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.
Direct Numerical Simulation of Drag-Reducing Turbulent Boundary Layer of Viscoelastic Fluid (2nd Report, Energy Transfer and Coherent Structures)