Universität Stuttgart
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Item Open Access Load-balancing for scalable simulations with large particle numbers(2021) Hirschmann, Steffen; Pflüger, Dirk (Prof. Dr.)Item Open Access Data transfer in partitioned multi-physics simulations : interpolation & communication(2019) Lindner, Florian; Mehl, Miriam (Prof. Dr. rer. nat. habil.)Partitioned multi-physics simulations allow to reuse existing solvers and to combine them to multi-physics scenarios. This provides not only greater flexibility and improved time-to-solution, but also helps to manage the increasing complexity of modern scientific software. This thesis sees itself as a continuation of the works of B. Gatzhammer and B. Uekermann who developed a comprehensive tool to couple independent simulation codes. I focus on the two important aspects of interpolation between non-matching grids as well as communication between several parallel codes and conclude with aspects of software development of the coupling library preCICE. The interpolation part puts special emphasis on radial-basis function interpolation. It starts with a thorough review of existing interpolation methods with special consideration of the black-box approach to multi-physics simulations and explores promising enhancements to RBF interpolation. Numerical experiments provide a rigorous testing for accuracy, stability and scaling behavior of different variants of RBF implementations. Following the insights gained from the numerical experiments, a highly-optimized parallel implementation for preCICE is developed, containing various measures to improve accuracy and stability of the interpolation. The communication part first defines the requirements for partitioned simulations in terms of communication. A new technique for peer-to-peer communication networks between distinct MPI domains is developed and evaluated against existing approaches. Furthermore, a fast method to establish connections via the file system is presented. Both measures optimize the initialization phase and achieve a considerable speedup. Finally, a strategy to fully decouple algorithmically independent participants on the communication protocol level is implemented and tested. In the last part, the software-related challenges in developing a parallel scientific application involving multiple independent solvers are outlined. I show how the preCICE project handles testing, profiling and integration of a large parallel scientific software with multiple participants. A profiling library for distributed applications has been developed and is extensively used in preCICE and potentially other projects.Item Open Access Performance-oriented communication concepts for networked control systems(2022) Carabelli, Ben W.; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h. c.)Networked control systems (NCS) integrate sensors, actuators, and digital controllers using a communication network in order to control physical processes. They can be found in diverse application areas, including automotive and aircraft systems, smart homes, and smart manufacturing systems in the context of Industry 4.0. Because control systems have demanding Quality of Service (QoS) requirements, the provisioning of appropriate communication services for NCS is a challenge. Moreover, the trend of steadily increasing digitization in many fields will likely lead to control applications with more complex system integration, especially in large-scale systems such as smart grids and smart cities. The proliferation of NCS in such an environment clearly depends on strong methods for integrating communication and control. However, there currently remains a gap between these two domains. On the one hand, the control-theoretic design and analysis methods for NCS have been based on simplistic and abstract network connection models. On the other hand, communication networks are optimized for conventional performance metrics such as throughput and latency, which do not readily translate into application specific Quality of Control (QoC) metrics. The goal of this thesis is to provide performance-oriented concepts for the design of communication services for NCS. In particular, methods for scheduling and routing the traffic of NCS and increasing their reliability through replication are developed on the basis of integrated models that capture the relationship between control-relevant characteristics of communication services and the methods that are used to provide those communication services in the network. This thesis makes the following contributions. First, we address the problem of optimally arbitrating limited communication bandwidth for a group of NCS in a shared network by designing a performance-aware dynamic priority scheduler. The resulting first scheduling policy provides asymptotic stability guarantees for each NCS and performance bounds on the joint QoC. While it is efficient to implement on the data link layer with stateless priority queueing, it requires a large optimization problem comprising all NCS to be solved initially for determining scheduler parameters. To increase the scalability, we therefore relax the scheduling problem by separating the NCS traffic into deterministic transmissions with real-time guarantees and opportunistic traffic used for QoC optimization. The resulting second scheduling policy imposes no QoS constraints on opportunistic traffic, yields less conservative stability guarantees, and allows scheduler parameters to be calculated for each NCS separately and thus much more efficiently. Second, we address the problem of optimally routing NCS traffic in networks with random latency distributions by designing a cross-layer communication service for stochastic NCS. The routing algorithm exploits trade-offs between delay and in-time arrival probabilities to find a route that provides a predefined level of QoC while minimizing network load. Third, we address the problem of active replication for controllers in order to increase the reliability of NCS subject to crash failures and message loss. While existing replication schemes for real-time systems focus only on ensuring that no conflicting values are sent to actuators, we develop stronger consistency concepts that provide replication transparency for control systems. We present a corresponding replication management protocol that achieves high availability and low latency at low message cost, and evaluate it using physical experiments.Item Open Access Modeling and simulation of cabin air filtration with focus on electrostatic effects(2019) Schober, Carolin; Mehl, Miriam (Prof. Dr. rer. nat. habil.)Cabin air filters serve to remove harmful pollutants from the air flow supplied to the car passenger compartment. Electrostatic charges on cabin air filter media significantly improve the degree of particle separation without compromising the air permeability, thus achieving superior filtration performance. In order to optimize the performance metrics, a basic understanding of electrostatic filtration effects is required. However, these effects are largely unexplored due to limited experimental measurement options. Numerical simulations allow a deeper insight into fundamental physical processes than the measurement of macroscopic quantities. However, the uni-directionally coupled status quo simulation approach leads to results deviating from experimental observations for electrostatically charged systems. Numerous unknown parameters such as the charge distribution on filter fibers and dust particles and the lacking implementation of all simultaneously effective electrostatic separation mechanisms cause these differences. This dissertation provides an enhanced fully-coupled modeling approach to simulate specific electrostatic filtration effects. The new simulation model includes the interaction of highly bipolar charged dust particles with each other, with filter fibers, and with the background air flow. Extensive studies demonstrate the necessity of this high level of detail in order to dissolve electrostatic agglomeration effects in the inflow area. In addition, combined numerical and experimental test scenarios provide qualitative results allowing to observe the effect of induced dipoles and mirror charges. A combination of the fully-coupled modeling approach with the status quo simulation method in a two-step procedure is highly recommended for further research studies.Item Open Access Using geographic models in the simulation of mobile communication(2008) Stepanov, Illya; Rothermel, Kurt (Prof. Dr. rer. nat. Dr. h. c.)Network simulation tools are frequently used for the performance analysis of mobile networks. Their common shortcoming lies within the approaches they use for the modeling of user mobility and radio wave propagation. The provided mobility models describe random movements within the area, which is similar to the motion of molecular particles. For the modeling of a radio channel, the tools assume a line of sight between communicating nodes, and thus, a simple dependency of the signal loss to the distance from the transmitter. These models poorly reflect real scenarios, in which the characteristics of the spatial environment have a significant impact on the network performance. In this thesis more realistic mobility and radio propagation models are described and integrated into a network simulation. These models are based on the solutions from related research areas like physics, transportation planning, traffic modeling, and electrical engineering, which have been validated against real-world data. They consider digital maps of the simulation area, which are taken from a geographic information system (GIS). This thesis analyzes common geospatial data standards to provide input to the used mobility and radio propagation models. The evaluations show significant differences between the simulation results obtained with simpler and more realistic models. It is caused by the changes in the distribution of network users due to their mobility in the area and the obstacles of the propagation environment, which simple models cannot reflect.