Production and survival of early turbulence due to flow modulation at the duct inlet

Abstract

The paper reports a fully resolved numerical simulation of turbulent duct flow developing under complex inlet flow conditions at very low Reynolds number $R{e}_m=1250-4070$ based on the channel width and bulk velocity. Turbulence was initiated by inflectional instabilities created at the duct center owing to the special inlet design. All simulations were performed using the open-source software OpenFOAM. Computations reveal that turbulence persisted over more than 60 turnover time. Its statistical features were characterized by distributions of the mean flow and turbulent stresses at multiple duct locations. The turbulent stresses were used to analyze the anisotropic nature of turbulence by evaluating trajectories of across the anisotropy-invariant map which bounds all realizable states of turbulence. The limiting behavior of turbulence stresses reveals explosive production of the dissipation at the wall. This evidence suggests that the chief mechanisms involved during the transition process and self-maintenance of turbulence at very low Reynolds numbers is related to the dynamics of the turbulent dissipation rate which attains the maximum amplification by approaching the two-component isotropic state at the wall.