Is the Volume-of-Fluid Method Coupled with a Sub-Grid Bubble Equation Efficient for Simulating Local and Continuum Aeration?

Abstract

Air entrainment is common in free surface flows in large hydraulic structures (e.g., spillways, chutes, energy dissipation structures) and must be considered to assure an effective and safe operation. Due to the large size of the prototype structures, it is infeasible to model individual air bubbles. Therefore, using the OpenFOAM toolbox, an efficient simulation model for aerated flows is developed for engineering purposes. The Reynolds-averaged Navier–Stokes equations and the volume-of-fluid method are coupled with a sub-grid bubble population model that simulates entrainment and transport. A comprehensive assessment of the effectiveness, computational cost, and reliability is performed. Local and continuum bubble entrainment are evaluated in two distinct flows: an impinging jet and along a spillway chute. Aeration is induced, respectively, by a shear flow and by the thickening of the turbulent boundary layer. Moreover, a detailed sensitivity analysis of the model’s parameters is conducted. Calibration and validation are performed against experimental and prototype data. Among the analyzed entrainment formulations, the one depending exclusively on the turbulent kinetic energy is the only applicable to different flow types. Good accuracy is found, meeting engineering standards, and the additional computation cost is marginal. Results depend primarily on the volume-of-fluid method ability to reproduce the interface. Calibration is straightforward in self-aeration but more difficult for local aeration.

Do the Volume-of-Fluid and the Two-Phase Euler Compete for Modeling a Spillway Aerator?

“…Despite not simulating the air-water mixture, the TPVoF may be an appropriate tool for preliminary analysis of flows with low air-concentrations (<20%), where the tracking of the free-surface is dominant. Further research should compare the efficiency of the CTPE with other successful approaches using the VoF or the mixture models combined with a sub-grid bubble model (e.g., [27,37]), in cases where the air-concentration is lower than 15-20%, or even for larger air concentrations, in light of some recent successful applications of the VoF and mixture models for highly aerated flows [16,18,24].…”