Particles in Wall-Bounded Turbulent Flows: Deposition, Re-Suspension and Agglomeration

The purpose of this chapter is to provide an introduction to the theme of particle deposition and, in that sense, to the various contributions compiled in this book. This is done by presenting various examples which are helpful to reveal the diversity through which particle deposition is manifested. This survey is also useful to clarify the aspects that are addressed and those that are outside of the present scope. Based on this overview, a framework is proposed where particle deposition is decomposed in elementary phenomena and fundamental interactions. Then, the characteristic rates of deposition, resuspension and agglomeration are introduced. As the phenomena making up particle deposition can be addressed from different modelling standpoints, the notion of microscopic, mesoscopic and macroscopic levels of description is introduced and the relations between these different points of view is sketched.

In this chapter two experimental imaging techniques, holography and particle image velocimetry (PIV), and their application to particle dispersed flows are discussed. Special emphasis is put on particle deposition, re-suspension and agglomeration processes. In the first two chapters the theoretical background of the techniques is presented indicating theoretical and practical limitations of both techniques. In consecutive chapters, several case studies are presented illustrating the use of both techniques. During the last decade tomographic PIV has become the leading technique in 3D flow measurements that opens up exciting new research possibilities in particle-dispersed flows. In addition, refractive index matched techniques are discussed enabling researchers to measure in detail the simultaneous coupling between finite-sized particles and turbulent flows.

Salient features of single-phase turbulent flow modelling are recalled first, including the closure problem, the statistical RANS models, the Lagrangian stochastic approach (one-point PDF method) together with its extension for near-wall turbulence, and the basics of Large-Eddy simulation (LES). In the second part of the chapter, two-phase dispersed turbulent flows in the Eulerian-Lagrangian approach are addressed. The issue of turbulent dispersion in RANS is succintly presented. Then, the subfilter dispersion in LES is discussed at length; functional and structural models are described, and some recent ideas about closures in terms of stochastic diffusion processes are discussed. Examples of computational results are presented for homogeneous isotropic and wall-bounded turbulence. At last, a specific modelling study of particle-laden channel flow is recalled where a low-order dynamical system with a reduced number of fluid velocity modes is constructed.

The objective of this chapter is twofold. First, it provides a general overview of the Eulerian-Lagrangian modelling approach to the numerical simulation of turbulent dispersed flows in the point-particle limit. Second it reviews the phenomenology of particle deposition and resuspension in wall-bounded turbulence as brought to light by Eulerian-Lagrangian studies over the last two decades. Specific interest is devoted to the case of inertial particles, which are ubiquitous in environmental and industrial flow-systems. Effects due to particle shape on deposition and resuspension mechanisms, as well as on numerical modelling are also addressed.

The purpose of this chapter is to provide an introduction to surface forces and to highlight their role in the context of particle deposition and resuspension. For that purpose, surface forces are first presented with a specific emphasis on the DLVO theory, which combines both van der Waals and electrostatic double-layer contributions within a single theory. The limitations and possible extensions of the DLVO theory are also briefly outlined, especially the role played by surface properties (such as surface roughness or surface charge heterogeneities). Then, the impact of such surface forces on particle deposition and resuspension is analysed with a brief review of some experimental results. Besides, the development of modelling approaches including the coupling and resulting effects of these various phenomena/mechanisms (with very different spatial- and time-scales) is illustrated with a thorough description of a one-point PDF modelling approach together with corresponding numerical results.