability analysis of RWs, close to the Hopf bifurcation point, by analyzing the leading unstable Floquet eigenmode.