Evaluation of Two-phase Flow Properties in Hydraulic Jumps Using CFD

Hydraulic jumps often occur naturally in rivers and streams and are also artificially created to dissipate energy. The hydraulic jump flow field is characterized by strong turbulence, air entrainment, free-surface fluctuations and energy dissipation. Due to two-phase (air-water) nature of the flow, conventional experimental techniques have their limitations in studying hydraulic jumps. Conductivity probes, fiber-optic probes and imaging techniques have been employed to understand the air-water flow characteristics in hydraulic jumps. While these studies have improved our understanding of the hydraulic jump, the complete internal turbulence structure is yet to be understood. Advancement in computing capabilities and progress in multiphase flow modelling have rendered CFD to be a viable option. In the present study, a high Froude number (F > 8) classical hydraulic jump is simulated using both the volume of fluid (VOF) multiphase model and the Eulerian (two-equation) multiphase model. The free-surface profiles and mean velocity predicted by the simulations are validated with available experimental data and are presented with pertinent discussions. The instantaneous oscillations of the jump toe along with the frequencies of free-surface fluctuations captured by models are also discussed. The difference between the models in predicting the bubble characteristics are also analyzed by examining the instantaneous flow field. Coherent structures in the flow are captured using vortex identification techniques. The dynamics of these structures that are responsible for air entrainment and bubble transport are also discussed. The study clearly brings forth the strengths and limitations of the two multiphase flow models for understanding the complex shear-induced and bubble-induced turbulence in classical hydraulic jumps.