TW 2008_02

Bart Verleye
Computation of the permeability of multi-scale porous media with application to technical textiles

Advisor(s): Dirk Roose, Stepan Vladimirovitch Lomov

Abstract

Technical textiles are used as reinforcement in composite materials. The permeability of the textiles is an important input parameter for the simulation of the impregnation stage of the Liquid Composite Moulding process, an often used production technique for composite materials.

Textile reinforcements are multi-scale porous structures and the permeability is considered on the dirent scales. The fibres inside the yarns determine the micro-scale properties of the textile. On the meso-scale level, we consider a unit cell of the textile, which has the same average permeability as the macro-scale textile layer.

We present a fast and accurate method to compute the permeability of textile reinforcements, based on the finite dirence discretisation of the Stokes equations. The input for the CFD simulations (Computational Fluid Dynamics), is a unit cell of the textile model, provided by textile modelling software like WiseTex and TexGen. If intra-yarn ow is taken into account, the Stokes equations are locally extended with a penalty term, resulting in the Brinkman equation. The penalty is computed analytically based on the properties of the fibres inside the yarn. We compare dirent formulas to compute this penalty.

We validate the results of our simulations with permeability values obtained by experiments. Results for woven fabrics, random structures and noncrimp fabrics are presented. Moreover, experimental and computational results are compared for a structure that is designed to minimise the experimental errors. The in uence of shear and nesting on the permeability of textiles is discussed.

To avoid the 3D-simulation problem, dirent methods that reduce the dimension of the problem are presented in literature. The Grid2D method reduces the 3D Stokes problem to a 2D Darcy problem. We explore this method, and compare the results with our CFD computations.

A second model-reduction method is the pore-network method. This method describes the porous medium as a network of pores, and computes the overall permeability via the law of conservation. The Stokes solver is applied to compute the conductivity of the pores of the network. We explain how our Stokes solver is used for this method. We also compare the results of the method with CFD computed values for samples of porous rock.

lirias 1979/1744 / Doctadmin 3E041034 / text.pdf (5.5M) / mailto: twr team