## The LIR Space Partitioning System applied to the Stokes Equations

- We consider two major topics in this thesis: spatial domain partitioning which serves as a framework to simulate creep flows in representative volume elements. First, we introduce a novel multi-dimensional space partitioning method. A new type of tree combines the advantages of the Octree and the KD-tree without having their disadvantages. We present a new data structure allowing local refinement, parallelization and proper restriction of transition ratios between nodes. Our technique has no dimensional restrictions at all. The tree's data structure is defined by a topological algebra based on the symbols \( A = \{ L, I, R \} \) that encode the partitioning steps. The set of successors is restricted such that each node has the partition of unity property to partition domains without overlap. With our method it is possible to construct a wide choice of spline spaces to compress or reconstruct scientific data such as pressure and velocity fields and multidimensional images. We present a generator function to build a tree that represents a voxel geometry. The space partitioning system is used as a framework to allow numerical computations. This work is triggered by the problem of representing, in a numerically appropriate way, huge three-dimensional voxel geometries that could have up to billions of voxels. These large datasets occure in situations where it is needed to deal with large representative volume elements (REV). Second, we introduce a novel approach of variable arrangement for pressure and velocity to solve the Stokes equations. The basic idea of our method is to arrange variables in a way such that each cell is able to satisfy a given physical law independently from its neighbor cells. This is done by splitting velocity values to a left and right converging component. For each cell we can set up a small linear system that describes the momentum and mass conservation equations. This formulation allows to use the Gauß-Seidel algorithm to solve the global linear system. Our tree structure is used for spatial partitioning of the geometry and provides a proper initial guess. In addition, we introduce a method that uses the actual velocity field to refine the tree and improve the numerical accuracy where it is needed. We developed a novel approach rather than using existing approaches such as the SIMPLE algorithm, Lattice-Boltzmann methods or Exlicit jump methods since they are suited for regular grid structures. Other standard CFD approaches extract surfaces and creates tetrahedral meshes to solve on unstructured grids thus can not be applied to our datastructure. The discretization converges to the analytical solution with respect to grid refinement. We conclude a high strength in computational time and memory for high porosity geometries and a high strength in memory requirement for low porosity geometries.

Author: | Sven Linden |
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URN: | urn:nbn:de:hbz:386-kluedo-39458 |

Advisor: | Hans Hagen, David Adalsteinsson, Andreas Wiegmann |

Document Type: | Doctoral Thesis |

Language of publication: | English |

Publication Date: | 2014/12/10 |

Year of Publication: | 2014 |

Publishing Institute: | Technische Universität Kaiserslautern |

Granting Institute: | Technische Universität Kaiserslautern |

Acceptance Date of the Thesis: | 2014/11/06 |

Date of the Publication (Server): | 2014/12/10 |

Tag: | Adaptive Data Structure; Direct Numerical Simulation; LIR-Tree; Partial Differential Equations; Stokes Equations |

Number of page: | VI, 64, III |

Faculties / Organisational entities: | Kaiserslautern - Fachbereich Informatik |

CCS-Classification (computer science): | G. Mathematics of Computing / G.1 NUMERICAL ANALYSIS / G.1.8 Partial Differential Equations / Domain decomposition methods (NEW) |

DDC-Cassification: | 0 Allgemeines, Informatik, Informationswissenschaft / 004 Informatik |

Licence (German): | Standard gemäß KLUEDO-Leitlinien vom 28.10.2014 |