Rettinger C (2023)
Publication Language: English
Publication Type: Thesis
Publication year: 2023
URI: https://open.fau.de/handle/openfau/22561
Fluid flows that transport rigid particles are ubiquitous in Nature and industrial processes where they exhibit a rich set of fascinating and complex dynamics. Developing a thorough understanding of these particle-laden flows is mainly based on studies at the microscale, where relatively small but representative systems are investigated in detail. Corresponding simulations that aim to assist this procedure are challenging due to the vast computational costs involved in accurately resolving all relevant physical effects. Their successful usage necessitates efficient algorithms designed for high-performance computing and extensive verification studies to ascertain the simulation's broad applicability and correctness.
This thesis develops a coupled fluid-particle simulation method that tackles these challenges and demonstrates its usability for such fully resolved simulations at the microscale.
To this end, we combine the lattice Boltzmann method with the discrete element method in the open-source framework waLBerla.
While the former method is used to efficiently represent the fluid flow dynamics,
the discrete element method accounts for frictional collisions between the rigid particles.
Their coupling is based on the momentum exchange between the fluid and the particle phase.
We provide a detailed description of the algorithmic components and all involved model parameters.
Focusing on the characteristics of the applied coupling approach, we present and compare several techniques to improve the simulation's accuracy and stability.
Measures include higher-order boundary conditions, temporal averaging of interaction quantities, and adaptions of the lattice Boltzmann method.
We assemble an extensive calibration and validation pipeline for all relevant aspects of dense particulate flows, universally applicable for such simulation approaches. This concept is utilized to gain valuable insight into our coupled algorithm's behavior and derive guidelines for the model parameterization. In particular, explicitly considering short-range hydrodynamic interactions between particles and an adequate temporal resolution of single collision events are found to be essential ingredients to obtain accurate collision dynamics of submerged particles.
The method's algorithmic components work on strictly localized subsets of the complete data. This characteristic feature is a prerequisite for efficient usage of the immense compute power offered by today's supercomputers. In many cases, the domain sizes and runtimes necessary for simulative microscale studies can only be reached through such massively parallel execution with thousands of processes. We explore the possibilities and limitations of our unified parallelization strategy via scaling experiments on up to 65536 processes. As fluid-particle simulations often exhibit a spatially and temporally varying workload, we show that performance improvements can be achieved via dynamic load balancing techniques. They quantify current imbalances between the processes and then re-distribute the workload.
We finally illustrate the predictive capabilities of our method by investigating the erosion and transport of densely packed sediment beds consisting of several ten thousand particles with vastly different sizes. The detailed information obtained from such a simulation allows us to study phenomena like size-based vertical sorting and derive rheological descriptions of the macroscale system dynamics. This thesis thus contributes a valuable building block in the multi-scale modeling framework of particulate flows.
APA:
Rettinger, C. (2023). Fully Resolved Simulation of Particulate Flows with a Parallel Coupled Lattice Boltzmann and Discrete Element Method (Dissertation).
MLA:
Rettinger, Christoph. Fully Resolved Simulation of Particulate Flows with a Parallel Coupled Lattice Boltzmann and Discrete Element Method. Dissertation, 2023.
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