More information
Description
The project aims to explore the properties of turbulent von K´arm´an swirling flow (VK flow). VK flow can be briefly described as flow within a vessel closed from both sides with impellers spinning in the opposite directions. The flow exhibits a characteristic mean pattern consisting of two, samesize, toroidal cells inside which fluid continually circulates. On top of that, fluid spins around the vessel axis, and the rotation rate changes gradually along the axis to match angular velocities of the impellers. As a result of this composition, the flow velocity vanishes close to the vessel centre, forming a stagnant region.
VK flow is one of the canonical flows in turbulence research. It has been a subject of multiple theoretical, experimental, and numerical works, spanning over a wide range of topics. There are a few features which make this flow configuration particularly appealing for researchers, e.g. highly energetic turbulent motion can be created in a relatively compact experimental apparatus. It is hardly arguable that VK flow is important for turbulence research, and the better it is understood, the better it can serve for all kinds
of turbulence-related research.
The particular aspect of VK flow that the project aims to investigate is a recently observed feature of the flow field, which appears in the vicinity of its stagnant region. The feature is the strongest close to the vessel centre, where it accounts for more than half of the turbulence energy, and weakens progressively away from the centre. The project goal is to address the following four research questions about the described flow feature: (i) is this phenomenon reproducible in different facilities, (ii) how sensitive is it to the history of the flow, (iii) how sensitive is it to the geometry of the vessel, (iv) can it be controlled via different stirring strategies. By answering these, the project is meant to broaden our understanding of VK flows.
Summary of project results
The project aimed to address critical challenges in understanding large-scale structures within baffled turbulent von Kármán swirling flows. One of the primary issues was the discrepancy in earlier findings, particularly concerning the persistence and behavior of large-scale flow phenomena. This called for a systematic investigation into the sensitivity of such structures to boundary conditions and stirring mechanisms. Additionally, the project faced challenges related to experimental setups, such as equipment malfunctions and facility-dependent results that affected the reproducibility of observations. These challenges were compounded by the complex interactions between large-scale flow features and small-scale turbulence dynamics, requiring innovative approaches and precise experimental controls to unravel these intricacies.
The project undertook a comprehensive approach to study the baffled turbulent von Kármán flow, beginning with the design and development of a cutting-edge experimental rig. This rig featured advanced control mechanisms, including integrated stepper motors and a LabVIEW-based system to manage steady, modulated, and stochastic stirring modes. Experimental campaigns were conducted to explore turbulence decay dynamics, the effects of baffle configurations, and the impact of stirring modulation on flow characteristics. Detailed analyses were performed to investigate the relationship between local flow topology and the turbulence kinetic energy budget, shedding light on both large- and small-scale turbulence interactions. In collaboration with the Norwegian University of Technology, complementary experiments were carried out to enhance the understanding of flow behavior under varying conditions. The project''s outputs included experimental data, analytical frameworks, and knowledge dissemination through presentations at international conferences and contributions to scientific publications.
The project yielded significant outcomes that advanced the understanding of turbulence dynamics and flow control mechanisms. It demonstrated how boundary conditions and stirring modulation influence large-scale flow phenomena, providing valuable insights into turbulence decay dynamics and the role of flow topology in turbulence energy distribution. These findings offered practical benefits for the scientific community, particularly researchers in fluid mechanics and turbulence modeling, by establishing benchmarks for evaluating turbulence theories and simulations. Industrial stakeholders also benefitted from actionable knowledge on optimizing mixing processes using modulated stirring techniques. Moreover, the project fostered capacity building by mentoring a young researcher in experimental techniques and turbulence theory, ensuring knowledge transfer and expanding expertise in the field. This collaborative effort also strengthened partnerships between Polish and Norwegian institutions, highlighting the project''s broader impact on the research community.