KTH Complex Fluids group is a research group at KTH Royal Institute of Technology, founded by Dr. Outi Tammisola, that focuses on understanding complex (non-Newtonian) fluid dynamics. The group investigates how fluid elasticity and plasticity (yield-stress behaviour) influence flow behaviour in various scenarios, from flows in porous media to mixing and turbulent flows. They aim to unravel fundamental mechanisms by which the synergy of a fluid’s elasticity and plasticity alters flow stability and patterns – for example, how combining these properties can trigger elastic flow instabilities even in regimes that would remain laminar for ordinary fluids. By exploring such effects across laminar, transitional, and turbulent regimes, the group’s work provides insight into phenomena like elasto-inertial turbulence and other non-linear flow responses that are important in industrial and biological contexts. This formal yet inquisitive approach to fluid mechanics enables them to address why complex fluids behave in ways that defy classical expectations.
To tackle these challenges, the group employs a dual approach of high-fidelity simulations and cutting-edge experiments. On the computational side, they have developed in-house numerical codes built on the open-source CaNS (Canonical Navier–Stokes) framework – a fast, massively-parallel solver for simulating canonical fluid flows. Using CaNS as a foundation, the team implements additional physics for complex fluids (e.g. viscoelastic and elastoviscoplastic models), allowing them to perform direct numerical simulations that capture the nuanced effects of elasticity and yield stress in flow. Additionally, the group employs Basilisk C, a versatile computational framework for simulating two-phase, surface-tension-driven flows. Basilisk C enables highly adaptive mesh refinement, making it particularly effective for resolving interfacial dynamics in multiphase flows. This allows the team to investigate droplet and bubble dynamics, and capillary-driven flows with high accuracy.These bespoke simulations are complemented by innovative experimental methods. The group designs laboratory experiments to validate and enrich their numerical findings, employing advanced flow diagnostics to observe the flow field. For instance, they use high-speed imaging and optical coherence tomography (OCT) to measure the flow. OCT provides non-invasive, high-resolution imaging, enabling the group to capture fine details of flow instabilities and deformation in real-time. Such experiments serve to cross-verify the computational models with physical reality. This combined approach ensures a deep and comprehensive understanding of complex fluid flows, where simulation and experiment go hand-in-hand.
A hallmark of the group’s work is its use of large-scale simulations on world-class supercomputing resources. The team leverages both European and national high-performance computing facilities to run their computationally intensive models. In particular, they have performed extensive simulations through PRACE (the Partnership for Advanced Computing in Europe) and via Sweden’s national supercomputing infrastructure NAISS. Through NAISS allocations, the group accesses leading HPC clusters like Dardel (at KTH/PDC) and Tetralith (at NSC Linköping), which provide the massive CPU/GPU power needed for their research. These resources enable simulations with billions of grid points or long temporal spans, capturing fine details of complex flow physics that would be impossible to resolve otherwise. By harnessing advanced HPC and experimental tools in tandem, Dr. Tammisola’s group pushes the frontiers of fluid mechanics – offering both fundamental insights and practical predictions about how elasticity and plasticity shape fluid flows in engineering and nature.