Turbulence attenuation in particle-laden flow in smooth and rough channels

A W Vreman

doi:10.1017/jfm.2015.208

Abstract

Results of point-particle direct numerical simulations of downward gas–solid flow in smooth and rough vertical channels are presented. Two-way coupling and inter-particle collisions are included. The rough walls are shaped as fixed layers of tiny spherical particles with diameter much smaller than the viscous wall unit. The turbulence attenuation induced by the free solid particles in the gas flow is shown to be enhanced with increasing wall roughness. The so-called feedback force, the force exerted by the free particles on the gas, is decomposed into three contributions: the domain average of the mean feedback force, the non-uniform part of the mean feedback force and the fluctuating part of the feedback force. Since the non-uniformity of the mean feedback force increases with wall roughness, the effect of the non-uniform part of the mean feedback force is investigated in detail. For both smooth and rough walls, the non-uniform part of the mean feedback force is shown to contribute significantly to the particle-induced turbulence attenuation.

Highlights

It is well known that small heavy particles with large Stokes number dampen turbulence (Tsuji, Morikawa & Shiomi 1984; Gore & Crowe 1989; Hetseroni1989; Elghobashi & Truesdell 1993; Kulick, Fessler & Eaton 1994; Li et al 2001; Yamamoto et al 2001; Ferrante & Elghobashi 2003; Mito & Hanratty 2006; Vreman2007)
In order to investigate the effect of wall roughness on particle-induced turbulence attenuation, both smooth and rough walls were applied in the simulations
A new element in the computational method was that wall roughness was taken into account with a deterministic instead of a stochastic model

Summary

Introduction

It is well known that small heavy particles with large Stokes number dampen turbulence Reτ = 642 is not high, the Reynolds number is approximately four times higher than in previous studies of turbulent channel flows using the same method (PP-DNS with two-way coupling and inter-particle collisions). The grid spacing in the normal direction varies from 0.99δν at the wall to 7δν at the centre of the channel These are standard grid sizes for DNS of single-phase channel flow (Moser, Kim & Mansour 1999) and are sufficiently small to compute mean velocity and Reynolds stress profiles accurately (Vreman & Kuerten 2014a,b). We introduce the domain average of a quantity Q: Q (t)

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Findings

Conclusions