Effect of the instantaneous turbulent flow structures on the particle distribution near the wall of a channel

We analyzed databases obtained from direct numerical simulations of the turbulent flow in a plane channel at low Reynolds numbers. Particles with different inertia were tracked using the dilute phase approximation. The main objective of this study is the determination of the fluctuations of concentration produced by two types of instantaneous near-wall flow structures. The first type of structure produces extreme negative fluctuations of the wall shear stress (τw′≪0) and the second type of structures are associated with large positive fluctuations (τw′≫0). A conditional sampling technique was used to educe the topologies of these two types of flow structures and the corresponding conditional averaged particle concentration fluctuations. The contribution of each type of flow event to the wall shear stress history is about 10% but their contribution to the particle velocity component perpendicular to the wall, in the near-wall region, is about 50% for the flow structures associated to events with τw′≪0 and about 40% for those with τw′≫0, indicating that the role of these structures is significant in the particle fluxes from and towards the wall. The flow structures consist in two parallel counterrotating streamwise vortices that, in the case of τw′≪0, create a flow ejection from the wall between them and in the case of τw′≫0, produce a sweep towards the wall. These events produce important fluctuations of the particle concentration near the wall, with intensities up to 200% for the flow structure associated with τw′≪0 and for the largest Stokes number considered (St=25).