Polymer stretching and alignment under the hierarchy of coherent vortices in turbulence

We perform Brownian dynamics simulations of the finitely extensible nonlinear elastic (FENE) dumbbells in spatially periodic turbulence to investigate the relationship between the dynamics of polymers and the hierarchy of coherent vortices. We decompose the velocity field into different scales to directly evaluate the effect of vortices with different sizes on dumbbells. Scale-decomposition analysis provides quantitative evidence that the smallest-scale vortices dominantly stretch dumbbells with a relaxation time shorter than the Kolmogorov time, whereas the contribution from large-scale vortices relatively increases when the relaxation time exceeds the Kolmogorov time. To explore the origin of this scaledependent stretching mechanism, we investigate the alignment between dumbbells and the scale-decomposed strain-rate tensor. We find that dumbbells with a shorter relaxation time than the Kolmogorov time preferentially align in the most extensional direction induced by the smallest-scale vortices. However, as the relaxation time increases, dumbbells tend to align in the most extensional direction induced by 2-4 times larger vortices than the smallest-scale vortices. We explain this relaxation-time dependence of the effect of vortices with different sizes on dumbbells by focusing on how persistently the vortices stretch dumbbells.