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Accueil > La recherche > Coupling solid and fluid mechanics

Coupling solid and fluid mechanics

Advancing in the knowledge of the rheology and the mechanical behaviour of fluid and solid mixtures would allow major progresses in a variety of fields related to environmental and industrial engineering. To achieve this objective, a multi-scale (from grains to phenomenon scales) and multi-physics (interfaces, solid fluid transitions...) approach involving a combination of advanced experiments and numerical modelling is required. In terms of engineering outcomes, the research will be more specifically focused on the prediction of geomaterial instabilities and related hazards and on industrial transformations of granular and fibrous materials.

In the engineering of geomaterials (soil, rock, concrete, snow, and more generally cohesive-frictional materials), macroscopic behaviour results from mechanisms of deformation at several smaller scales. Improved modelling of geomaterials thus requires multi-scale models, which in turn require a better understanding of pertinent phenomena at the different scales. An obvious example is the case of granular materials where contacts between grains determine the response at large scale and this behaviour can be modified by attrition, grain breakage, fluid menisci or cementation (e.g., during bio-remediation). All these aspects need better characterization to feed the development of more refined modelling approaches. A more complex example is presented by clay rocks, e.g., in the context of nuclear waste storage. Clay rock structures, and thus the mechanisms of deformation, involve different structures across a wide range of scales (from nm to mm), including clay platelets, clay aggregates, mineral inclusions and cracks. These structures and their interactions determine the long term behaviour of the material and its ability to act as a fluid barrier and thus ensure containment of stored waste. Whilst significant inroads have recently been made using x-ray tomography (Lenoir et al. 2006, Hall et al. 2010), the multi-scale characterization of the deformation in geomaterials is still a big challenge and requires new experimental tools and approaches working at a range of scale lengths.

Rapid mass movements (e.g., avalanches and debris flows) are another challenging area of the engineering of geosystems. Their modelling must account for all phases involved in the phenomena : triggering and release, propagation and run-out plus the possible influence of obstacles. Such issues are tackled in particular at Irstea (Cemagref), e.g., solid-fluid transition and rupture in geomaterials (Nicot et al. 2007) under heterogeneous and stratified conditions (snowpack, armoured torrential beds, etc..).

The mechanical performance of concrete structures is now appreciated to depend on the behaviour of concrete at several scales (from centimetres for gravel, millimetres for sand, tens of microns for limestone filler, down to nanometres for C-S-H hydrates). One noticeable demonstration is the so-called size effect (the larger the structure, the lower its strength). Addressing such issues implies the use of theories that take into account the microstructure in one way or another (non local approaches, higher order continua, e.g., Pijaudier-Cabot & Dufour 2010). This is particularly relevant when addressing the durability of structures such as confinement vessels of nuclear plants, or liquid natural gas reservoirs, where preventing crack propagation and leakage is essential (e.g., Dufour et al. 2008, Pijaudier-Cabot et al. 2009). Similar challenges exist for others engineered barriers such as clay-based nuclear waste disposal sites. In all these cases, engineering profits from the low permeability of the material, but must insure its durability with respect to hydro mechanical loading and all sort of chemical aggression. More generally, vulnerability and management of large, aging and at-risk infrastructures (dams, nuclear power plants, bridges, levees, protection structures, wind turbines, off-shore oil platforms, etc.) are major technical and social concerns for the future.

In mechanics of industrial complex media (paste, fiber suspensions, porous media…), the study of the interactions between multi-physics phenomena (solid and fluid mechanics, thermal transfer, phase change,…) and microstructure (pores, fibers, grains,…) on the overall properties of heterogeneous materials is a long-standing problem (Auriault et al. 2009) concerning a wide range of applications (transport, health, energy, food industry…). The assessment of predictive models in order to manufacture materials with a desired set of properties suitable for a given application requires a better knowledge of the microstructure and the basic micro-mechanisms arising at different scales. For example, the main goal of paper and composite forming techniques is to optimize forming processes, by handling the coupling between the rheology and the complex arrangements of short or longer fibers embedded (or not) in a non-Newtonian (or not) matrix. Their understanding and modelling involve challenging problems at several scales : the fibres, bundle and textile scales (Le and al. 2008, Orgeas et al. 2008, Dumont et al. 2009, Chalencon et al. 2010). Similar challenges arise to develop biomimetic materials, i.e. seeking to replicate or mimic biological processes and materials, for artificial organ implants or scaffolds.

