Learning Forum

[scabo]

Hi

I have an air-water pipe flow where the water can have different filling ratios. What can be the order of magnitude of the shear stress applied by the air on water surface? Is it very small and can be neglected? Is it similar to open channel flows?

thanks

[Ahmed Hussien]

The shear stress applied by air on the water surface depends on the relative velocity between the phases and the flow regime. This interfacial shear is typically much smaller than the wall shear stress acting on the water but may still be significant, particularly when the air velocity is high, the water fill level is low, or the flow is counter-current. So, if the air velocity is relatively low, you may neglect it. When using the VOF model in Fluent, this shear stress is automatically accounted for through the coupled solution of the momentum equations across phases, with no assumptions required.