Mechanisms of core perturbation growth in vortex-turbulence interaction

We study mechanisms of coherent structure decay via direct numerical simulations (DNS) of a vortex column interacting with external, fine-sale turbulence. Ensemble-averaged statistics show growth of strong core (Kelvin) waves induced by the external turbulence - surprising, given the stabilizing effect of rotation in the vortex core. We explore two potential mechanisms of perturbation growth and core transition: (i) resonant forcing of Kelvin waves by turbulence filaments wrapping the column, and (ii) growth of optimal transient perturbations. We demonstrate the possibility of ring-vortex wave resonance even for relatively weak rings. Resonance in the form of amplifying core dynamics results in sheath-like structures in the core, known to be unstable to a Kelvin-Helmholtz-like instability. However, this process requires sustained organized ring-like structures over several vortex turnover times. Amplification of core perturbations in optimal transient modes also occurs through resonant forcing. Several orders of magnitude growth is possible at even moderate Re (̃ 10⁴) before the inevitable (linear) decay. We briefly examine the nonlinear evolution of optimal bending modes and show that such growth reproduces features of vortex interaction with turbulence: enhanced core diffusion, core perturbation growth, and circulation overshoot. Results from transient growth analysis suggest the importance of optimal transient modes in governing the decay of turbulent vortices.