Computational Fluid Dynamics
Fabian is actively working on coupled finite-volume frameworks for multiphase flows in general and interfacial flows in particular, and the integration of different multiphase flow modelling approaches with each other.
Fabian is one of the main contributors, and has led this project for many years, to the development of a novel coupled balanced-force numerical framework for flows at all speeds on unstructured meshes, including the incompressible, weakly compressible and fully compressible flow regimes for multiphase flows. The coupled numerical framework provides a strong, implicit pressure velocity coupling and accurately accounts for source terms using a novel balanced-force discretisation, which is of distinct advantages in flows with large source terms, such as multiphase flows or porous media, reduces errors caused by a force imbalance at the interface to solver tolerance and the fully-coupled methodology provides a strong pressure-velocity coupling. In particular, the ability to conduct high-fidelity simulations of multiphase flows on unstructured meshes in complex geometries is a unique feature. The discretisation of the governing equations on unstructured meshes, such as tetrahedral meshes or polyhedral meshes, involves additional difficulties compared to the discretisation on structured meshes, due to the additional topological complexity of unstructured meshes.
Capillary waves and instablities
Capillary-driven waves and instabilities are ubiquitous in flows where two immiscible fluids interact. Prominent examples are short surface waves for which the influence of gravity is negligible or the capillary-driven breakup of liquid jets. The understanding of these capillary-driven interface phenomena are important in microfluidic interfacial flow applications, the atomisation of liquid jets (e.g. fuel injection) as well as ink-jet printing or interfacial flows in porous media, to name just a few examples. Our research on capillary-driven waves and instabilities aims to answer questions relating to the application of direct numerical simulation methods (e.g. spatiotemporal resolution requirements) as well as gaining new physical insight by studying these phenomena using advanced numerical methods.
Falling liquid films
A falling liquid film is an open-flow hydrodynamic system that is convectively unstable to long-wave perturbations, exhibiting a rich variety of spatiotemporal structures and wave instabilities which are generic to a large class of hydrodynamic and other non-linear systems. The hydrodynamics of falling liquid films are governed by a complex interplay of inertia, gravity, viscosity and capillary effects. Hence, a falling liquid film can serve as a canonical reference system for the study of spatiotemporal chaos as well as for the development of new numerical methodologies to predict interfacial flows. Due to their typically small flow rates and low pressure drops, their large contact area and their significantly improved heat and mass transfer, falling films are utilised in a wide range of engineering applications, such as evaporators, heat exchangers or chemical reactor columns. Our research is focused on the complex hydrodynamics of solitary waves and the application of this knowledge to improve the heat transfer characteristics of these flows using direct numerical simulation.
Foam formation and stability
The formation and stability of foams is governed by complex physicochemical mechanisms and the current understanding of the formation and stability of foams is very limited and predominantly based on empirical observations. Foams are multiscale and multiphase systems with significant geometrical constraints. On the surface of each bubble, the dynamics range from a predominantly creeping fluid flow, to capillary wave behaviours and to quantum mechanical fluctuations down to the molecular range. A detailed understanding of these phenomena and the mechanisms leading to foam formation and supporting the stability of foams is critical to many industrial applications. Foams are, for instance, successfully used to fight fires, in wastewater treatment, in hydraulic fracturing (fracking) or in the decontamination of nuclear reactors. In other applications, however, the formation of foam has severe adverse effects and the development of antifoaming agents (e.g. specialised chemical solutions) has become an industry of itself. The formation of foams in lubricants, for example, can lead to a critical decrease in lubrication efficiency, oxidation and cavitation, affect the lubricant supply and act as a thermal insulator.
Our research involves a combined theoretical, numerical and empirical approach to study critical stages of the evolution of foams, from formation to collapse. Our numerical and theoretical tools include full-scale direct numerical simulations to study the hydrodynamics of foams as well as thin-film approximations and elastic membrane theory to study the stability of foams.