Jean-Louis Marié

An upward turbulent bubbly boundary layer along a vertical flat plate

Abstract The structure of an upward wall-bounded bubbly flow is investigated in the simple case of a turbulent boundary layer developing on a vertical flat plate. The data reported is part of a research program currently under progress. They concern the void fraction distribution, the wall shear stress, and the mean liquid velocity profiles. It is shown that depending on their mean diameter, a significant fraction of the bubbles deflected towards the wall. This migration, together with a significant deceleration of the bubbles at the surface, prove to be the two main mechanisms responsible for the so-called void peaking phenomenon. Besides, the skin friction coefficient which depends both on the amplitude of the peak, and on the free-stream velocity is found to increase in the presence of the dispersed phase. This increase is linked to a modification of the universal logarithmic law of the wall, and to a depression of the wake.

Similarity law and turbulence intensity profiles in a bubbly boundary layer at low void fractions

This paper follows on from the research recently published by Moursali et al. (1995). While the first data reported were relative to the void migration, the wall shear stress and in a very limited extent to the liquid velocity, the present results provide new information on the kinematic and turbulent structure of a bubbly boundary layer at low air concentrations. The mean liquid velocity is shown experimentally to obey a modified logarithmic law of the wall in the presence of millimetric bubbles. An expression for this modified law is derived by simple analytical considerations and non-dimensional scaling. The skin friction calculated on this theoretical basis fits the measurements satisfactorily. Longitudinal turbulence intensity profiles are also obtained. They show that the turbulence is increased by two relatively uncoupled mechanisms: a modification of the wall production and the creation of pseudo-turbulence in the external layer. Finally, the mixing length inferred from the data is given and compared with some of the models which are proposed in the literature.

Three-dimensional reconstruction of particle holograms: a fast and accurate multiscale approach.

In-line digital holography is an imaging technique that is being increasingly used for studying three-dimensional flows. It has been previously shown that very accurate reconstructions of objects could be achieved with the use of an inverse problem framework. Such approaches, however, suffer from higher computational times compared to less accurate conventional reconstructions based on hologram backpropagation. To overcome this computational issue, we propose a coarse-to-fine multiscale approach to strongly reduce the algorithm complexity. We illustrate that an accuracy comparable to that of state-of-the-art methods can be reached while accelerating parameter-space scanning.

Drag and lift forces on interface-contaminated bubbles spinning in a rotating flow

The equilibrium position of a spherical air bubble in a solid body rotating flow around a horizontal axis is investigated experimentally. The flow without bubbles is checked to be solid body rotating. The area of influence of the bubble is characterized to determine for each bubble whether the incoming flow is perturbed or not. The demineralized water used is shown to Tbe contaminated, and spinning of the bubbles interface is observed and measured. From the measurement of the bubbles equilibrium position, drag and lift coefficients are determined. They appear to be dependent on two dimensionless numbers. Eo the E tv s number and Rω the rotational Reynolds o o number (or Taylor number Ta) can be varied independently by changing the control parameters, and for that reason are the convenient choice for experiments. (Re, Ro) with Ro the Rossby number is an equivalent choice generally adopted in the literature for numerical simulations, and Re denotes the Reynolds number. When using this second representation, the Ro number appears to be an indicator of the influence on the force coefficients of the shear, of the curvature of the streamlines of the flow and of the bubbles spinning. The bubbles spinning effect on the lift force is far from trivial. Its contribution explains the important gap between lift values for a bubble (not spinning) in a clean fluid and for a bubble (spinning) in a contaminated fluid as present.