Computational fluid dynamic modelling of a plunging jet

There is a growing interest towards Computational Fluid Dynamic (CFD) modelling to analyse and solve problems related to fluid flows. There are many flows of interest with scientific relevance and engineering/industrial application. This research includes a CFD modelling of one of such flows; plunging jets, which is widely occurring around us. Plunging jets can be found in drop structures, aeration systems in wastewater treatment plants, dam outlets, oceanic breakwaters, etc. It is a complex phenomenon with turbulence, air entrainment and multiphase (air-water mixture) flow. To investigate it numerically, a model for a vertically supported plunging jet was developed in OpenFOAM. RANS approach was applied and k-ε turbulence model was utilized to simulate the flow. InterFoam was used as the solver for the multiphase flow which uses volume of fluid (VOF) method for interface tracking. A mesh sensitivity analysis was done with three different mesh sizes and the finest mesh was selected for further study. The modelling results were also compared against the experimental results. The modelling outputs showed that the numerical model produced reasonable results in terms of velocity distribution. Similarly, the maximum air concentration was over predicted by the model. However, flow field was qualitatively well reproduced by the model. The bubble entrainment mechanism and its subsequent movement inside the plunge pool was thoroughly discussed. The numerical model contributed to the understanding of the flow field in the vertically supported plunging jet. The air entrainment occurs around the perimeter of the jet in the form of a bubble. The continuous entrainment of bubbles and their further breakdown leads to a bubbly mixture of air and water. The maximum velocity occurs at the jet centreline and decays with the depth almost in a linear way. The presence of a shear layer forming a conical shape is well supported by velocity profiles. The maximum air concentration is present near the impingement point and it decreases in vertically downward and radial direction. It was also observed that the VOF method is not capable of resolving sharp interfaces between air and water. That results in the numerical diffusion of the bubble interface which is one of the drawbacks of this method