Maurizio Picciotto

Characterization of near-wall accumulation regions for inertial particles in turbulent boundary layers

In this paper, we examine particle distribution in the wall region of turbulent boundary layers, considering specific flow conditions (Reτ=150) 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.

Particle dispersion and wall-dependent turbulent flow scales: implications for local equilibrium models

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 wall-bounded 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.

Turbulence Modulation by Micro-Particles in Boundary Layers

Turbulent dispersed flows over boundary layers are crucial in a number of industrial and environmental applications. In most applications, the key information is the spatial distribution of inertial particles, which is known to be highly non-homogeneous and may exhibit a complex pattern driven by the structures of the turbulent flow field. Theoretical and experimental evidence shows that fluid motions in turbulent boundary layers are intermittent and have a strongly organized and coherent nature represented by the large scale structures. These structures control the transport of the dispersed species in such a way that the overall distribution will resemble not at all those given by methods in which these motions are ignored.

Quantification of Particle and Fluid Scales in Particle-Laden Turbulent Channel Flow

In this work, we study the dispersion of inertial particles in fully-developed turbulent channel flow to evaluate the relationship between particle and fluid time scales, and to identify suitable scales for parametrization of near-wall particle behavior. Direct Numerical Simulation (DNS) and Lagrangian particle tracking are used to build a complete and homogeneous dataset which covers a large target parameter space and includes statistics of particle velocity and particle concentration at steady state. Our results show that the Lagrangian integral time scale of the fluid is adequate to characterize particle wall deposition and that such fluid time scale will be different when sampled at the position of either fluid particles or inertial particles. Differences become particularly evident in the range 5 < St < 25. These observations can be crucial to improve the accuracy of engineering models for particle deposition.Copyright

Interaction between Turbulence Structures and Inertial Particles in Boundary Layer: Mechanisms for Particle Transfer and Preferential Distribution

Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward the wall and away from the wall and favour particle segregation in the viscous region. In this work we examine turbulent transfer of heavy particles to the wall and away from the wall in connection with the coherent structures of the boundary layer. First, a detailed analysis of wall turbulence phenomena in a boundary layer will be provided. We will focus on the evolutionary dynamics of the structures populating the boundary layer: according to Schoppa & Hussain (1996, 1997), we will identify the following turbulence regeneration cycle: (i) low-speed streaks generate quasi-streamwise vortices, (ii) quasi-streamwise vortices generate sweeps and ejections, (iii) sweeps and ejections contribute to maintain the low-speed streaks.