A Morphology-Adaptive Multifield Two-Fluid Model: Recent developments and applications

Physical phenomena in industrial gas-liquid flows typically span a wide range of length
and time scales. Individual flow regimes are usually modelled using specific approaches,
which are mainly characterised by the level of detail the existing interfaces are handled
with. The cross-scale nature of multiphase flows requires the simultaneous application and
flexible switching between these methods in a single common framework.
For this reason, a morphology-adaptive model is established by combining the Euler-
Euler model with the Volume-of-Fluid (VOF) model. Interactions and transitions between
different morphologies and scales are taken into account by dedicated models. This work
gives an overview over recent advances towards a fully scalable morphology-adaptive
multiphase model (MultiMorph).
In order to be applicable to realistic, large-scale problems, special care is required to
ensure a robust model behaviour, even if the spatial resolution is not optimal in terms of
the respective flow phenomena. Large interfaces might be represented on coarse numerical
grids. The usual VOF model typically over-predicts the interfacial shear stress in such
a situation, resulting in unrealistic interface dynamics. Instead, a resolution-adaptive
interfacial coupling is proposed. In that way the phases may slip along each other in the
direction parallel to the interface, improving the prediction, i.e., of interface shape or of
bubble rising velocity.
A central building block of morphology-adaptive methods is the ability of structures to
evolve from one morphology to another. For example, unresolved bubbles may coalesce,
grow, or enter highly-refined mesh regions, such that a resolved representation becomes
possible. Therefore, a transition to a continuous representation is realised, to make optimal
use of the available numerical degrees of freedom. The opposite case, the transition from a
continuous to a dispersed representation, is handled as well. This becomes relevant in case
of mesh coarsening or if large continuous structures disintegrate into to smaller particles
which cannot longer be resolved by the given spatial resolution.
Another important feature of the model is the ability to track large numbers of dispersed
particles. A class-method based solution approach is included, providing complete
information about the size distribution, a necessity for modelling the number-conservative
transition between dispersed and resolved structures. However, the associated computational
cost is significant. Fortunately, the solution of the population balance equation could
be parallelised by outsourcing it to graphics processing units, which leads to a significant
improvement in performance.
The MultiMorph model is implemented in the software released by the OpenFOAM
Foundation with strong focus on sustainable research, including a state-of-the-art IT
approach. Both the source code and a comprehensive suite of simulation cases are publicly
available. Several multi-scale applications are presented, featuring for example a distillation
column, a swirl separator, and a impinging jet. Further details can be found at
www.hzdr.de/multimorph.