Passive-scalar dispersion in floating-roof tank cavities: Aspect ratio dependence and actionable insights

Fugitive emissions from floating-roof petrochemical tanks pose serious environmental and public health risks, yet the mechanisms governing pollutant retention and dispersion within these open-topped cylindrical cavities remain insufficiently understood. Focusing on tracer transport characteristics, this study investigates how subtle geometric variations - characterised by the cavity depth-to-diameter aspect ratio (AR^') - influence pollutant dispersion pathways, concentration decay and timescales. Large-eddy simulations reveal that shallow cavities (AR^'=0.25) enable rapid pollutant evacuation through entrainment into the overlying flow. By contrast, deep cavities (AR^'=0.75) exhibit dispersion dominated by closed internal circulation, leading to slower pollutant removal and increased lateral spread. While external momentum fluxes remain broadly consistent across cases, the cavity-internal turbulent flux decreases with AR^', reflecting increasing confinement. Scalar variance shows a sharp initial decay, largely independent of AR^', indicating effective early-stage mixing; however, late-stage variance exhibits AR^'-dependent fluctuations, with pronounced resurgence at shallower cavity depths (AR^'=0.25, 0.375), reflecting contrasting transport regimes: advection-dominated in shallow cavities versus diffusion-limited in deeper ones. Mean tracer age increases systematically with AR^', correlating with diminished turbulent momentum fluxes and vertical confinement of pollutant centroids within the cylindrical cavity. Discrepancies between decay-based retention times and tracer ages highlight the coexistence of rapid initial ventilation and longer-lived pollutant retention. These findings provide physical insight into how cavity geometry shapes pollutant dispersion and retention, informing preliminary risk assessment, adaptive monitoring and geometry-specific response windows.