preCICE

The new software FEniCS–preCICE is a middle software layer, sitting in between the existing finite-element library FEniCS and the coupling library preCICE. The middle layer simplifies coupling (existing) FEniCS application codes to other simulation software via preCICE. To this end, FEniCS–preCICE converts between FEniCS and preCICE mesh and data structures, provides easy-to-use coupling conditions, and manages data checkpointing for implicit coupling. The new software is a library itself and follows a FEniCS-native style. Only a few lines of additional code are necessary to prepare a FEniCS application code for coupling. We illustrate the functionality of FEniCS–preCICE by two examples: a FEniCS heat conduction code coupled to OpenFOAM and a FEniCS linear elasticity code coupled to SU2. The results of both scenarios are compared with other simulation software showing good agreement.

preCICE is a free/open-source coupling library. It enables creating partitioned multi-physics simulations by gluing together separate software packages. This paper summarizes the development efforts in preCICE of the past five years. During this time span, we have turned the software from a working prototype-sophisticated numerical coupling methods and scalability on ten thousands of compute cores-to a sustainable and user-friendly software project with a steadily-growing community. Today, we know through forum discussions, conferences, workshops, and publications of more than 100 research groups using preCICE. We cover the fundamentals of the software alongside a performance and accuracy analysis of different data mapping methods. Afterwards, we describe ready-to-use integration with widely-used external simulation software packages, tests and continuous integration from unit to system level, and community building measures, drawing an overview of the current preCICE ecosystem.

Nuclear fusion technology is projected to play a major role as a source of clean and safe energy in the future. The immediate challenge is to develop sustainable fusion reactors. In the process of converting complex physical theories to working engineering applications, modelling and simulation assumes a vital position. While simulating nuclear fusion de- vices, the physical and geometrical complexity arising from different scales and physical regimes needs to be addressed. Specifically for tokamak devices, the regimes are broadly classified into core and edge regions. Simulating both regions in a single software is a laborious task and mostly segregated analysis is pursued. The edge and core regions can be coupled in a way that the individual analysis remains the same and some form of data communication across a physical boundary takes place. To perform this coupling, a partitioned black-box approach is pursued using the open-source coupling library preCICE. A model diffusion problem is simulated in the edge physics code GRILLIX having a Cartesian grid and a core physics code having a polar coordinate system. The edge region is simulated by the GRILLIX code and the core region is simulated by a custom-built code as a part of this thesis. A coupling in which the core is modelled with a polar coordinate system and the edge with a Cartesian grid is shown to be first order convergent. Global and local Radial-basis function mapping schemes available in preCICE are tested. A comparative analysis of mapping entities within GRILLIX and doing the same operation with preCICE is shown. In the last part, a strategy for coupling with diverted geometries in cylindrical and curvi- linear coordinate systems is presented.

We present novel coupling schemes for partitioned multi‐physics simulation that combine four important aspects for strongly coupled problems: implicit coupling per time step, fast and robust acceleration of the corresponding iterative coupling, support for multi‐rate time stepping, and higher‐order convergence in time. To achieve this, we combine waveform relaxation – a known method to achieve higher order in applications with split time stepping based on continuous representations of coupling variables in time – with interface quasi‐Newton coupling, which has been developed throughout the last decade and is generally accepted as a very robust iterative coupling method even for gluing together black‐box simulation codes.We show convergence results (in terms of convergence of the iterative solver and in terms of approximation order in time) for two academic test cases – a heat transfer scenario and a fluid‐structure interaction simulation. We show that we achieve the expected approximation order and that our iterative method is competitive in terms of iteration counts with those designed for simpler first‐order‐in‐time coupling.

We present novel coupling schemes for partitioned multi-physics simulation that combine four important aspects for strongly coupled problems: implicit coupling per time step, fast and robust acceleration of the corresponding iterative coupling, support for multi-rate time stepping, and higher-order convergence in time. To achieve this, we combine waveform relaxation -- a known method to achieve higher order in applications with split time stepping based on continuous representations of coupling variables in time -- with interface quasi-Newton coupling, which has been developed throughout the last decade and is generally accepted as a very robust iterative coupling method even for gluing together black-box simulation codes. We show convergence results (in terms of convergence of the iterative solver and in terms of approximation order in time) for two academic test cases -- a heat transfer scenario and a fluid-structure interaction simulation. We show that we achieve the expected approximation order and that our iterative method is competitive in terms of iteration counts with those designed for simpler first-order-in-time coupling.

