Modelling of Multiphase Flows with OpenFOAM
Over the last decade, OpenFOAM became the leading open source software package for numerical simulations of fluid flows (Computational Fluid Dynamics, CFD) in engineering applications. One of the major advantages of open source software is the accessibility of the source code ensuring full transparency of the underlying algorithms and models, which is of importance in the design processes of process and energy related applications. Furthermore, it is a prerequisite for following the FAIR principles (findability, accessibility, interoperability, and reusability) of research software and publications.
The majority of the developments for numerical simulation of multiphase flows at HZDR are implemented in the open source library OpenFOAM. By signing the Contributors Agreement with the OpenFOAM Foundation in 2017, HZDR has the unique opportunity to actively participate in the development of OpenFOAM. An example is the contribution of the population balance modelling framework in 2017 together with VTT Technical Research Centre of Finland Ltd, which is continuously developed and improved since.
Besides, HZDR develops an extension for multiphase flows, the HZDR Multiphase Addon for OpenFOAM, and a validation database, the HZDR Multiphase Case Collection for OpenFOAM, both published via Rossendorfer Data Repository. Since 2019, HZDR coordinates the development work for OpenFOAM_RCS (RCS - Reactor Coolant System), which is an extension for OpenFOAM to simulate the primary cooling circuit in a nuclear power plant. As a member of the OpenFOAM Process Engineering Consortium, HZDR has established a strong cooperation with the chemical and process engineering industry to ensure that the research done on multiphase flows fits industrial needs.
Agile software development for the CFD library OpenFOAM
OpenFOAM is developed by the OpenFOAM Foundation following the concept of continuous integration. For contributors and developers of an addon this development strategy is challenging. The Helmholtz Cloud Services provide a perfect environment for a high level of automation in the software development process and allow an efficient usage of the available resources. The addon developed at HZDR serves for feasibility prototyping as well as for production software. The usage of Gitlab and the Mattermost chat system are a key for collaborative software development and both tools support the communication with international partners.
Scientific Workflow for Automation of Baseline Cases
The baseline strategy requires the analysis and evaluation of a multitude of bubbly flow cases that are part of an extensive case repository established at the CFD department. For the efficient validation of new models, an automated workflow environment is being developed that aims to allow the simulation of all ~400 cases and their joint evaluation in a single report. This enables the sustainable development and assessment of new models.
Large-Eddy Simulations (LES) of Resolved Interfaces
Following the idea of scale separation, turbulent eddies are classified into large and small, sub-filter scales. In LES, large-scale structures are resolved in space and time, while the small turbulent structures require modelling. In the framework of multiphase flows in a two-fluid model, additionally to the "classical" turbulent stress, a number of sub-filter scale contributions are present, which describe the dynamics and interaction of interfaces and turbulence. For these sub-filter contributions, different models and their combinations are assessed in a-posteriori investigations for application to technically relevant simulations.
Modelling of Bubble Dynamics under Pool Scrubbing Conditions
Pool scrubbing is an effective method for removing radioactive aerosol particles during severe nuclear accidents. Bubble dynamics including growth and detachment at the nozzle, oscillation and deformation as the bubble rises through the pool and as well as coalescence and breakup are crucial factors affecting the decontamination. The work mainly focuses on the study of bubble dynamics under pool scrubbing conditions using the interface-tracking method in OpenFOAM. It is a part of the international project IPRESCA (Integration of Pool scrubbing Research to Enhance Source-term Calculations).
OpenFOAM-Hybrid: A Morphology Adaptive Two-fluid Model
The two-fluid model (Euler-Euler) is modified to allow for the numerical simulation of co-existing resolved (continuous) and small-scale (poly-disperse) structures. Therefore, the solution procedure in OpenFOAM is adopted to the state-of-the-art numerical developments (Cubero et al., Computers & Chemical Engineering 62, 96 - 107, 2014) and new, generalized models for the resolved interface, e.g., a resolution-adaptive drag formulation, models for the interaction between turbulent eddies and the interface, and mass- and morphology transfer between the adjacent phases (entrainment, detrainment, impingement, detachment and liquid film) are derived.
OpenFOAM_RCS: Sustainable Development of Simulation Software for Modeling of Reactor Coolant Systems
Due to the growing importance of CFD for reactor safety research, there have been activities aimed at qualifying the associated methods since many years. This entails the development and validation of models on the basis of detailed experimental data, generated in comprehensive projects. Multiphase flows play an important role in many accident scenarios in the reactor coolant system (RCS). In order to be able to use the model developments and validation data generated throughout the various projects funded by the German Federal Ministry for Economic Affairs and Energy in the long term, these are carried out using the reference code OpenFOAM, which is thereby qualified for application. The project presented here has the objective of gathering and maintaining software and simulation setups from partner institutions in a common repository.
Population Balance Modelling for Poly-disperse Two-phase Flows
A large number of two-phase flows occurring in nature and technology are characterized by the fact that one phase, referred to as continuous, occupies a coherent region of space, while the other so-called disperse phase is present in the form of bubbles, drops or particles. The size of the particles is usually distributed in which case the two-phase flow is referred to as polydisperse. At HZDR, methods are developed which enable simulation of the spatial and temporal evolution of particle size distributions in technical applications. The corresponding code is made available open source.
Particle-center-averaged Euler-Euler model
In the phase-averaged Euler-Euler model, each bubble force is a function of the local gas volume fraction. These closure models are developed with the assumption that the forces act on the bubbles' centres of mass. In simulations, this inconsistency can lead to gas over-concentration for high mesh resolutions, for example in the centre or near the wall of a channel. The particle-centre-averaged Euler-Euler model avoids this consistency. A bubble number density is introduced in the solution process and a Gaussian convolution method is employed to calculate the gas volume fraction from it. Moreover, the bubble dimension and the bubble deformation can be fully considered by an anisotropic diffusion.