Learning Forum

[scabo]

Hi

I ran a Volume of Fluid simulation of air-water in a 3D circular pipe. The water is about 50% filling the pipe diameter. I want to calculate the shear force/stress applied by the air on the water surface for the entire 3D cicular pipe. How can i calculate it? Do i need to use any UDF or anythng like that?

thanks

[Rob]

You'll likely need a UDF. The VOF model free surface isn't a geometry body (read up on the maths) so you may need to calculate from what you can access from the solver. 

[scabo]

Can i somehow calcualte the velocity gradient from the 2D data along a vertical line at the interface and apply stress=mu*velocity gradient? and then apply it for the entire 3D case?

[scabo]

Hi

I got this https://innovationspace.ansys.com/knowledge/forums/topic/how-to-calculate-the-shear-stress-of-the-fluid-in-ansys-fluent/

If i follow this and do the strain rate and then extract the value of shear stress at the interface, should i get the shear stress acting on the interface? Is this the shear acting on the water given by the air?

[scabo]

Will the above give the shear stress at the interface due to both air and water phase? But i want the shear exerted only by the air-phase.

[Rob]

That looks to take the bulk values. You need the surface effect but there is no surface to effect in the VOF model. Hence this being very non-trivial and needing UDFs. Multifluid VOF may be easier to work with as the phases have different momentum values: but you're then comparing phase momentum which adds another layer of complexity. 

[scabo]

I somehow understand, but in the above i am plotting the shear stress on the air-water interface itself (on an iso-surface with volume fraction=0.5). What do u mean by bulk values?

[Rob]

The shear stresses from your link - they're likely bulk cell values linked to viscosity. 

[scabo]

Oh so you are saying they are not at the interface??

[Rob]

The free surface is a plotted iso-surface there's no special treatment at that location other than within VOF to figure out where the wet bit starts. The shear in the contours panel (post processing) is turbulence/viscosity related: check the definition. 

[scabo]

Hi

But shear stress is defined like that: mu*velocity gradient in air phase at the interface. If i get this, wont it give the shear stress at the interface? Now VoF has a shared vel and visc field, so something with that? it will be good if you can explain a bit more clearly what do you mean..thanks

[Rob]

Plot a contour of phase through the free surface, node values off. Where is the free surface on the plot? 

[scabo]

Okay but in one of the ansys discussion forums called 'Strain rate/Shear stress at the interface in VoF simulations', they suggested to take mu*du/dy in the air-phase at the interface to calculate the shear stress applied by air on water. but i can understand what you are trying to tell.

[Rob]

So you're now opening two threads to ask the same question? Do what Ahmed says, and don't double post. 

[scabo]

Okay- but i did not mean to double post i thought i am asking a different type of question

[Rob]

Both are asking about surface shear - look pretty similar! No worries.