Coupling OpenFOAM solvers and external solvers for Conjugate Heat Transfer, Fluid-Structure Interaction, and other problem types using the free/open-source coupling library preCICE. Because of the advances coupling, mapping, and communication features of preCICE, the coupled simulations are high-performance and scalable.

Fluid-structure interaction problems are solved by applying either the monolithic or the partitioned approach [1]. While the monolithic approach usually provides more stable solutions, the partitioned approach has many advantages from a software engineering perspective as, for example, the reuse of well-tested existing solvers (participants) in a black-box fashion. However, this typically results in a degradation of time stepping to first order if applied in the standard way [2] - even if the used solvers are of higher order. In this talk, the convergence order of time-stepping for a simple 1D model problem is investigated. In our partitioned approach, the solvers are considered as black-boxes: Only nodal data at the wet interface between the fluid and the structure region is exchanged between the solvers. The aforementioned effect of order degradation is reproduced for state-of-the-art explicit (weak) and implicit (strong) coupling. Finally, an order conserving coupling scheme is introduced. This scheme allows to couple participants using arbitrary time stepping schemes and differing temporal meshes. Here, dense output is used to construct high-order local interpolants between the timesteps at low cost. Waveform relaxation iteratively improves the approximative, time-continuous solution at the boundary and windowing techniques are used to allow non-matching temporal discretization. In the future, we plan to implement this mechanism in the open source coupling library preCICE [3] to be able to solve complex multi-scale-multi-physics scenarios, such as turbulent fluid-structure interaction or fluid-structure-acoustics interaction. Currently, very small time steps are needed in this area due to degradation of convergence order and stability properties of the time-stepping scheme if a partitioned approach is in use. References [1] C Michler et al. "A Monolithic Approach to Fluid-Structure Interaction". In: Comput. Fluids (2004) [2] C. Farhat and M. Lesoinne. "Two efficient staggered algorithms for the serial and parallel solution of three-dimensional nonlinear transient aeroelastic problems". In: Comput. Methods Appl. Mech. Eng. (2000) [3] Hans-Joachim Bungartz et al. "preCICE - A fully parallel library for multi-physics surface coupling". In: Comput. Fluids (2016)

We present the coupling library preCICE providing full functionality to couple different black-box solvers to a partitioned multi-physics simulation environment. Generally there are two approaches for solving multi-physics problems: monolithic approaches and partitioned approaches. While monolithic solutions seem to be less problematic regarding stability issues, the possibility of using existing solvers makes partitioned approaches very attractive. In order to use previously developed solvers in multi-physics simulations, a coupling tool is necessary to connect different solvers to each other. The coupling library preCICE provides communication, data mapping and equation couplings for surface coupled multi-physics applications in a modular way. Up to now preCICE has been used to couple various open-source solvers Open-FOAM, SU2, foam-extend and Calculix and commercial solvers Fluent, FEAP and COMSOL. The applications studied by preCICE include, but not limited to, fluid-structure interaction, fluid- structure-acoustics interaction and heat transfer problems. To obtain accurate results, high grid resolution is required and accordingly multi-physics simulations are run on massively parallel computers and this would result in a large number of communications. A scalable coupling tool must be able to efficiently handle such a large number of communications. This presentation is aimed to introduce different features of preCICE with focus on new communication schemes. Substituting central communication with point-to-point communication and using better communication initialization approaches enhance the preCICE library’s scalability.

Using the partitioned approach, specialized single-physics solvers with different numerical properties may be combined to solve a multi-physics setup. This allows us to use expert solvers for every considered phenomenon. To handle their respective problems in the most efficient way solvers use non-matching meshes-both in the spatial and temporal domain (multi-scale)-and different discretization techniques (inhomogeneous setup). The coupling library preCICE provides methods for an appropriate treatment of non-matching spatial meshes, but it currently lacks advanced techniques for the treatment of non-matching temporal discretization. To maintain order and stability of time stepping, special care must be taken and simply exchanging nodal values at the coupling interface does not suffice. This contribution presents different setups that arise in partitioned multi-physics problems. Different temporal scales and different discretization techniques are considered. Challenges and requirements that stem from the partitioned approach are derived and various candidate techniques for multi-rate time stepping are introduced and evaluated. Convergence studies for a simple benchmark scenario are performed and implementation aspects are presented-with the final goal of implementing multi-rate time stepping in preCICE.

