Coherent structure-turbulence interaction studied via a vortex column embedded in fine-scale turbulence

The coupling of large-scale coherent structures (CSs) with fine-scale turbulence and its possible relevance to cascade are explored via numerical simulations of a vortex column as an idealized CS embedded in homogeneous, isotropic turbulence. LES of this high Reynolds number (Re) vortex-turbulence interaction supports the turbulence cascade scenario – showing that finer-scale filaments arch over vorticity thread dipoles which themselves wrap around the column, hence the vorticity portrait of the cascade process. LES of a ring, idealizing a thread around a column, embedded in turbulence shows external turbulence and ring-perturbed core fluid stripped from the column are organized into threads acting as an oppositely signed, coaligned pseudo-ring, hence forming a dipole encircling the column. This evolving dipole then reconnects and breaks up, generating new turbulence around the column – a possible scenario for turbulence cascade. Furthermore, contrary to the expected viscous decay within the column due to its no core strain rate, core turbulence intensifies via radial stretching due to the column’s “core dynamics,” further generating new turbulence, now surprisingly within the column. The breakup of the threads is delineated via reconnection of two antiparallel vortices embedded in turbulence. Slender orthogonal filaments form between these vortices, perturbing the collision of the vortices, generating additional threads and new turbulence structures – providing further insight into the cascade process. Hence, studying vortex-turbulence interaction and that of the hierarchy of the resulting threads reveal further steps in the cascade process, seemingly consistent with our vortex cascade scenario of shear flow turbulence.