Abstract Summary (Max 250 words)
Single-use stirred tanks are increasingly adopted in biopharmaceutical manufacturing, however, despite their industrial relevance their flow behaviour has received limited detailed investigation. The gas-liquid hydrodynamics of a laboratory-scale single-use stirred tank with square cross-section and a bottom-mounted radial impeller is investigated using a Detached-Eddy Simulation coupled with a Volume-of-Fluid formulation (DES–VOF). Numerical predictions are quantitatively validated against stereoscopic PIV measurements, showing good agreement for both mean and fluctuating velocities. A harmonic-regression-based decomposition is introduced to separate the velocity field into mean, impeller-locked periodic, and stochastic turbulent components without requiring phase-synchronised data. The method provides a geometry-independent alternative to classical phase-averaging and is benchmarked against Proper Orthogonal Decomposition. The analysis confirms that impeller-locked fluctuations are confined to the immediate vicinity of the blades and account for only a minor fraction (≈3%) of the total kinetic energy, whereas stochastic turbulence dominates the region beneath the free surface. This separation clarifies the relative roles of coherent forcing and turbulence in gas–liquid mixing. Free-surface fluctuations are strongly correlated with residual turbulent intensity, while no measurable contribution from the periodic component is observed, indicating that surface dynamics is driven by turbulence, rather than by deterministic impeller forcing. The proposed framework enables a physically consistent partition of flow contributions in multiphase single-use systems and is directly applicable to both numerical simulations and experimental velocity datasets, supporting improved analysis and scale-down strategies for bioprocessing applications.