The propagation of uncertainty in physical parameters of fluid-structure interaction problems is a challenging task-both mathematically and in terms of computational workload. In this paper, we employ nonintrusive polynomial chaos expansion and model the uncertainty in five independent input parameters that characterize both fluid and structure. We propose a novel methodology to compute the expansion’s coefficients using spatially adaptive sparse grids and products of one-dimensional integrals, which exploits the tensor structure of both sparse grids and probabilistic space. Furthermore, with spatial adaptivity and modified basis functions, we keep the number of sparse grid points small. We test our approach in two test cases: (i) an elastic vertical flap in a channel flow and (ii) a computationally challenging, well-established benchmark. The outputs of interest are the x-deflection and total force on the structure. In the first test case, we consider six implementations of our methodology and two established methods based on sparse grid quadrature. We evaluate the performance (in terms of number of runs of the numerical solver) and accuracy of all eight methods. The results show that one variant of our approach outperforms all the other implementations. We apply our best variant to the benchmark scenario and address the high computational demands of the resulting problem using three levels of parallelism. Importantly, our approach is not restricted to fluid-structure interaction problems. We can address a broad spectrum of computationally expensive problems, provided that sparse grid approximations can be employed.

Simulating multiple interacting phenomena at the same time can help us predict their effects more precisely. It is possible to perform a multi-physics simulation by coupling pieces of single-physics simulation software with preCICE, a free library for black-box, partitioned surface coupling. An adapter connects a solver to preCICE, allowing preCICE to access the required data elements and steer the coupled simulation. Individual solvers of the computational fluid dynamics simulation software OpenFOAM have been adapted in the past, but there is still a need for a flexible adapter that would be ready to work with any OpenFOAM solver, in order to eliminate the duplication of development effort and improve the user experience. This thesis presents a general, solver-agnostic preCICE adapter for OpenFOAM, which requires no changes in the individual solvers. The adapter can be loaded at runtime using the existing controlDict configuration file. A wide variety of standard solvers are supported, while the proposed design makes the adapter compatible with any similar in-house solver. While we focus on conjugate heat transfer, the adapter is extensible to other types of problems, such as mechanical fluid-structure interaction. The adapter is an OpenFOAM function object: a shared library whose methods are called from predefined points in a solver's code. The boundary conditions are general and do not bind to specific solvers, but support a wide range of compressible or incompressible flow solvers, as well as basic solvers. The required fields and parameters are drawn from the object registry or provided in a separate adapter configuration file. This thesis continues previous work on solver adaptation to preCICE and the proposed adapter is validated against the (already validated) previous adapters.

To deal with the increasing complexity of today’s multiphysics applications, the reuse of existing simulation software often becomes a necessity. Coupling to open-source simulation codes, in particular, is a time-efficient way to tackle new applications. The open-source coupling library preCICE enables such coupling in a minimally-invasive way. In this contribution, we give an overview on ready-to-use preCICE adapters for standard open-source solvers, namely CalculiX, Code Aster, OpenFOAM, and SU2.

The partitioned simulation of fluid-structure interactions offers great flexibility in terms of exchanging flow and structure solver and using existing established codes. However, it often suffers from slow convergence and limited parallel scalability. Quasi-Newton or accelerated fixed-point iterations are a very efficient way to solve the convergence issue. At the same time, they stabilize and speed up not only the standard staggered fluid-structure coupling iterations, but also the variant with simultaneous execution of flow and structure solver that is fairly inefficient if no acceleration methods for the underlying fixed-point iteration are used. In this chapter, we present a review on combinations of iteration patterns (parallel and staggered) and of quasi-Newton methods and compare their suitability in terms of convergence speed, robustness, and parallel scalability. Some of these variants use a so-called manifold mapping that yields an additional speedup by using an approach that can be interpreted as a generalization of the multi-level idea.

For multi-field simulations involving a larger number of different physical fields and in cases where the involved fields or simulation codes change due to new modelling insights, e.g., flexible and robust partitioned coupling schemes are an important prerequisite to keep time-to-solution within reasonable limits. They allow for a fast, almost plug-and-play combination of existing established codes to the respective multi-field simulation environment. In this paper, we study a class of coupling approaches that we originally introduced in order to improve the parallel scalability of partitioned simulations. Due to the symmetric structure of these coupling methods and the use of 'long' vectors of coupling data comprising the input and output of all involved codes at a time, they turn out to be particularly suited also for simulations involving more than two coupled fields. As standard two-field coupling schemes are not suited for such cases as shown in our numerical results, this allows the simulation of a new range of applications in a partitioned way.

