Jet-induced mixing mechanisms in a novel pond geometry

This study examines the mixing performance of a novel raceway pond design actuated by lateral oscillating jets, using computational fluid dynamics simulations with passive scalar transport modeling as a theoretical framework for quantifying homogenization. Two jet operation strategies are investigated: a fixed single-configuration mode (case 1) and a time-alternating configuration mode (case 2). Mixing efficiency is evaluated through time-resolved concentration fields and quantified using complementary indicators, namely, the scalar variance, the L2 norm of concentration gradients, and the mix-norm. The results show that alternating jet actuation (case 2) significantly enhances homogenization. Specifically, the scalar variance decays 41% faster, and the mix-norm decreases over twice as rapidly compared to case 1. The gradient norm exhibits a pronounced rise followed by rapid decay once jet switching is initiated at 20 s, indicating accelerated filament stretching and breakup of unmixed regions. As a result, case 2 reaches a near-uniform concentration state approximately 30% earlier than case 1. The improved performance is attributed to the periodic disruption of coherent vortical structures, which intensifies Lagrangian stretching and folding and thereby accelerates diffusion-driven mixing. These findings highlight the potential of time-modulated jet operation to improve operational efficiency in microalgal raceway ponds while offering a mechanically flexible alternative to conventional mixing technologies.