Laurence Bergougnoux

Spreading fronts and fluctuations in sedimentation

A diffuse interface or “front” at the top of the suspension is investigated experimentally and numerically. The width of the front is found to grow linearly in time, mainly due to a polydispersity of particle size in the very dilute experiments, and due only to fluctuations in particle density in the simulations. Away from the front, the fluctuations in the particle velocities are found not to decay.

The motion of solid spherical particles falling in a cellular flow field at low Stokes number

We present a jointed experimental and numerical study examining the influence of vortical structures on the settling of solid spherical particles under the action of gravity at low Stokes numbers. The two-dimensional model experiment uses electroconvection to generate a two-dimensional array of controlled vortices which mimics a simplified vortical flow. Particle image-velocimetry and tracking are used to examine the motion of the particles within this vortical flow. Particle motion is compared to the predictions of a numerical simulation inspired by the model equation developed by Maxey [“The motion of small spherical particles in a cellular flow field,” Phys. Fluids 30, 1915 (1987)].

Size, velocity, and concentration in suspension measurements of spherical droplets and cylindrical jets.

The principle of an optical technique for simultaneous velocity, size, and concentration in suspension measurements of spherical droplets and cylindrical jets is proposed. This technique is based on phase Doppler anemometry working in the dual burst technique configuration. The particle size and velocity are deduced from the reflected signal phase and frequency, whereas the amplitude ratio between the refracted and the reflected signals is used for measuring the concentration of small scatterers inside the particles. Numerical simulations, based on geometrical optics and a Monte Carlo model, and an experimental validation test on cylindrical jets made of various suspensions, are used to validate the principle of the proposed technique. It is believed that this new technique could be useful in investigating processes in which liquid suspensions are sprayed for surface coating, drying, or combustion applications.

On stratification control of the velocity fluctuations in sedimentation

We have tested whether stratification can govern local velocity fluctuations in suspensions of sedimenting spheres. Comparison of the proposed scaling for local control of fluctuations by stratification to experimental data demonstrates that this mechanism cannot account for the reduction of the observed velocity fluctuations.

Characterization of an optical fiber bundle sensor

An optical fiber sensor made of a central emitting fiber and two concentric crowns of receiving fibers is presented. This sensor is mainly used to measure high solid volume concentrations in suspension. The aim of this article is to accurately characterize the geometrical properties of this sensor comparing experimental data and simulation. Experimentally, the sensor is used as a displacement sensor with a metallic mirror as a target. This geometrical problem is simulated by a Monte Carlo method, which takes into account: (1) The experimental determination of the angular distribution of light intensity for the emitting fiber; (2) The finite size of fibers by means of a numerical integration procedure; and (3) The multiple reflections of light on the fiber sections with an explicit use of Fresnel’s factors. By fitting the experimental data with the results of the simulation, we can determine the geometrical properties of the probe and photon efficiency (voltage output versus number of launched photons) of ...

Spreading fronts in sedimentation of dilute suspension of spheres

The thickness of the diffuse front between a sedimenting dilute suspension and the clear fluid above grows linearly in time due to polydispersity in the size of the particles and due to a hydrodynamic effect in which randomly heavy clusters fall out of the front leaving it depleted. Experiments and simplified point-particle numerical simulations agree that these two effects are not simply linearly additive.

Fluctuations and stratification in sedimentation of dilute suspensions of spheres

We have tested in experiments and simulations whether stratification can control velocity fluctuations in suspensions of sedimenting spheres. The initial value and early decay of the velocity fluctuations are not affected by stratification. On the other hand, in the descending front where the stratification is strong and well defined, the velocity fluctuations are inhibited according to a previously proposed scaling. In between, after the initial decay and before the arrival of the front, the local value of the stratification does not always play a role.

Non-Poisson statistics of settling spheres

Direct tracking of the particle positions in a sedimenting suspension indicates that the particles are not simply randomly distributed. The initial mixing of the suspension leads to a microstructure which consists of regions devoid of particles surrounded by regions where particles have an excess of close neighbors and which is maintained during sedimentation.