Mechanisms of particle deposition in a fully developed turbulent open channel flow

Abstract

Particle dispersion and deposition in the region near the wall of a turbulent open channel is studied using direct numerical simulation of the flow, combined with Lagrangian particle tracking under conditions of one-way coupling. Particles with response times of 5 and 15, normalized using the wall friction velocity and the fluid kinematic viscosity, are considered. The simulations were performed until the particle phase reached a statistically stationary state before calculating relevant statistics. For both response times, particles are seen to accumulate strongly very close to the wall in the form of streamwise oriented streaks. Deposited particles were divided into two distinct populations; those with large wall-normal deposition velocities and small near-wall residence times referred to as the free-flight population, and particles depositing with negligible wall-normal velocities and large near-wall residence times (more than 1000 wall time units), referred to as the diffusional deposition population. Diffusional deposition (deposition induced by the small residual turbulent fluctuations near the wall) is found to be the dominant mechanism of deposition for both particle response times. The free-flight mechanism is shown to gain in importance only for τp+=15 particles. For τp+=5 particles only 10% deposit because of free flight, whereas the fraction is around 40% for τp+=15 particles. This result runs counter to the widely held opinion that free flight is the dominant mechanism of deposition in wall-bounded flows and clearly quantifies the relative importance of the two mechanisms. A simple relationship between the particle wall-normal velocity on deposition and the residence time for free-flight particles is presented. Particle deposition locations over the period of the entire simulation reveal that, while diffusional deposition occurs mostly along streamwise oriented lines below the near-wall particle accumulation patterns, free-flight particles deposit more evenly over the wall.

Experimental measurements and numerical simulations of particle transport and distribution in ventilated rooms

“…Nevertheless, the trap treatment worked well when the near wall grid was sufficiently fine, like that which was used in the DNS simulation (McLaughlin, 1989;Narayanan et al, 2003). The trap treatment, however, is not suitable for the current situation which uses a high Reynolds number k-ε model.…”

Langevin PDF simulation of particle deposition in a turbulent pipe flow

“…In order to further support this argument, we have computed the mean near-wall residence time (in the layer y + < 30) of deposited particles, for each class of diameters. We have chosen to monitor the particle residence-time because this quantity has been found to properly distinguish different deposition mechanisms 13 . In Table II, the results obtained for each class of diameters are given for the simulation with all exact fluid profiles.…”

“…Therefore, they are found to deposit mainly with negligible wall-normal velocities and large near-wall residence time. This mechanism of deposition has been called diffusional 13 . On the contrary, heavy particles are not so well correlated with turbulent structures and their motion is less influenced by them in the near-wall region.…”

“…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.…”

“…On the contrary, heavy particles are not so well correlated with turbulent structures and their motion is less influenced by them in the near-wall region. Therefore, heavy particles deposit with large wall-normal velocities and small near-wall residence time, that is by the so-called free-flight mechanism 10,13 .…”