Inertial particles in turbulence

In a collaboration with LEGI (Laboratoire des Ecoulements Géophysiques et Industriels, Grenoble, France), I have started studying particle laden turbulent flows during my post-doc in Grenoble in 2008. I aim at characterising heavy particles dynamics in homogeneous isotropic turbulent flows. Since 2011, in collaboration with Anne Dejoan from CIEMAT (Madrid, Spain), we are addressing the settling velocity enhancement of particles in turbulent fields from a numerical point of view. More recently, in collaboration with David De Souza and Till Zurner, I have designed at ENSTA a new experiemnt, in water. With Marc Massot from Ecole Polytechnique and Aymeric Vié from CentraleSupelec, we are more interested on the back-reaction of the particle on the carrier fluid.

Particle laden flows are linked to many industrial and environmental issues and the comprehension of the basic mechanisms underlying their dynamics is of prime interest regarding design and optimisation of industrial processes (oil transport, combustion, chemical reaction...) as well as the understanding of natural phenomenons (plankton dispersion in ocean, rain formation, pollution of atmosphere...). For the last forty years, many fundamental studies have allowed observation and characterisation of many properties of these flows. When particles size is much smaller than smaller turbulent scales and when their density matches the fluid's one, they behave as tracers and follow the fluid dynamics. Nevertheless, when they have an inertial behaviour due to their size or density, they do not follow the fluid any more and dynamic is more complex: terminal velocity of these particles is enhanced in the gravity field, the concentration field presents strong inhomogeneities and cluster formation is observed.

Clustering and LEGI experiment:

How cluster emerges? Do they have a specific organisation and dynamics? How do they interact with the carrier flow? Here are many basic questions that are still open. In collaboration with people from LEGI (Laboratoire des Ecoulement Géophysiques et Industriels in Grenoble, France) and with the support of French ANR, we studied the dynamics of a cloud of water droplets in an air flow. The carrier flow was a grid generated isotropic homogeneous turbulence already widely used in LEGI. Measurements were performed by Particle Tracking Velocimetry using an ultra-fast phantom v12 camera at a rate of 10kHz. We aimed at being able to characterise the effect of clustering not only on a statistical basis, but also on a dynamical point of view.

Experimental wind tunnel at LEGI, Grenoble, France.

Measuring concentration

A key point is to be able to measure precisely the concentration field. In that scope, we proposed a new approach to analyse particle concentration fields based on Voronoï tessellations which give a measure of the local concentration field at inter-particles length scale. By itself, this data processing technique is particularly enlightening to explore and quantify the preferential concentration phenomenon while, combined to Lagrangian tracking, it also gives access to simultaneous measurements of velocity, acceleration and local concentration along particles trajectories which are crucial for a better insight of clusters and particles dynamics.

Example of a Voronoï tessellation in experimental data from LEGI.

Disentangling the role of numerous control parameters and ENSTA experiment

An other crucial issue is to disantangle the respective role of the numerous control parameters (Stokes, Froude, Rouse and Reynolds numbers, density ratio, volume fraction). To achieve that we work with a broad range of particle types in water (ceramic, glass, tungsten, steel...). Particles are fed from top. They are tracked while PIV is used to measure the fluid velocity field.

Our new experiment at ENSTA. Two oscillating facing grids generate moderate Reynolds number turbulence in water. Flow is seeded with particles of various densities and diameters.

Simultaneous fluid and particle measurements

A key point to understand the settling velocity alteration of particles due to turbulence or to assess the effect od the back-reaction of the particles on the carrier fluid is to be able to combined Lagrangian tracking of inertial particles and eulerian fluid velocity field measurements. We have designed a setup to achieve this goal.

Example of a simultaneous particle and fluid measurement in ENSTA experiment.

© Romain Monchaux
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