First hydrodynamic instability and flow transitions between concentric vertical cylinders

This comparative experimental and numerical study investigates the first hydrodynamic instability and flow transitions in the annular gap between concentric vertical cylinders, with the inner cylinder rotating and the outer cylinder stationary. The study contrasts the behavior of a non-Newtonian fluid, carboxymethyl cellulose (CMC), with that of a Newtonian fluid, potassium sulfate (K2SO4). The influence of inner cylinder rotational speed, represented by Reynolds (Re) and Taylor (Ta) numbers, and filling rates (Γ) on the first instability thresholds and flow modes is examined for both fluids. Experimental visualization and numerical simulations illustrate distinct flow characteristics, emphasizing the significant impact of CMC's non-Newtonian properties on flow structure formation and instability thresholds. Rheological analysis identifies instability thresholds specific to CMC at a 0.2% concentration. The study further analyzes azimuthal and axial wave numbers (n) and (λ) relative to Reynolds number (Re) and filling rates (Γ) to characterize flow dynamics comprehensively.