Boundary-layer dispersion of near-wall injected particles of various inertias

A direct numerical simulation approach was employed along with a Lagrangian particle tracking technique to investigate dilute particle motion and dispersion in a horizontal turbulent boundary layer (Re-r = 270) with no streamwise pressure gradient. Particle inertias based on inner Stokes numbers St(+) (based on friction velocity) ranging from 10(-2) to 10(2) were investigated. The particles were injected near the wall at a height of four wall units (with elastic wall collision specified at one wall unit), and the terminal velocity was kept small so that particle-eddy interaction would be the primary dispersion. The results showed that particles having St(+) < I behave approximately as fluid tracers with respect to the large-scale turbulent structures. Particles with a significant inertia effect (St(+) > 1) tended to yield increased near-wall concentrations and wall collisions, qualitatively consistent with previous channel flow experiments and simulations, but particle bounce velocities were significantly different due to wall reflections and near-wall injection. Lagrangian statistics of the transverse fluid velocity deviated substantially from the Eulerian statistics due to asymmetric diffusion. In addition, particle relative velocities far exceeded the terminal velocity for moderate and large inertia particles, which was explained by a simple theoretical model.