Thomas Zauner

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Dipl.Phys. Thomas Zauner
visiting scientist
Office:1.076
Phone:+49 711 685-63932
Fax:+49 711 685-63658
Email:Thomas.Zauner _at_ icp.uni-stuttgart.de
Address:Dipl.Phys. Thomas Zauner
Institute for Computational Physics
Universität Stuttgart
Allmandring 3
70569 Stuttgart
Germany



Research Overview

Sandstone is a very common natural porous medium. ©wikipedia

The general field of my research is the computational investigations of natural porous media. A typical example of such a natural porous medium is sandstone. It consists of very small consolidated grains that form a solid backbone but, due to the space between individual grains, is permeable for fluids. Physical transport phenomena of natural porous media, such as flow rates, trapping behaviour and prediction of effective transport coefficients, have always been of great practical interest for the oil and gas industry and has currently drawn public attention in the context of CO2 sequestration. Many industrial applications use porous materials. Examples are fuel cells, filtration processes, catalyst in chemical reactions or building materials with special physical properties.


The scientific problems I address in my work are:

  • Developing algorithms that generate realistic and macroscopic computer models for granular porous media. These models are made of several hundred millions of individual grains whose positions, orientations and sizes must fulfil certain correlations and constraints. The associated large computational demand requires efficient and highly scalable parallelized algorithms.
  • Identifying and evaluating numerical methods that allow accurate and predictive calculations of physical properties of porous media. These calculations typically require numerically solving partial differential equations within a chosen discretization scheme and at a specific resolution or order of accuracy.


Realistic computer models for granular porous media

In the reconstruction procedure a dense packing of spheres is created using a Monte-Carlo or Discrete-Element-Simulation. The packing must fulfil given constraints such as density, overlap, size distribution and others. The spheres are later substituted by more complex geometrical objects determined by the shapes of the grains used. One option is to use polyhedrons as grain shapes. The resulting model is a continuum model. The grain centres, sizes and orientations are continuous in space and the grain shapes are analytical defined. An example of such a model for Fontainebleau sandstone is shown in figure 1. The continuum computer model can then be discretized at any desired resolution (figure 2) for further analysis and computer simulations.

Figure 1: Three dimensional rendering [2] of a model for Fontainebleau sandstone. The model contains over 1.2 million quartz grains. Each grain is modelled as a polyhedron. A crosssection is shown in orange.
Figure 2: Two dimensional crosssections of the discretized model.The sidelength of the crosssection is L and the resolution a. The porespace is black and the grains are gray. At the highest resolution even narrow channels are resolved by more than 20 lattice nodes. This allows high precision flow simulations.

Flow Simulations

The permeability of a porous medium is an example of a physical transport parameter. It describes the mediums ability to transmit fluid flow through it. Using Darcy's law the permeability of a porous medium is calculated from the velocity and corresponding pressure field on the porescale [1,3]. These fields are solutions of porescale hydrodynamic partial differential equations. Different numerical methods that solve different hydrodynamic equations can be used to obtain the velocity and pressure field. Possibilities are Navier-Stokes simulations and Lattice-Boltzmann simulations. Our Lattice-Boltzmann implementation, for example, numerically solves the Boltzmann equation and is run until a stationary solution has been reached. From this stationary solution the hydrodynamic fields are calculated. Simulations in stochastic geometries often require very large system sizes to obtain representative solutions. In our investigations, typical sizes of the discretized system are in the range 1024³-4096³ voxel.

Very small subvolume (128³ voxel) of a sandstone at a resolution of 5µm. The porespace-matrix interface is shown as a mesh. The velocity magnitude in lattice units, as calculated by a Lattice-Boltzmann simulation, is shown within the porespace. Areas of slow flow are blue, fast flow is shown in red. Empty porespace indicates areas where the fluid is at rest. Only a small portion of the porespace contributes to the mass transport. Media:poremovie.avi

Publications

[1] A. Narvaez, T. Zauner, F. Raischel, R. Hilfer, J. Harting, “Quantitative analysis of numerical estimates for the permeability of porous media from lattice-Boltzmann simulations”, Journal of Statistical Mechanics, P11026, 2010, Preprint

[2] S. Grottel, G. Reina, T. Zauner, R. Hilfer, T. Ertl, "Particle-based Rendering for Porous Media", SIGRAD, 2010, Preprint

[3] J. Harting, T. Zauner, R. Weeber, and R. Hilfer "Numerical modeling of fluid flow in porous media and in driven colloidal suspensions" in High Performance Computing in Science and Engineering '08, edited by W. Nagel, D. Kröner, M. Resch, Springer ,2008. Preprint

[4] R. Hilfer and T. Zauner "High precision synthetic computed tomography of reconstructed porous media" in Physical Review E, vol.84, p. 062301 (2011)