Systems involving mixtures of fluids and granular materials represent another important (and rapidly evolving) multidisciplinary research area. Dilute multiphase flows (involving mixtures of gas, solid and liquid) are fundamental to many industrial and natural systems. Major challenges are related to particle-turbulence interactions and unstructured interface topologies. Flows of dense mixtures of fluids and solids (Cartellier et al. 2009, Rognon et al. 2007, Catalano et al. 2010) constitute an emerging research topic, which is important for a number of applications related to natural hazards, process engineering (paper, fibrous materials, food processing…), blood flow, civil and environmental engineering (liquefaction of soils - Michallet et al. 2009, Mory et al. 2007, sediment transport in rivers and ocean, water treatment…).

There is thus a wide range of problems involving both fluid and solid mechanics : fluid flow in a solid, flow of solid-fluid mixtures, and granular flows. Solid mechanics has traditionally focused on the mechanics of granular and porous systems excluding strong reorganization of the granular structure in which the fluid flows. The study of solid-fluid mixtures in fluid mechanics has long regarded the solid phase merely as a concentration with no solid-like behaviour. Interconnecting knowledge from the two disciplines is necessary to tackle problems where the fluid phase is intimately related to the solid phase or when the solid phase behaves as a fluid (granular flows). Traditionally the two disciplines have focused on different length scales. Multi-scale modelling of multi-phase materials/flows requires overcoming the borders between the disciplines for a full integration of the models on different length scales into a coherent framework. That project will be developed under the following three main directions.

Solids in flows

Some unresolved issues in this topical area concern conditions in which the scale of the solid structures and the scale of the system are not clearly separated. Such situations arise in confined flows (by boundaries, by a free surface) or in presence of large solid inclusions or solid aggregates as for instance in natural debris flows in mountainous areas (Chambon et al. 2009), in flows of complex fluids (Le Corre et al. 2010), in concentrated fiber suspensions (Dumont et al. 2003, Orgeas et al. 2008) encountered in process engineering. From an experimental standpoint, these particular flows must be further studied in order to better lunderstand deformation mechanisms at the particle scale : when combined with in situ micro-rheometry, X-ray tomography is a possible relevant tool to reach this goal (Le et al. 2008, Chalencon et al. 2010). From a theoretical and numerical standpoint, if the scale separation is weak, standard upscaling techniques must be reconsidered either by accounting for enriched micromechanics (Le Corre et al. 2004, Dumont et al. 2009) or by searching for higher order terms in the upscaling process (Auriault et al. 2005) : in both cases, standard equivalent continua arising at the macro-scale must be replaced by generalized continua. If there is no clear scale separation, rigorous upscaling techniques become inappropriate, and direct simulation techniques based on discrete element simulations (for example) are certainly a promising approach in order to build relevant phenomenological macro-scale models.

Other important issues originate from situations where the solid phase concentration exhibits strong heterogeneity produced by segregation in the presence of fluid or size sorting by differential transport (Jossic et al. 2002). Granular micro-mechanics is in this case the most promising approach to derive constitutive relationships at the macro-scale.

Simple situations of mixture flows such as down-slope uniform flows have received much attention. However from the engineering standpoint, non-stationary dense fluid mixtures in which the solid phase inertia are also (and possibly more) important (Grasso et al. 2009, 2010). Rapid dense non-stationary flows in strong interaction (erosion, deposition) with the underlying solid substratum (sheet flows, sedimentary plug flows …) are prone to instabilities that can also trigger suspensions. Accounting for particle inertia, fluid acceleration effects but also turbulence collective effects in these flows is becoming tractable by two-phase flow approaches (Chauchat & Médale, 2010).

All these issues require experimental characterization of the kinematics fields of both the particles and the fluid, however severe experimental bottlenecks require using cutting-edge instrumentation such as X-ray tomography at high spatial resolution or acoustic probing techniques where usual optical methods fail because of high particulate concentration (Hurther & Lemmin, 2010).

