Mechanism of fluid infiltration and mixing in plane jets with applications to environmental flow

Air curtains (AC) are contemporary devices that are used to minimize buoyancy-driven exchange flow in buildings. When physical barriers are impractical e.g. in cases of human and vehicular traffic, air curtains serve as an effective alternative to mitigate such exchange flows while maintaining accessibility. These devices generate a high-momentum vertical plane jet that suppresses transverse fluid exchange, thereby assisting towards preservation of indoor air conditions. Their performance is generally quantified in terms of a separation effectiveness, E, which compares the exchange flow rates with and without an air curtain. 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. We further explain the saturation of sealing effectiveness at 85% by comparing transport characteristics of an AC with those of non-buoyant 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. Present study thus highlights that it is inherent entrainment of the ambient fluid by the curtain, which unavoidably results in a sealing effectiveness of less than 100%.