Tanmay Agrawal

The analysis of gravity currents in a stratified ambient (relevant to geophysical and environmental applications) is complicated by the relatively large number of physical input parameters that allow for various scalings of the data; this is exacerbated for particle-driven flows where additional parameters are involved. This creates a lack of consistency, or even confusion, concerning the interpretation and prediction power of the available knowledge. We discuss the propagation of lock-release (Boussinesq, inertial–buoyancy) gravity currents at the bottom, top, and middle (intrusion) of a linearly stratified ambient tank. We focus attention on the evaluation of the constant speed of propagation in the initial phase (or stage) called “slumping.” Two major methods have been used in previous studies: (a) empirical or data-processing analysis (EDP) and (b) shallow-water (SW) theory analysis. The methods use different definitions of the stratification parameter and different scalings of the variables (including the speed of propagation). We show that the SW method, with a small extension, is physically and mathematically more convenient; results of experiments, simulations, and approximate models can be consistently unified for both top and bottom gravity currents, with and without particles. This generalization improves the insights and the prediction power concerning these phenomena.

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.