In this paper, we examine particle distribution in the wall region of turbulent boundary layers, considering specific flow conditions ͑Re = 150͒ and spanning two orders of magnitude of particle inertial parameter-the particle timescale. First, we identify the flow timescales that govern particle distribution, examining the degree of particle preferential concentration and determining the optimum in connection with particle timescale. Second, we identify which of the flow variables may be used to control particle distribution. These are the streamwise and spanwise shear stress components at the wall, which correspond to the only nonvanishing elements of the fluctuating fluid velocity gradient tensor.Controlling particle distribution in turbulent flow near a wall could improve the efficiency of heat, mass, and momentum transfer processes in a number of industrial and environmental applications, such as multiphase flow reactors and combustion or post-combustion devices. As shown by several experiments 1,2 and numerical simulations, 3-5 particle distribution in turbulent shear flows is known to be highly inhomogeneous due to the action of near-wall coherent flow structures. These structures are responsible for particle accumulation and segregation into elongated clusters, which correlate well with the instantaneous position of the low-speed streaks at the wall. 1,2 Based on this observation, particle distribution control may be achieved by boundary layer manipulation. A possible strategy is to treat the turbulent boundary layer as a black box and to observe the structural changes when the boundary conditions are modified. Looking at boundary layers in this fashion is certainly a way forward. Still, current literature lacks objective criteria for particle accumulation, which should go beyond observing long-term particle clustering along low-speed streaks. 1-5 We thus need to define clearly and in a quantitative way which of the flow variables is best suited for devising feasible flow-control strategies. This is the final aim of our research.In this work, we focus precisely on the wall region ͑z + Ͻ 5 in nondimensional units along the wall-normal direction͒, where the flow is depleted from strongly coherent structures yet it is complicated enough to induce particleselective distribution. 1-5 Rouson and Eaton 6 ͑R&E hereinafter͒ investigated particle distribution in connection with the flow topology classification based on the invariants of the velocity gradient tensor. 7 R&E 6,8 found that moderate-to high-inertia particles in turbulent channel flow sample preferentially convergence rather than vortical-flow regions. Different from these previous works in channel flow, 6,8 we consider a range of particle parameters matching the timescale range of the fluid structures in the wall region. First, we will examine the relationship between convergence-flow regions and particle instantaneous location in the viscous sublayer. Second, we will try to establish a connection between convergence regions and regions of particle accu...

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Theoretical models for particle dispersion in turbulent wall layers are based on closure assumptions for particle-turbulence correlations which strongly depend on the wall-normal coordinate. This paper presents new data for the near-wall dispersion of inertial particles in fully developed turbulent channel flow, obtained using direct numerical simulation (DNS) with a one-way point-particle approach. The link between wall-dependent flow time scales and particle time scales is discussed addressing further the issue of using integral flow time scales to parametrize particle behavior. This is fundamental to validate closure relations in theoretical local-equilibrium models for particle dispersion in wallbounded flows, where the reduction of the fluid velocity fluctuation in the wall-normal direction is not accompanied by an equivalent reduction of the particle velocity fluctuation.

Preferential concentration of inertial particles in turbulence is studied numerically by evaluating the Lagrangian compressibility of the particle velocity field using the "full Lagrangian method." This is compared with the "mesoscopic Eulerian particle velocity field" both in a direct numerical simulation of turbulence and in a synthetic flow field. We demonstrate that the Lagrangian method, in contrast to the Eulerian, accurately predicts the compressibility of the particle velocity field even when the latter is characterized by singularities. In particular we use the method to evaluate the growth rates of spatial moments of the particle number density which reflect the fractal structure of segregation and the occurrence of singularities.