Fluids in solids

Although coupled analysis of fluid flow in porous media may appear somehow a classic problem, it still poses a number of challenges which have a direct impact on environmental- and bio-engineering applications :

  • Interaction of crack/damage propagation and strain localization with pore fluid(s) , especially in the case of progressive de-saturation – with associated permeability changes – in cohesive geomaterials (e.g., clay rocks and concrete). Failure/damage propagation and simultaneous dynamics of fluid front(s) will be experimentally investigated by applying state-of-the-art full-field measurement techniques (X-ray and neutron tomography) in-situ, i.e., scanning the material while it deforms under load (Desrues et al. 1996, Lenoir et al. 2007, Viggiani & Hall 2008). Modelling hydro-mechanical (HM) coupling in the presence of localized damage/deformation will be achieved either through HM coupling at the micro-scale (double scale models based on homogenization schemes with several micro structures, Dascalu et al. 2008, Dascalu and Bilbie 2007), or by using enriched continua formulations (e.g., local second gradient models, Chambon et al. 2001, 2004, Collin et al. 2006) for two- or three-phase materials.
  • Mechanical behaviour and structural evolution of anisotropic fibrous media (e.g., paper) in evolving environmental conditions (e.g., wetting, drying), which is strongly influenced by the intrinsic mechanical properties of the fibers or their chemical treatment. Several advanced observation techniques (Tof-SIMS, TEM, AFM, ESEM, X-ray nano- and micro-tomography) will be used to experimentally inspect (in 3D and at several scales) the structure of the fibrous media, possibly under applied load. This will allow for obtaining a precise geometrical description of the fibers skeleton (Rolland du Roscoat et al. 2005, 2007, Vernhes et al. 2008, Charvet et al. 2010, Considine et al. 2010), which is, together with the interaction between the elements of the fibrous media, one of the major ingredients for realistic modelling. Micro-meso mechanical models will be developed based on these observations, using homogenization methods for discrete or continuous media (Rolland du Roscoat et al. 2008, Koivu et al. 2010, Viguié et al. 2010).
  • Flow of particulate or structured fluids (containing polymers, nanoparticles, nanocrystals, …) in porous media (Auriault et al. 2002, Orgéas et al. 2006, 2007, Loix et al. 2008, 2009), with the dilute particulate phase scale corresponding to the size of the pores. These flows are relevant for a number of technological processes (filtration, impregnation (Denneulin et al. 2008), petroleum exploration and recovery, underground waste disposal …) but also for the study of bio fluids in the human body. The challenge here is to understand the interactions between fluid and porous matrix at the macro- and micro-scale, which requires the characterization of the fluid at all scales (structure, micro-rheology, ..), as well as a fine description of the matrix and the determination of the physico-chemical interactions between the fluid and the matrix.

Solids and fluids

The interaction of rapid gravitational flows
(avalanches, mudslides) with obstacles (buildings, protection structures) is a booming research domain raising important scientific issues : (i) how do obstacles influence the flow in terms of energy flux deviation and energy dissipation ? (ii) what are the forces exerted by the flow on the obstacle ? (iii) what is the dynamical response of the obstacle submitted to such flows ?

The first objective is to provide a detailed characterization of the triggering of such dense gravitational flows and of the dynamics of structure-free flows (analog and real materials, granular materials, and muds) at flow scales ten times greater than today’s scale models. The triggering processes are strongly related to instabilities and solid-fluid transition (Nicot et al. 2007), which will be analyzed experimentally and numerically. The next objective is to study the interaction of these flows with obstacles and to measure the pressure fluctuations over time and space on a relatively large structure (Faug et al. 2003, 2009). The results of these measurements will be used directly to simulate the behaviour of a structure subjected to these same loadings to validate the hypothesis of obstacles remaining fixed and rigid during the experiments. This experimental work will result in the reliable characterization of spatiotemporal loadings while taking into consideration fluid–structure coupling. Grenoble research units are already very active in this domain and have been recognized for their work on the flow–obstacle interaction also by using numerical approaches such as Saint-Venant modelling (Naaim et al. 2003, 2004), SPH modelling (Laigle et al. 2005) and discrete numerical simulations (Faug et al. 2008)

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