Re-picturing viscoelastic drag-reducing turbulence by introducing dynamics of elasto-inertial turbulence

TL;DR

This paper re-examines viscoelastic drag-reducing turbulence by integrating elasto-inertial turbulence dynamics, revealing new insights into flow regimes, energy transfer, and the transition from inertial to elasto-inertial turbulence.

Contribution

It introduces a new energy self-sustaining process framework that explains the transition from inertial turbulence to elasto-inertial turbulence in viscoelastic flows.

Findings

EIT-related SSP can survive at moderate Re, preventing relaminarization.

Flow transitions from laminar to MDR involve EIT dynamics at higher elasticity.

Streamwise velocity fluctuations lag wall-normal fluctuations, explained by the new energy picture.

Abstract

Recently, the nature of viscoelastic drag-reducing turbulence (DRT), especially maximum drag reduction (MDR) state, has become a focus of controversy. It has long been regarded as polymers-modulated inertial turbulence (IT), but is challenged by the newly proposed concept of elasto-inertial turbulence (EIT). This study is to re-picture DRT in parallel plane channels by introducing dynamics of EIT based on statistical and budget analysis for a series of flow regimes from the onset of DR to EIT. Energy conversion between velocity fluctuations and polymers as well as polymeric pressure redistribution effect are of particular concern, based on which a new energy self-sustaining process (SSP) of DRT is re-pictured. The numerical results indicate that at low Reynolds number (Re), the flow enters laminar regime before EIT-related SSP is formed with the increase of elasticity, whereas, at moderate Re, EIT-related SSP can get involved and survive from being relaminarized. This somehow explains the reason why relaminarization is observed for small Re while the flow directly enters MDR and EIT at moderate Re. Moreover, with the proposed energy picture, the newly discovered phenomenon that the streamwise velocity fluctuations lag behind those in wall-normal direction can be well explained. The re-pictured SSP certainly justify that IT nature is gradually replaced by that of EIT in DRT with the increase of elasticity.