Air curtains (AC) are contemporary devices that are used to minimize buoyancy-driven exchange flow across the doorway of a building. Under optimal conditions, they restrict up to 85% of outdoor fluid from infiltrating the indoor region. In the present study, we use large-eddy simulations and a Lagrangian analysis framework to investigate the spatial origin and transport mechanisms of fluid that penetrate an air curtain operating at Reynolds number $\mathrm{Re}=4000$ and Froude number ${\mathrm{Fr}}_0=28.4$. We further explain the saturation of sealing effectiveness at 85% by comparing transport characteristics of an AC with those of nonbuoyant jets and gravity currents. We observe that prior to the curtain establishment, most infiltration occurs via a gravity current, where particles initially near the wall are transported indoors along near-straight trajectories. Once the AC establishes, an entrainment-detrainment pathway becomes dominant. The outdoor fluid is first entrained into the turbulent core and subsequently detrained into the indoor region, either before reaching the wall or after its impingement. The present study thus highlights that it is the inherent entrainment of the outdoor fluid by the curtain which results in a loss of sealing by about 15 to 20%.

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• Wakes & jets
• Lagrangian particle tracking
• Large eddy simulations