Tanmay Agrawal

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 𝑥𝑐⁢𝑙, which follows a noticeably steep power law (𝑥𝑐⁢𝑙 ∝𝑧2) 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.

Air curtain devices (ACD) are commonly installed in domestic and commercial buildings to suppress the buoyancy-driven exchange flow through a doorway opening. Generally, the operating density of an ACD is equal to that of the indoor space making it neutrally buoyant. In the present study, we evaluate the performance of heavy air curtains where the operating density of the ACD is higher than that of the ambient fluid. The primary objective is to quantify the air curtain effectiveness, E, that determines the thermal comfort of building occupants based on the mean temperature inside the interrogated region. Experiments and numerical simulations are conducted and validated for various values of deflection modulus, D_m, that compare the relative magnitude of the jet momentum and transverse stack effect due to buoyancy. The other important non-dimensional parameter is the density ratio, S, which compares the extent of added buoyancy in ACD to that of across the doorway. In addition, the velocity dynamics of the air curtains are compared with an isothermal jet to understand the underlying effects that the buoyancy causes on the jet development. The general structure of air curtains that characterize the jet inclination and penetration is visualized through injecting a dye, and it agrees very well with the buoyancy distribution obtained using simulations at different D_m. Upon introduction of an assisting buoyancy, it has been found that the infiltration reduces by 25% compared to a neutrally buoyant air curtain for practical values of D_m.

Gravity currents produced by a lock-exchange flow are studied using high-resolution molecular tagging techniques. Instead of employing salt to produce density stratification, an initial temperature difference is introduced in the system to generate the ensuing gravity currents. The experiments focus on the interface between the hot and cold fluids to characterize the resultant mixing across the interface. The present measurements spatially resolve the flow to smaller than the Kolmogorov scale and close to the Batchelor scale. This enables reasonably accurate estimates of velocity and density gradients. The measured density (temperature) distribution allowed estimation of the background potential energy of the flow that is used to quantify mixing. These measurements yield a mixing efficiency of about 0.13 with a standard deviation of 0.05 for the present Reynolds number range. An analysis combining flow visualization and quantitative measurements reveals that spatially local values of high mixing efficiency occur after the occurrence of certain dissipative stirring events. These events, largely associated with vortical overturns, are commonly observed near the interface between the two fluids and are a precursor to locally efficient mixing.