Strain rate/Shear stress at the interface in VoF simulations

[scabo]

Hi

I have run a 3D air-water pipe flow simulation where the water free surface is about 50% filling the pipe diameter. I want to calculate the shear stress applied by the air on the water interface. For that i have turned on the strain rate in Fluent post-proc. After that i have multiplied by the total viscosity to get the shear stress at the interface. But i am confused which shear stress value it is? Or which strain rate is acting on the interface? Is it only air or only water or because of both air and water? I want the shear applied by air on water surface only. How can i get that?

any help is appreciated..thanks

[Ahmed Hussien]

To accurately calculate the shear stress applied by air on the water surface in a VOF-based air-water simulation, you need to evaluate the shear stress specifically at the interface, typically defined where the water volume fraction is around 0.5. The correct approach is to use only the air viscosity, not the total or mixture viscosity, and multiply it by the velocity gradient at the interface on the air side. If turbulence modeling is used (e.g., k-ε or k-ω), you should use the total viscosity of air (i.e., laminar + turbulent viscosity) because turbulent effects contribute to the effective shear stress in the flow. Create a custom field function in Fluent or extract data on the interface isosurface to ensure you’re capturing the interfacial shear contributed by the air phase alone.

[scabo]

thanks, i have created the custom field fun of strain rate at alpha=0.5 and then multiplied it by total viscosity of mixture because that is what is available in Fluent options. There is no effective viscosity of only air available to choose from. 

As a alternative, i can extract the velocity gradient seperaely at the interface (by exporting the data along a vertical line) and multiply it by total air viscosity and  integrate it along the width of the pipe to get the shear stress,. Does this make sense? Its slightly confusing so if can make any comments, thanks

[scabo]

I was just saying: if i define a custom field func like: air viscosity*strain rate and show the contour result on the air-water interface, will I get the shear stress applied by air on water surface?

[Ahmed Hussien]

Yes, that approach is reasonable, but to accurately estimate the shear stress applied by air on the water surface, you need to use the velocity gradient normal to the interface. If the interface is horizontal, this would be the vertical gradient of the horizontal velocity. Also, make sure to use the effective viscosity of air, which includes both laminar and turbulent components. Defining a custom field function as air viscosity multiplied by strain rate and displaying it at the interface (e.g., where the volume fraction is 0.5) can give a good estimate, as long as the strain rate is based on the correct directional gradient.

[scabo]

Yes i have multiplied the velocity gradient du/dy by effective viscosity of mixture because there is no effective air viscosity to choose from. Or i can simply multiply du/dy by 1.8e-5? In the former doesnt fluent automatically choose effective air viscosity when it comes to air phase?

[Ahmed Hussien]

You should use the effective viscosity in the air region, especially near the interface where the air volume fraction is high. In this case, the mixture effective viscosity closely approximates the air phase viscosity and can be used to estimate air-side shear stress.

[scabo]

Okay thanks, the thing is there is no option to choose only the effective viscosity of air-phase. There is only mixture option.