Hydrodynamics of transitional flow in a stirred-tank equipped with an A320 hydrofoil impeller: PIV–POD mapping across flow regimes

Transitional flow in mechanically agitated vessels remains difficult to characterise and model, despite its prevalence in industrial mixing applications involving moderate–to–high viscosity fluids. This study presents an experimental investigation of the hydrodynamics in a baffled stirred tank equipped with an A320 hydrofoil impeller across a transitional Reynolds number range of 8 ≤ 𝑅 ⁢ 𝑒 ≤ 3 5 , 1 0 0 , using planar Particle Image Velocimetry (PIV). Averaged velocity fields and turbulence kinetic energy distributions reveal that transition from laminar to turbulent flow is gradual and strongly spatially heterogeneous, with velocity fluctuations emerging first near the impeller and extending into the bulk flow at higher Reynolds numbers. Proper Orthogonal Decomposition (POD) demonstrates a redistribution of energy across increasing numbers of modes, a loss of coherent impeller–forced structures, and a transition from impeller–locked to low–frequency dominated temporal dynamics. The results show that Reynolds number alone is insufficient to characterise transitional stirred–tank flows. The study provides a valuable experimental validation dataset for Computational Fluid Dynamics (CFD) model development and validation under industrially relevant transitional conditions.