Modification of particle-laden near-wall turbulence: effect of Stokes number

TL;DR

This study uses direct numerical simulations to explore how particles with different Stokes numbers modify turbulence in wall-bounded flows, revealing that small Stokes particles enhance turbulence while larger ones suppress it.

Contribution

It provides new insights into the Stokes number-dependent effects of particles on turbulence modification in channel flows, using detailed numerical simulations.

Findings

Small Stokes particles enhance turbulence by promoting vortices.

Large Stokes particles attenuate turbulence through preferential concentration.

Energy transfer from particles to fluid increases instability in high-speed streaks.

Abstract

Turbulent channel flows laden with particles are investigated using direct numerical simulation with a point-force approximation for small, heavy particles with a diameter smaller than the Kolmogorov length scale of the fluid. The Stokes numbers based on the wall units considered in our study are $St^+=0.5, 5, 35$ and 125. The main purpose of this study is to examine the effect of Stokes number on turbulence modification in a channel. We found that particles with $St^+=0.5$ enhance turbulence by increasing the occurrence of quasistreamwise vortices, while larger-Stokes-number particles attenuate turbulence. When $St^+=0.5$, kinetic energy is transferred from the particles to streamwise fluid velocity fluctuations in the high-speed regions and low-speed streaks, which may increase the instability of the low-speed streaks responsible for the birth of new quasistreamwise vortices. On the other hand, the preferential concentration of larger-Stokes-number particles in low-speed streaks is responsible for turbulence attenuation, and the slow response of the particles to the fluid produces feedback against the fluid velocity associated with quasistreamwise vortices.