Study on the hydrodynamic thresholds of Cyclotella growth based on CFD simulations and annular flume experiments

Abstract With the aggravation of eutrophication in lakes and reservoirs, the frequent occurrence of algal blooms has become a significant aquatic ecological challenge. To reveal the regulatory mechanisms of hydrodynamic conditions on the growth of Cyclotella , a CFD-based hydrodynamic simulation was carried out to investigate the flow characteristics and its influence on the growth of Cyclotella (centric diatom) in an annular tank reactor. This study combined indoor annular flume experiments with three-dimensional computational fluid dynamics (CFD) numerical simulations. By setting different stirring speed gradients, this study systematically investigated the growth dynamics, nutrient metabolism characteristics, and flow field response patterns of Cyclotella under multi-gradient hydrodynamic conditions. Experimental and simulation data confirm that hydrodynamics exert a typical dual regulatory effect on the growth and metabolism of Cyclotella : moderate flow circulation significantly promotes algal cell proliferation and nitrogen/phosphorus assimilation, whereas excessive mechanical shear and intense turbulent dissipation induce severe physiological stress, leading to a substantial inhibition of biomass and metabolic activity. Based on a dual-coupling analysis of the macroscopic velocity field and the microscopic turbulent dissipation field, this study defines the suitable hydrodynamic niche for Cyclotella . The optimal growth range was identified as a mean flow velocity of 0.30–0.40 m/s, with a mean turbulent dissipation rate below 6.89 × 10 −3 m 2 /s 3 . Within this optimal range, the water flow acts as a “material transporter” by thinning the diffusive boundary layer on the cell surface and enhancing vertical suspension; however, high-intensity turbulence exceeding the critical threshold transforms into a “structural destroyer,” causing mechanical damage to the algal cells. This mechanism elucidates the physical essence of water flow regulating microalgal growth from the perspective of multiphase fluid mechanics, providing important quantitative indicators and a theoretical basis for the early warning of diatom blooms and hydraulics-based ecological prevention in natural water bodies.