2.5D turbulence in shear-thinning jets

TL;DR

This study investigates the transition from three-dimensional to two-dimensional turbulence in shear-thinning jets under vertical confinement, revealing a mixed 2.5D state with unique energy cascades and scale interactions.

Contribution

It introduces the concept of a mixed 2.5D turbulence state in shear-thinning jets and elucidates how confinement influences the dimensional transition and energy cascade processes.

Findings

Mixed-dimensional state is more energetic than purely 2D or 3D turbulence.

Transition from 3D to 2D turbulence depends on the level of vertical confinement.

Coexistence of direct energy cascade at small scales and enstrophy cascade at large scales.

Abstract

The dimensional transition in turbulent jets of a shear-thinning fluid is studied via direct numerical simulations. Our findings reveal that under vertical confinement, the flow exhibits a unique mixed-dimensional (or 2.5D) state, where large-scale two-dimensional and small-scale three-dimensional structures coexist. This transition from three-dimensional turbulence near the inlet to two-dimensional dynamics downstream is dictated by the level of confinement: weak confinement guarantees turbulence to remain three-dimensional, whereas strong confinement forces the transition to two-dimensions; the mixed-dimensional state is observed for moderate confinement and it emerges as soon as flow scales are larger than the vertical length. In this scenario, we observed that the mixed-dimensional state is an overall more energetic state and it shows a multi-cascade process, where the direct cascade of energy at small scales and the direct cascade of enstrophy at large scales coexist. The results provide insights into the complex dynamics of confined turbulent flows, relevant in both natural and industrial settings.