Philippe Séchet

Bursting phenomenon and incipient motion of solid particles in bed-load transport

In this paper, we present the results of an experimental investigation aiming at understanding the interaction between near-wall coherent structures (or burst) and bedload transport in an open channel flow. This experiment associates Laser Doppler Anemometry measurements of the instantaneous velocity near the wall with real time measurement of sand particle trajectories on the smooth bed of a hydraulic flume. We will first give a description of the bursting phenomenon and we will present the experimental process. Then, we will give some details about the specific signal processing used in this work to detect coherent structures in the velocity signal. To conclude we will show that some characteristic scales of the particle motion are commensurate with those of the bursts.

The role of near wall turbulent structures on sediment transport

In this paper, we present the results of an experimental investigation designed to analyze the interaction between near wall coherent structures (or burst) and bedload transport in an open channel flow. Laser doppler velocimetry (LDV) measurements of the instantaneous velocity near the wall were coupled with real time measurements of sand particle trajectories on the bottom of an hydraulic flume. We will first give a description of the bursting phenomenon and we will present the experimental design. Then, we will give some details about the specific signal processing techniques we used in this work to detect coherent structures on the velocity signal. The third part of this paper will present our results. We will show that the period between two consecutive displacements of a solid particle can be compared to the mean period between two consecutive ejections. This would suggest that these structures are involved in bedload transport. For small displacements, the time distribution between two consecutive movements shows two modes. One would be associated to particles whose mean deposit time corresponds to the ejection period, the second would be associated to particles whose mean deposit time is close to the sweep period. We made the assumption that there exist two transport modes at the wall: the dominant one was transport by ejections, but sweeps can be involved in the removal of particles whose friction coefficient with the wall is high.

Application of synchrotron X-ray microtomography for visualizing bacterial biofilms 3D microstructure in porous media.

The development of reliable models to accurately predict biofilm growth in porous media relies on a good knowledge of the temporal evolution of biofilms structure within the porous network. Since little is known on the true 3D structure of biofilms developed in porous media, this work aimed at developing a new experimental protocol to visualize the 3D microstructure of bacterial biofilms in porous media. The main originality of the proposed procedure lies on the combination of the more recent advances in synchrotron microtomography (Paganin mode) and of a new contrast agent (1-chloronaphtalene) that has never been applied to biofilm visualization. It is shown that the proposed methodology takes advantage of the contrasting properties of 1-chloronaphtalene to prevent some limitations observed with more classical contrast agents. A quantitative analysis of the microstructural properties (volume fractions and specific surface area) of bacterial biofilms developed in columns of clay beads is also proposed on the basis of the obtained 3D images.

A Modified Model for the Prediction of Bioclogging in Saturated Porous Media

Bioclogging or the process that biofilm is conditioned for the development in porous media, leading the biomass accumulation in pore spaces and reducing the permeability of porous media that impedes fluid flow. Numerous research has focused on the study of bioclogging, but the process is very complicated and still poorly understood. This results in various permeability models that are only capable of predicting under certain conditions. In the study, we aim to mathematically develop a macroscopic model to predict permeability reduction in the saturated biofilter. The model adopts the concept that permeability reduction results from two mechanisms: pore radius reduction and pore plugging by mass accumulation. In the former, porosity, grain sizes, tortuosity and bulk factor determine the magnitude of permeability reduction. The pore plugging, in the other hand, interprets recent experimental findings of straining process in deep bed filtration that clogging in pore throat can be induced by deposited particles with the diameter many times smaller than that of pore throat. The model proves its capacity by obtaining a good match to a wide range of experimental data in predicting: permeability of clean porous media and magnitude of bioclogging in the biofilter.

A One Dimensional Two-Fluid Model for Bubbly Flows in Fixed Beds

Pressure drop and gas void fraction are important parameters for the design of multiphase packed bed reactors which are widely used in petrochemical industry. Several experimental studies have been devoted to the hydrodynamics of two-phase cocurrent upflow or downflow through fixed beds, and various correlations of limited range of validity are available in the literature. However, there is not yet a clear agreement on the form of the momentum equations to be used in such systems. Early attempts devoted to the pressure drop estimate were based on an extension of the Lockhart-Martinelli approach (Sweeney 1967), Rao et al. 1983). More recently, Attou at al. (1999) proposed the first serious attempt to adapt the Eulerian two-fluid model to cocurrent bubbly flows through packed beds. From an analysis of their proposal, it happens that the basic mechanical equilibrium for the gas phase needs to be reconsidered. In this scope, we derived a new model on the basis of the so-called hybrid approach initially developed for bubbly flows in ducts in absence of shear-induced turbulence (Achard and Cartellier 2000). As a first application, we considered a mean unidirectional flow of a bubbly mixture through a porous medium composed of beads uniform in size. For steady and fully established flows, and assuming a flat void fraction (α) profile, the resulting momentum equations for each phase write: Liquid phase: −dpdz = ρLg + fLS − fLG1 − α (1)Gas phase: −dpdz = ρGg + fLS + fLGα (2) where fLS is the resultant of the liquid shear stress exerted on beads surface and on exterior walls, and where the quantity fLG = α F* / Vp represents the interaction force density between the gas and the liquid (F* is the mean force on bubbles and Vp = 4πa3 /3 denotes the bubble volume, a being the bubble radius). The main difference with the model derived by Attou et al. is the presence of the fLS term in the gas phase equation. Without this term, the relative velocity of bubbles would be controlled by the axial pressure gradient dP/dz even in non accelerating flows which is unphysical. On the opposite, in the present model (1–2) the relative movement of bubbles is simply due to buoyancy. The set of equations (1–2) provides a mean to exploit the experimental data to derive the required closures, namely the evolution of the friction fLS with the gas content and that of the momentum exchange between phases fLG . Notably, from (1) and (2), one gets fLG = α(1 − α)(ρL − ρG)g (3) In order to establish reliable closures, available experimental data of the literature are currently revisited under this framework. For the friction term, which is the principal contribution to the pressure drop, the usual closure law for fLS as given by an Ergun equation adapted to two-phase flows is under analysis. For the interfacial momentum transfer, the objective is to evaluate an “apparent” drag coefficient defined as Cd = F*/[ρL Ur 2 π a2 / 2] where the mean relative velocity Ur is defined as the difference between the mean gas and liquid velocities averaged over a volume. Indeed, paralleling an approach already exploited for bubbly flows in ducts (Riviere and Cartellier 1999), it happens that the mean void fraction can be derived from equations (1) and (2) assuming a flat void fraction profile: β(1 − β) − α(1 − α) =(4π/3) α (1 − α)[g δ2VSL νc] (aδ)2fd (4) where δ is the typical size of the pores and where fd = (π/2) Rep Cd is expected to be a function of the bubble size, the porosity e and the void fraction. To extract fd or Cd from (4), a characteristic bubble size must be specified. As shown Fig.1, the bubble size is controlled by the bed geometry and evolves between 0.2 δ and 3 δ in the dilute limit (Bordas et al. (2001)). Analysis of the existing data will be presented based on these size estimates, and comparison will be performed of this “apparent” drag with values measured for isolated bubbles in fixed beds (Fig.2).Copyright