Inertial particle accelerations in a turbulent boundary layer

Two dimensional Lagrangian acceleration statistics of inertial particles in a turbulent boundary layer with free stream turbulence are determined by means of a high speed particle tracking technique (Ayyalasomayajula et al. PRL, 95, 144507, 2006). The boundary layer is formed above a flat plate, and water droplets are fed into the flow from sprays placed down-stream from an active grid, and from tubes fed into the boundary layer from humidifiers. The free stream Stokes number is varied from 0.035 to 0.47. As the boundary layer plate is approached, the tails of the pdfs narrow, become negatively skewed, and their peak occurs at negative accelerations (decelerations in the stream-wise direction). The mean deceleration and its r.m.s. increase to large values close to the plate and are more pronounced with increasing Stokes number, in marked contrast to what is found in isotropic turbulence where the acceleration r.m.s. decreases with increasing Stokes number. A model shows the significance of the combined effects of shear and gravity on the acceleration statistics. The work is funded by the US NSF.