Bursting of columnar structures in forced rotating turbulence

Abstract

In this study, we report intermittent bursting of cyclonic columnar structures in direct numerical simulations of forced rotating turbulence in a three-dimensional (3D) periodic box. Columnar structure formation is associated with dominant inverse cascade of energy in addition to the typical forward cascade at large wavenumbers. The forcing is random and isotropic, applied in two distinct wavenumber bands (kf = [3, 4] and [6, 7]). Notably, bursting is observed only for small forcing wavenumber, kf = [3, 4] and not at kf = [6, 7], for the same rotation rate and initial conditions. These bursting events are associated with forward cascade of energy in contrast to the inverse cascade of rotating turbulence, as confirmed from the ring spectrum and time-series of modal kinetic energy of modes with kz = 2, 3. Identifying some of the triads involved in the energy transfer from modal analysis, we also calculate the mode-to-mode energy transfer. We find that, before the event, the circular cyclonic vortex becomes elliptical due to cyclone-anticyclone interactions leading to elliptical instability. We argue that energy transfer preceding the bursting events to modes with higher kz triggers the Crow instability. These instabilities ultimately lead to bursting of the columnar vortex, which subsequently forms again due to the external rotation. Our results portray a tug-of-war between the destabilizing effect of elliptical instability and the stabilizing effect of external rotation.