Modification of turbulence and stratification of stably stratified turbulent channel flows by finite-size particles

Direct numerical simulations are carried out for thermally stratified turbulent channel flow laden with finite-size particles using an immersed boundary method that was developed for use in nonuniform grids. We consider the stratification effect at friction Richardson numbers Riτ=0 and 20 with a friction Reynolds number Reτ=180. The particle diameter is chosen as 0.21δ and the global volume fraction of the particles ranges between 0 and 8%, where δ is the channel half gap. Particles are neutrally buoyant or slightly heavier than fluids. We find that neutrally buoyant particles make the internal gravity waves stronger. The vertical motion of the particles is limited to each half of the channel due to the gravity wave, which acts as a wall at the center of the channel. If the particles are slightly heavier than the fluid, the turbulence structures are greatly modified by the particles. The particle movements in the near-wall and core regions are statistically analyzed using particle velocities or residence times, and individual particle movements are presented. The particles stay in the high-speed streak region for a relatively long time and disappear rapidly in the near-wall region when encountering the ejection event of the turbulent flow. Analysis of the vortex structure with the heat flux shows that the particles play a role in suppressing the vertical heat transport. Additional budget analysis of heat flux is performed for a detailed investigation.