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

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 tau(p)(+)=15 particles. For tau(p)(+)=5 particles only 10% deposit because of free flight, whereas the fraction is around 40% for tau(p)(+)=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.