Entrainment characteristics of a swirling liquid jet plunging in a quiescent pool of liquid

Waterfalls are one of example which leads to river oxygenation to sustain underwater ecosystem. In the present work, we have explored the near-field entrainment characteristics of a swirling liquid jet plunging into a quiescent liquid pool. We have carried out three-dimensional numerical simulations using the open source solver Gerris coupled with Volume of Fluid method for a range of swirl numbers 0.5 ≤S≤1.6 and Reynolds number 4000≤Re≤10000. The main objectives of the present work are: (1) To explore, if swirl can enhance the entrainment of the ambient air inside the quiescent liquid pool and (2) To understand the role played by the vortex pairs to enhance the engulfment of ambient fluid inside the quiescent liquid pool. The results indicate that, larger amount of air is dragged inside the liquid pool at a higher swirl number for a fixed Reynolds number. We have also found that the maximum entrainment occurs at the interface between the swirling plunging jet and ambient fluid present in the liquid pool. Furthermore, we also observed the evolution of Kelvin-Helmholtz instabilities near the interface which triggers vortex pairs causing early entrainment of the ambient fluid present inside the pool. Moreover, we also made a comparison between the entrainment characteristics of non-rotating plunging jet with the entrainment characteristics of a swirling plunging jet by calculating the entrainment coefficients for both the cases.