The simulation of multi-physics scenarios, in particular fluid-structure interaction has gained more and more importance in the last years due to increasing accuracy requirements for a large range of applications from biomedical fields to technical design problems. At the same time, this type of simulation has become feasible due to the increased computing power of modern supercomputers. Note that only the combination of a highly accurate and, thus, highly resolved, discretization with the multi-physics model yields more detailed and more realistic results than a simple single-physics simulation. However, modern computing architectures require a good scalability of simulation methods on massively parallel systems. For fluid-structure interactions, if done in a partitioned way using separate fluid and structure codes, in particular the usually applied staggered scheme executing fluid and structure solver one after the other hinders a good scalability. This is due to the in general largely different computational costs of the two solvers. In this paper, we propose two new coupling schemes for an implicit coupling of black-box fluid and structure solvers that execute the two solvers in parallel and still yield good convergence and stability even for incompressible fluids which is shown by means of numerical results for the flow through a flexible tube.

One of the great prospects of exascale computing is to simulate challenging highly complex multi-physics scenarios with different length and time scales. A modular approach re-using existing software for the single-physics model parts has great advantages regarding flexibility and software development costs. At the same time, it poses challenges in terms of numerical stability and parallel scalability. The coupling library preCICE provides communication, data mapping, and coupling numerics for surface-coupled multi-physics applications in a highly modular way. We recapitulate the numerical methods but focus particularly on their parallel implementation. The numerical results for an artificial coupling interface show a very small runtime of the coupling compared to typical solver runtimes and a good parallel scalability on a number of cores corresponding to a massively parallel simulation for an actual, coupled simulation. Further results for actual application scenarios from the field of fluid–structure–acoustic interactions are presented in the next chapter.

The partitioned simulation of fluid–structure interactions offers great flexibility in terms of exchanging flow and structure solver and using existing established codes. However, it often suffers from slow convergence and limited parallel scalability. Quasi-Newton or accelerated fixed-point iterations are a very efficient way to solve the convergence issue. At the same time, they stabilize and speed up not only the standard staggered fluid–structure coupling iterations, but also the variant with simultaneous execution of flow and structure solver that is fairly inefficient if no acceleration methods for the underlying fixed-point iteration are used. In this chapter, we present a review on combinations of iteration patterns (parallel and staggered) and of quasi-Newton methods and compare their suitability in terms of convergence speed, robustness, and parallel scalability. Some of these variants use the so-called manifold mapping that yields an additional speedup by using an approach that can be interpreted as a generalization of the multi-level idea.

Interpolation based on radial basis functions (RBF) is a standard data mapping method used in multi-physics coupling. It works on scattered data without requiring additional mesh topology or neighborhood information of support points. However, system matrices of the equations for the coefficients tend to be ill-conditioned. In this work, we illustrate the problem by a simple example and discuss possible remedies. Furthermore, we investigate the numerical performance of this method on uniform and non-uniform meshes with a particular focus on the coupling of black-box components where typically no information about the underlying discretization can be extracted. Radial basis function interpolation usually uses an enhancement of the radial basis functions by a global polynomial in order to properly capture constant components and linear trends in the given data. We present a method that determines this polynomial independent from the radial basis function ansatz, which substantially improves the condition number of the remaining RBF system. Furthermore, we show that a rescaling approach can be used to either increase the accuracy or improve the condition number even further by choosing radial basis functions with a smaller support radius. The results represent an intermediate state with the aim to be integrated into the multi-physics coupling library preCICE.

In the emerging field of multi-physics simulations, we often face the challenge to establish new connections between physical fields, to add additional aspects to existing models, or to exchange a solver for one of the involved physical fields. If in such cases a fast prototyping of a coupled simulation environment is required, a partitioned setup using existing codes for each physical field is the optimal choice. As accurate models require also accurate numerics, multi-physics simulations typically use very high grid resolutions and, accordingly, are run on massively parallel computers. Here, we face the challenge to combine flexibility with parallel scalability and hardware efficiency. In this paper, we present the coupling tool preCICE which offers the complete coupling functionality required for a fast development of a multi-physics environment using existing, possibly black-box solvers. We hereby restrict ourselves to bidirectional surface coupling which is too expensive to be done via file communication, but in contrast to volume coupling still a candidate for distributed memory parallelism between the involved solvers. The paper gives an overview of the numerical functionalities implemented in preCICE as well as the user interfaces, i.e., the application programming interface and configuration options. Our numerical examples and the list of different open-source and commercial codes that have already been used with preCICE in coupled simulations show the high flexibility, the correctness, and the high performance and parallel scalability of coupled simulations with preCICE as the coupling unit.