Bubbles Motion Under Water: With and Without Shear Stress

Bubbles Motion under the Water with/without Shear Stress CFD Simulation, ANSYS Fluent Training

Description

Bubble dynamics sit at the heart of chemical engineering, since the rise, deformation, and coalescence of bubbles govern the gas-liquid interfacial area — and therefore mass transfer and reaction rates — in equipment such as bubble columns, aeration tanks, and gas-liquid contactors. This project simulates bubble motion rising through water over a plate, comparing cases with and without surface tension to assess how significantly this interfacial force affects bubble shape and behavior when the free surface between phases matters.

The 2D geometry was built in SpaceClaim, spanning 50 mm long by 65 mm high, and meshed in ANSYS Meshing using a structured grid totaling 81,250 elements.

Methodology

The Volume of Fluid (VOF) model tracks the two Eulerian phases — air and water — using a sharp interfacial interface with explicit formulation, alongside a laminar viscous model, an initial bubble size of 2 × 10⁻⁴ m, and gravity applied at -9.81 m/s² along the Y-axis. The case was solved as transient using a pressure-based solver, with SIMPLE pressure-velocity coupling, PRESTO! for pressure discretization, second-order upwind for momentum, and a compressive scheme for volume fraction. The domain was initialized as fully patched with water and run over 1,900 adaptive time steps of 0.0002 s each. Two otherwise identical cases were compared, differing only in whether surface tension was included.

Conclusion

The resulting bubble shapes reveal how significantly the simulation is affected when surface tension is omitted: without it, the bubble fails to hold its form and collapses under the surrounding water pressure, rather than maintaining the coherent shape that surface tension would otherwise sustain. This underlines the importance of including interfacial forces when modeling bubble behavior — a key consideration for accurately predicting interfacial area and mass transfer in chemical-process equipment.