R. Richter (MPI Halbleiterlabor, München)
Design and simulation of a silicon radiation detector for the Belle II experiment

Semiconductor radiation detectors exploit the photoelectric effect for light or particle detection in which the detector performance is given by the signal to noise (S/N). The obtainable signal is given by the energy necessary to generate one electron-hole pair which is in silicon only 3.6eV. With modern silicon technologies very low noise detectors can be fabricated.
The Depleted Field Effect Transistor (DEPFET) is a semiconductor detector concept which combines detection and amplification within one device. Due to it's low input capacitance the intrinsic S/N is very high. In addition it can be operated at very low power. Both features open a lot of application fields in High Energy Physics and astronomy. Optimization of technology and design need elaborated simulation tools. The talk covers design and simulation of a DEPFET detector array for the Belle II experiment at KEK in Japan.

Priv.-Doz. Dr. A. Glitzky (WIAS, FG1)
Existence of bounded steady state solutions to spin-polarized drift-diffusion systems

We study a stationary spin-polarized drift-diffusion model for semiconductor spintronic devices. This coupled system of continuity equations and a Poisson equation with mixed boundary conditions has to be considered in heterostructures. In 3D we prove existence and boundedness of steady states. If the Dirichlet conditions are compatible or nearly compatible with thermodynamic equilibrium the solution is unique. The same properties are fulfilled for a space discretized version of the problem: Using a Scharfetter-Gummel scheme on 3D boundary conforming Delaunay grids, the existence, boundedness and, for small applied voltages, the uniqueness of the discrete solution is obtained.

Dr. H. Si (WIAS, FG3)
On 3D Delaunay mesh generation

This talk discusses the problem of generating 3D tetrahedral meshes whose elements satisfy the Delaunay criterion. It is motivated by finite volume algorithms for the numerical solution of PDEs.
In this talk, we present a practical technique for solving this problem. Our method first recovers domain boundary by constructing a constrained Delaunay tetrahedralization, then it improves the mesh quality by a variant of Delaunay refinement algorithm. An implementation is available in the software TetGen.

H.-J. Diersch (DHI-WASY GmbH)
Computational aspects in porous media problems Name

Flow, mass and heat transport through porous media encounter in many branches of engineering and science. Of particular concern are those processes occurring beneath the surface of the earth's ground, that means subsurface flow and transport in geologic media with their complexity and uncertainty. Men are also looking for new technologies of exploiting geothermal energy and storing fluids in reservoirs. Industries are developing new materials with improved properties for which a greater understanding of flow and energy transport is required. There is a wide spectrum in porous media flow modeling ranging from multi-phase flow via chemical reaction systems, fracture flow modeling, deformation processes to different numerical approaches.
DHI-WASY is the developer of the commercial finite-element simulator FEFLOW. We briefly characterize the status quo regarding the capabilities available in FEFLOW, numerical features and inherent software technologies. The present paper will discuss computational aspects along three chosen fields of porous media problems for 2D and 3D applications.

(1) Multi-diffusive fingering convection processes: New powerful features of the simulator are presented. It covers extensions to multi-species reactive transport equations, species and temperature related multiple density expansion parameters and multiple fluid viscosity relations.

(2) Unsaturated hysteretic flow in absorbent swelling porous media: The flow and deformation processes in swelling porous media are modeled for absorbent hygiene products (e.g., diapers, wipes, papers). The governing modeling equations are strongly nonlinear and requires advanced numerical strategies for solving. Mesh movement and mesh-refinement strategies are incorporated. Spline approximations are used for better and more flexible descriptions of experimental data and measured relations.

(3) Modeling of borehole heat exchanger (BHE) systems: We briefly discuss fundamental equations for BHE systems and their finite-element representations. Improved relationships for thermal resistances of BHE has been developed. The numerical solution of the final 3D problems is performed via a widely non-sequential (essentially non-iterative) coupling strategy for the BHE and porous medium discretization. Practical application to a borehole thermal energy store (BTES) consisting of 80 BHE is given for the real-site BTES Crailsheim, Germany. The simulations are controlled by the specifically developed FEFLOW-TRNSYS coupling module. Scenarios indicate the effect of the groundwater flow regime on efficiency and reliability of the subsurface heat storage system.

Prof. H. Gajewski (WIAS)
On a variational approach for domain separation

We are interested in algorithms for constructing surfaces Γ of possibly small measure that separate a given domain Ω into two regions of equal measure. Using the integral formula for the total gradient variation, we show that such separators can be constructed approximatively by means of sign changing eigenfunctions of the p-Laplacians, p → 1 , under homogeneous Neumann boundary conditions. These eigenfunctions turn out to be limits of steepest descent methods applied to suitable norm quotients.