Statistics of velocity and preferential accumulation of micro-particles in boundary layer turbulence

The distribution of inertial particles in turbulent flows is strongly non-homogeneous and is driven by the structure of the underlying carrier flow field. In this work, we use DNS combined with Lagrangian particle tracking to characterize the effect of inertia on particle preferential accumulation. We compare the Eulerian statistics computed for fluid and particles of different size, and observe differences in terms of distribution patterns and deposition rates which depend on particle inertia. Specifically, different statistics are related to the selective interaction occurring between particles and coherent flow structures. This selective response causes a preferential sampling of the flow field by the particles and eventually leads to the well-known phenomenon of long-term particle accumulation in the boundary layer. We try to measure particle preferential accumulation with a Lagrangian parameter related to the rate of deformation of an elemental volume of the particle phase along a particle trajectory. In the frame of the Lagrangian approach, this parameter is mathematically defined as the particle position Jacobian, J(t), computed along a particle path. This quantity is related to the local compressibility/divergence of the particle velocity field. Lagrangian statistics of J(t) show that compressibility increases for increasing particle response times τp+ (up to τp+=25 and within the time span covered by the simulation).