Modelling Flow of Water and Air in Reconstructed Structures of Porous Media

P. Lehmann 1, M. Krafczyk 2, A.Gygi 1, A. Flisch 3, P. Wyss 3 and H. Flühler 1

1Institute of Terrestrial Ecology, Swiss Federal Institute of Technology, Grabenstrasse 3, 8952 Schlieren, Zurich, Switzerland, lehmann@ and gygi@ and fluehler@ito.umnw.ethz.ch

2Lehrstuhl für Bauinformatik, University of Technology Munich, Arcisstrasse 21, 80290 Munich, Germany, krafczyk@bv.tum.de

3Centre for Non-Destructive Testing, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland, alexander.flisch@ and peter.wyss@empa.ch

ABSTRACT

Flow of water, air and transport of dissolved pollutants in soils and porous media in general, is controlled by the complexity of pore space geometry. To quantify the pore space, we scanned a sand column (2.5 cm in diameter and height) by means of microtomography with a resolution of 60 µm.

For the reconstructed geometry we calculated the coupled water and air dynamics. The flow dynamics of the system was modelled by the Navier-Stokes equations complemented by appropriate multiphase extensions and the resulting discretized equations were solved using a Lattice-Boltzmann approach. The predictions of this method were compared with data from laboratory experiments.

The size of objects dominating flow and transport in soils under water unsaturated conditions are in the range of less than 100 µm. But the structures relevant for fast processes under saturated conditions may have a dimension of several centimeters. So, to quantify both types of structure, we need a possibility to scan larger probes with high resolution. In addition, only larger samples can be used for laboratory experiments and it would be more adequate to scan such samples to explain the experimental results. We reconstructed the scanned geometry of a sand sample with dimensions suitable for laboratory experiments (5.25 cm in height and diameter) by means of Fourier transformation with a resolution of 70 µm.

Keywords soil structure, hysteresis, Lattice-Boltzmann method, Fourier transformation