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