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# dr inż. Pawła Baj, Badanie wielkoskalowych struktur w turbulentnym wirowym przepływie von Kármána z przegrodami

Investigating large-scale behavior in fluid flows is crucial for understanding

complex phenomena like turbulence. We focus on the von Kármán swirling flow,

generated by counter-rotating impellers, known for its significance in studying

turbulence. Our analysis, conducted in two similar rigs at comparable Reynolds

numbers, identifies a clear peak in power spectral density (PSD) function near the tank

center, associated with a radially-oriented velocity field. Proper Orthogonal

Decomposition (POD) reveals two dominant modes linked to this spectral peak,

collectively capturing a significant portion of velocity fluctuations energy. The

characteristic length of the structure, approximately 0.4 times the tank radius, signifies

its LSM nature. Further insights into the structure's evolution are gained through

consideration of associated evolution equations, contributing to a comprehensive

understanding of baffled VK flow phenomena. The second part of our research

considers the decay of stationary, homogeneous, high Reynolds number turbulence

produced in the von Kármán flow rig. Using stereoscopic particle image velocimetry

(PIV), we observe distinct phases in turbulent kinetic energy (TKE) decay, with an initial

inertia-driven phase followed by classical power-law decay. Our analysis reveals

different decay exponents across velocity components, highlighting the complex nature

of turbulence decay. Furthermore, we investigate the reversal of mean flow patterns

during the transition phase, providing insights into turbulence dynamics in confined

geometries.