Mechanisms for particle transfer and segregation in a turbulent boundary layer

Abstract

Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward and away from the wall and favour particle segregation in the viscous region, giving rise to non-uniform particle distribution profiles which peak close to the wall. The object of this work is to understand the reasons for higher particle concentration in the wall region by examining turbulent transfer of heavy particles to and away from the wall in connection with the coherent structures of the boundary layer. We will examine the behaviour of a dilute dispersion of heavy particles – flyashes in air – in a vertical channel flow, using pseudo-spectral direct numerical simulation to calculate the turbulent flow field at a shear Reynolds number Reτ = 150, and Lagrangian tracking to describe the dynamics of particles. Drag force, gravity and Saffman lift are used in the equation of motion for the particles, which are assumed to have no influence on the flow field. Particle interaction with the wall is fully elastic. As reported in several previous investigations, we found that particles are transferred by sweeps – Q2 type events – in the wall region, where they preferentially accumulate in the low-speed streak environments, whereas ejections – Q4 type events – transfer particles from the wall region to the outer flow. We quantify the efficiency of the instantaneous realizations of the Reynolds stresses events in transferring different size particles to the wall and away from the wall, respectively. Our findings confirm that sweeps and ejections are efficient transfer mechanisms for particles. In particular, we find that only those sweep and ejection events with substantial spatial coherence are effective in transferring particles. However, the efficiency of the transfer mechanisms is conditioned by the presence of particles to be transferred. In the case of ejections, particles are more rarely available since, when in the viscous wall layer, they are concentrated under the low-speed streaks. Even though the low-speed streaks are ejection-like environments, particles remain trapped for a long time. This phenomenon, which causes accumulation of particles in the near-wall region, can be interpreted in terms of overall fluxes toward and away from the wall by the theory of turbophoresis. This theory, proposed initially by Caporaloni et al. (1975) and re-examined later by Reeks (1983), can help to explain the existence of net particle fluxes toward the wall as a manifestation of the skewness in the velocity distribution of the particles (Reeks 1983). To understand the local and instantaneous mechanisms which give rise to the phenomenon of turbophoresis, we focus on the near-wall region of the turbulent boundary layer. We examine the role of the rear-end of a quasistreamwise vortex very near to the wall in preventing particles in the proximity of the wall from being re-entrained by the pumping action of the large, farther from the wall, forward-end of a following quasi-streamwise vortex. We examine several mechanisms for turbulence structures near the wall and we find that the mechanism based on the archetypal quasi-streamwise structures identified by Schoppa & Hussain (1997), the parent–offspring regeneration cycle for near-wall quasi-streamwise vortices discussed by Brooke & Hanratty (1993), and the mechanism based on coherent packets of hairpin vortices, the fundamental super-structure characterized by Adrian, Meinhart & Tomkins (2000), all depict the same characteristic pattern which is responsible for particle trapping very near to the wall.

Langevin PDF simulation of particle deposition in a turbulent pipe flow

“…In this way, we propose a first link between a statistical model, such as the present Langevin model, and some geometrical features recently found out by DNS analysis [11][12][13][14] . The goal of the work is therefore to propose simple and phenomenological models and, also, to indicate whether introducing geometrical features in a Lagrangian stochastic approach can be useful for particle deposition simulations.…”

“…The turbulent near-wall structures have been found to have a main role on the mechanism of particle deposition 11,13 . For our purpose, the most interesting aspect is that depositing particles can be divided into two categories.…”

“…It is largely accepted that particle transfer in the wall region and also deposition onto walls are processes dominated by near-wall turbulent coherent structures (sweeps and ejections), which are instantaneous realizations of the Reynolds stresses, and that particles tend to remain trapped along the streaks when in the viscous-layer [3][4][5]8,11 . However, the importance of these mechanisms for particle deposition depends on particle inertia.…”

Frazil dynamics and precipitation in a water column with depth-dependent supercooling

“…The definition of Ri is now modified to provide a single dimensionless quantity that represents the effects of frazil-seawater mixture viscosity, shear and stability on turbulence. This quantity is then evaluated in the ecntre of the bursting layer (z + = 35) to assess the viability of the vortices thought to be responsible for bursting (Marchioli & Soldati 2002). In order to assess the viscous effects of increased frazil ice concentrations in the bursting layer, it is assumed that the shear stress exerted there by a steady externally driven flow is unchanged by an increase in the fluid viscosity:…”

Can turbophoresis be predicted by large-eddy simulation?

“…1,2 This phenomenon has been measured in, e.g., vertical turbulent pipe flow 3 and calculated by direct numerical simulation (DNS) in vertical turbulent pipe and channel flow. 4,5 In these numerical simulations the fluid is modeled as a continuous phase, while for each particle an equation of motion is imposed. The combined effect of drag force on a particle and inertia leads to turbophoresis.…”