Numerical study of the near-wall vortical structures in particle-laden turbulent flow by a new vortex identification method-Liutex

Abstract

This study investigates turbulent particle-laden channel flows using direct numerical simulations employing the Eulerian-Lagrangian method. A two-way coupling approach is adopted to explore the mutual interaction between particles and fluid flow. The considered cases include flow with particle Stokes number varying from St = 2 up to St = 100 while maintaining a constant Reynolds number of Re_τ = 180 across all cases. A novel vortex identification method, Liutex (Rortex), is employed to assess its efficacy in capturing near-wall turbulent coherent structures and their interactions with particles. The Liutex method provides valuable information on vortex strength and vectors at each location, enabling a detailed examination of the complex interaction between fluid and particulate phases. As widely acknowledged, the interplay between clockwise and counterclockwise vortices in the near-wall region gives rise to low-speed streaks along the wall. These low-speed streaks serve as preferential zones for particle concentration, depending upon the particle Stokes number. It is shown that the Liutex method can capture these vortices and identify the location of low-speed streaks. Additionally, it is observed that the particle Stokes number (size) significantly affects both the strength of these vortices and the streaky structure exhibited by particles. Furthermore, a quantitative analysis of particle behavior in the near-wall region and the formation of elongated particle lines was carried out. This involved examining the average fluid streamwise velocity fluctuations at particle locations, average particle concentration, and the normal velocity of particles for each set of particle Stokes numbers. The investigation reveals the intricate interplay between particles and near-wall structures and the significant influence of particles Stokes number. This study contributes to a deeper understanding of turbulent particle-laden channel flow dynamics.