Increase in scale of fluid motion and decrease in fluid turbulence by addition of particles

Large contributions of large-scale fluid motions, that are characterized by their large Lagrangian time scales, to turbulent transport and sustenance of wall turbulence are presented using a direct numerical simulation of fluid turbulence flowing downward through a vertical channel in which a small amount of particles are added from uniformly-distributed point sources. Feedback effect of particles on fluid turbulence is considered using a point force method. Lagrangian measurements are done by releasing passive particles, that do not exert forces on the fluid, and fluid particles from point sources, located at several distances from the wall and several times. The objective of the present study is to reconcile the Lagrangian measurement with the Eulerian measurement and to have an insight into the mechanism, that sustains wall turbulence, and how it is attenuated by the addition of particles. The measurements indicate that wall turbulence is sustained by large-scale fluid motions with multiple flow directions, of which variety is due to small-scale turbulence structures, such as vortices, and that the dissipation of small-scale structures, caused by particles, reduces wall-normal velocity components of large-scale fluid motions, which results in damping of fluid turbulence.