Browsing by Author "Graf, Tobias"

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Open Access
Multiphasic flow processes in deformable porous media under consideration of fluid phase transitions
(2008) Graf, Tobias; Ehlers, Wolfgang (Prof. Dr.-Ing.)
Within this contribution, a multiphasic, continuum mechanical model for the description of porous materials with several fluid constituents under consideration of non-isothermal conditions and phase transition processes between liquid and gaseous pore water was presented based on the well-founded framework of the Theory of Porous Media (TPM). The required thermodynamically consistent constitutive relations were derived via an evaluation of the entropy inequality. In the following, this general model was reduced to a triphasic one consisting of a solid, a liquid water and an overall gas phase, which was built by water vapor and air. Furthermore, a special attention was taken on the numerical treatment of multiphasic flow processes. Finally, the presented initial-boundary value problems showed the capability of the discussed model to simulate engineering problems of practical relevance. In particular, concerning the derived continuum mechanical model, each phase of the porous material is governed by its individual temperature. The solid skeleton is assumed to behave like an elasto-viscoplastic, the pore fluids like viscous materials. Furthermore, the solid skeleton as well as the pore liquids are described in a mechanical sense as materially incompressible constituents, whereby their effective densities are only a function of the respective temperatures. The gaseous pore fluid constituents are assumed to behave like ideal gases building together one overall pore gas phase. It could be shown that the ratio between the partial pressure of a gaseous component within the overall gas phase and the overall effective gas pressure is given by the respective molar fraction, whereas the overall effective gas pressure is given by the sum of the partial pressures of the gaseous constituents, which corresponds directly to Dalton's law. The numerical treatment of the presented triphasic model is based on the finite element method (FEM), whereas extended Taylor-Hood elements with quadratic ansatz functions for the solid displacement vector and linear ansatz functions for the pore fluid pressures and saturations as well as the temperatures are used. Furthermore, the special numerical treatment of multiphasic flow processes in porous materials was discussed, which led to the application of a certain stabilization technique to overcome the occurring numerical problems. Finally, the presented multiphasic porous media model was applied to several two- and three-dimensional initial boundary-value problems, where the FE tool PANDAS/M++ was used. Particularly, typical pollutant infiltration and slope failure problems, injection processes of heated pore gas into a water saturated porous material and the so-called heatpipe problem were discussed. It could be shown that the model is capable to describe the strong interaction between the pore fluids flow and the deformable soil matrix as well as the occurring thermal effects and phase transitions processes between liquid and gaseous pore water.