Effects of lateral stratification on the dynamics of turbulent impinging line fountains

Turbulent line fountains are often operated in environments with a lateral density difference which are integral to engineering applications such as air curtains and bubble screens. While the dynamics of fountains in a uniform ambient (UA) has been widely studied, the underlying flow physics of their interaction with a lateral density stratification remain poorly understood. This study addresses this knowledge gap through a synergistic investigation combining high-fidelity large-eddy simulations and time-resolved particle image velocimetry measurements. We quantify the trajectory and spreading characteristics, flow statistics and entrainment dynamics of line fountains for a wide range of lateral density stratification and compare them with the canonical case of a UA. Our results reveal a new scaling law for the fountain’s trajectory $x_{_{cl}}$ , which follows a noticeably steep power law ( $x_{_{cl}} \propto z^2$ ) that is fundamentally different from jets in a unidirectional cross-flow. This is due to the bidirectional non-uniform lateral forcing and an adverse pressure gradient generated at the impingement surface. These mechanisms also oppose the fountain motion and result in a reduction in their centreline velocity compared with the UA cases. The spatial development of fountains is also affected by the lateral density stratification as it spreads more on the buoyant side exhibiting an asymmetry in the local half-width. The mean velocity statistics was shown to be symmetric if appropriate length scales are chosen for normalization on the buoyant and non-buoyant side. However, the turbulence profiles exhibit an asymmetric behaviour. A quantitative analysis of the fountain’s unsteadiness from experiments reveals that the lateral stratification actively energizes the natural, low-frequency flapping mode. The quantification of entrainment coefficient confirms that the fountains in laterally stratified ambient entrains up to 20 % more fluid than in a UA. These findings provide a deeper understanding of the structural and dynamical aspects of line fountains in complex, stratified environments.