DNS of horizontal open channel flow with finite-size, heavy particles at low solid volume fraction

TL;DR

This study uses direct numerical simulation to analyze the behavior of finite-size, heavy particles in turbulent open channel flow, revealing their preferential concentration near the wall and their interaction with vortices.

Contribution

It introduces a new method to define the fluid velocity 'seen' by finite-size particles and investigates their preferential localization and dynamics in turbulent flow.

Findings

Particles tend to concentrate near the bottom wall.

Particles lag slightly behind the fluid velocity, especially in low-speed streaks.

Spanwise particle motion correlates with nearby vortices.

Abstract

We have performed direct numerical simulation of turbulent open channel flow over a smooth horizontal wall in the presence of finite-size, heavy particles. The spherical particles have a diameter of approximately 7 wall units, a density of 1.7 times the fluid density and a solid volume fraction of 0.0005. The value of the Galileo number is set to 16.5, while the Shields parameter measures approximately 0.2. Under these conditions, the particles are predominantly located in the vicinity of the bottom wall, where they exhibit strong preferential concentration which we quantify by means of Voronoi analysis and by computing the particle-conditioned concentration field. As observed in previous studies with similar parameter values, the mean streamwise particle velocity is smaller than that of the fluid. We propose a new definition of the fluid velocity "seen" by finite-size particles based on an average over a spherical surface segment, from which we deduce in the present case that the particles are instantaneously lagging the fluid only by a small amount. The particle-conditioned fluid velocity field shows that the particles preferentially reside in the low-speed streaks, leading to the observed apparent lag. Finally, a vortex eduction study reveals that spanwise particle motion is significantly correlated with the presence of vortices with the corresponding sense of rotation which are located in the immediate vicinity of the near-wall particles.