Stabilising large-scale motions to enhance passive scalar transport in spanwise rotating plane Poiseuille flows

A recent study by Zhang et al. (2024, J. Fluid Mech. , vol. 979, A43) introduced an effective control strategy, namely streamwise-uniform spanwise equally distributed injection/suction slots on the pressure (unstable) wall, to enhance passive scalar transport in spanwise rotating plane Poiseuille flows (RPPFs). In this work, we employ direct numerical simulations to further investigate the scalar transport increase rate ( $\textit{STI}$ ) under different slot configurations. Two distinct configurations are investigated, namely uniform-width slots, where injection and suction slots share identical dimensions, and non-uniform-width slots, where their widths vary independently. The former is to examine the effect of slot width, whereas the latter is devoted to distinguishing the individual roles of injection versus suction. While the slot widths change, the root mean square wall-normal velocity is maintained at a fixed minimal value. For uniform configurations, $ \textit{STI}$ increases monotonically with slot width, nearly doubling as the width grows from $\pi /8$ to $\pi /2$ . In contrast, non-uniform configurations exhibit a complex, non-monotonic dependence on slot dimensions. Spectral, quadrant and zonal-conditional quadrant analyses reveal that injection and suction slots play distinct roles in modulating near-wall dynamics. Injection enhances ejection events ( $Q2$ ), promoting local plume detachment, and facilitating the formation of large-scale ascending plume currents. Suction, conversely, strengthens sweep events ( $Q4$ ), suppressing plume detachment while intensifying descending currents. This dual mechanism organises turbulent structures into more stable large-scale structures, thereby improving scalar transport efficiency. A decomposition of $ \textit{STI}$ based on clustering analysis confirms that the enhancement stems primarily from increased occurrence possibilities and improved transport capacities of dominant clusters. These findings establish flow stabilisation through selective slot control as an effective mechanism for enhancing passive scalar transport in RPPFs.