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Fundamental aspects of hydrodynamic instabilities in multiphase and multicomponent systems

Research project P4/06 (Research action P4)

Persons :

Description :

The network, composed of five teams (ULB, ULg, UMH, UCL, and VUB), studies a wide range of instability problems in fluid mechanics in systems composed of one or several liquid phases in contact with solid or gaseous phases. It focuses on the properties of such systems, particularly their behaviour at interfaces. The various phases may have several components and the focus includes coupling between different transport processes such as heat and mass diffusion, momentum transport, thermodiffusion, and phase change (solidification, vaporisation).

The primary research objective is to improve modelling of such phenomena by combining theoretical, numerical, and experimental approaches. Fundamental research emphasises the study of instability phenomena in non-equilibrium fluid systems, phenomena such as pattern formation, oscillatory behaviour, and the onset of turbulence. The theoretical aspects of these topics are studied by the ULB, ULg, and UMH teams, using recent techniques provided by modern hydrodynamic stability theory. In parallel, the VUB and UCL teams are developing very sophisticated numerical techniques and adapting them to the study of these problems. The ULB and UMH teams, furthermore, are conducting joint experiments; in addition to ground-based measurements, space experiments are being conducted in the framework of the microgravity programme of the European Space Agency. The latter make it possible to eliminate the disrupting effect of gravity-induced flows on the behaviour of interfaces or on diffusional transport processes.

Experiments are in preparation for space missions, such as heterogeneous nucleation in crystal growth from a vapour phase or surface-tension-induced instabilities in layers of non-isothermal fluids. Very interesting time-dependent phenomena were observed during a space experiment in which surface-tension-induced flows were studied in systems composed of several layers of immiscible liquids.

Understanding the behaviour of macroscopic non-equilibrium systems requires correct modelling of phenomena occurring at smaller scale, such as those observed near interfaces. The network has thus undertaken to describe interface phenomena both macroscopically (interfaces being treated as surfaces of discontinuity between different media) and mesoscopically (interfaces being viewed as three-dimensional regions where fluid composition and density vary at very short distance). The theoretical and numerical modelling of interface phenomena done at the ULB, ULg, UCL, and VUB benefits from the experimental measurements of interface properties and transport coefficients carried out by the ULB and UMH teams. The ULg team applies the tools of thermodynamics specifically to systems involving very short times and short characteristic lengths, as in the case of interfaces.

Recent experimental results are used to model flows in porous media (glass spheres or fractal networks). These experiments and numerical calculations are done by the ULB and UMH teams (buoyancy-induced filtration, one and two-phase capillary creeping, verification of macroscopic laws from modelling at the pore level).

Axes of collaboration between ULB, ULg and UMH concern modelling of mass transport of multicomponent non-isothermal systems, thermocapillary flows in porous media, instabilities at fluid-fluid interfaces in multi-layer systems, modelling of turbulent interfacial flows, and explanation of specific pattern transitions between hexagonal and square convective structures.

Further results obtained at the VUB concern macroscopic simulation of flow in a porous medium, simulation of natural convection and non-Newtonian fluid flows, and modelling of biphasic flow patterns in a tube.

A promising area of research, developed jointly by the ULg, UCL, and VUB teams, concerns numerical modelling of turbulent flows occurring in solidifying liquid mixtures.

In the long run, interactions between the various network teams should lead to developing numerical tools for studying a wide range of problems related to fluid interfaces, instabilities and phase change phenomena…

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