We present version 3 of the open-source simulator for flow and transport processes in porous media DuMu x . DuMu x is based on the modular C++ framework Dune (Distributed and Unified Numerics Environment) and is developed as a research code with a focus on modularity and reusability. We describe recent efforts in improving the transparency and efficiency of the development process and community-building, as well as efforts towards quality assurance and reproducible research. In addition to a major redesign of many simulation components in order to facilitate setting up complex simulations in DuMu x , version 3 introduces a more consistent abstraction of finite volume schemes. Finally, the new framework for multi-domain simulations is described, and three numerical examples demonstrate its flexibility.

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Efficient multiphysics (or hybrid) models that can adapt to the varying complexity of physical processes in space and time are desirable for modeling fluid migration in the subsurface. Vertical equilibrium (VE) models are simplified mathematical models that are computationally efficient but rely on the assumption of instant gravity segregation of the two phases, which may not be valid at all times or at all locations in the domain. Here we present a multiphysics model that couples a VE model to a full multidimensional model that has no reduction in dimensionality. We develop a criterion that determines subdomains where the VE assumption is valid during simulation. The VE model is then adaptively applied in those subdomains, reducing the number of computational cells due to the reduction in dimensionality, while the rest of the domain is solved by the full multidimensional model. We analyze how the threshold parameter of the criterion influences accuracy and computational cost of the new multiphysics model and give recommendations for the choice of optimal threshold parameters. Finally, we use a test case of gas injection to show that the adaptive multiphysics model is much more computationally efficient than using the full multidimensional model in the entire domain, while maintaining much of the accuracy.

Vertical equilibrium (VE) models are computationally efficient and have been widely used for modeling fluid migration in the subsurface. However, they rely on the assumption of instant gravity segregation of the two fluid phases which may not be valid especially for systems that have very slow drainage at low wetting phase saturations. In these cases, the time scale for the wetting phase to reach vertical equilibrium can be several orders of magnitude larger than the time scale of interest, rendering conventional VE models unsuitable. Here we present a pseudo‐VE model that relaxes the assumption of instant segregation of the two fluid phases by applying a pseudo‐residual saturation inside the plume of the injected fluid that declines over time due to slow vertical drainage. This pseudo‐VE model is cast in a multiscale framework for vertically integrated models with the vertical drainage solved as a fine‐scale problem. Two types of fine‐scale models are developed for the vertical drainage, which lead to two pseudo‐VE models. Comparisons with a conventional VE model and a full multidimensional model show that the pseudo‐VE models have much wider applicability than the conventional VE model while maintaining the computational benefit of the conventional VE model.

Hydrogen is a promising energy carrier on a path toward a decarbonized society and economy. Green hydrogen (produced by water electrolysis) and turquoise hydrogen (produced by high temperature methane pyrolysis) are attractive CO 2 -free power-to-gas technologies in periods with a surplus of power from renewable energies. Hydrogen can be stored and subsequently converted back to electricity using a fuel cell at times of additional demand. As such, hydrogen, among other energy storage techniques like pumped hydroelectric storage and largescale batteries, can buffer the periodic fluctuations of renewable energies to stabilize the power grid (Bünger

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