[hitzhwan]

Can anyone share the udf of drag force？Thanks

[mCiFlDk]

Originally Posted by hitzhwan

Can anyone share the udf of drag force？Thanks

Hi hitzhwan,

You mean, share the UDF equivalent to this equation?

Regards

[vinerm]

What's the last term, ? If that term is neglected, then the drag equation is standard. So, all you need to do is that determine that term and put it as drag modifier.

[hitzhwan]

Originally Posted by mCiFlDk

Hi hitzhwan,

You mean, share the UDF equivalent to this equation?

Regards

Yes， could you help me ?

[hitzhwan]

Originally Posted by vinerm

What's the last term, ? If that term is neglected, then the drag equation is standard. So, all you need to do is that determine that term and put it as drag modifier.

It represents the gradient of liquid volume fraction.

[vinerm]

Then you can just refer the example given in Fluent's UDF manual.

https://www.afs.enea.it/project/nept...udf/node61.htm

For calculating gradient, you have to use C_VOF_RG. You can use C_VOF_G but that could lead to very high drag coefficient values, hence, not recommended.

[hitzhwan]

Originally Posted by vinerm

Then you can just refer the example given in Fluent's UDF manual.

https://www.afs.enea.it/project/nept...udf/node61.htm

For calculating gradient, you have to use C_VOF_RG. You can use C_VOF_G but that could lead to very high drag coefficient values, hence, not recommended.

I use the VOF model， not the mixture or Eulerian Model, can I use an udf？

[vinerm]

In VOF, interface is resolved, hence, drag is not modeled but simulated. You cannot hook a drag model UDF for VOF since it is not required.

[hitzhwan]

Originally Posted by vinerm

In VOF, interface is resolved, hence, drag is not modeled but simulated. You cannot hook a drag model UDF for VOF since it is not required.

But I want to input a shear stress on the inter-surface defined by myself，such as in the figure before

[vinerm]

And where is that shear stress coming from?

You can do that by using source terms in the cells containing the interface.

[hitzhwan]

Originally Posted by vinerm

And where is that shear stress coming from?

You can do that by using source terms in the cells containing the interface.

It comes from the counter-gas flow， do you have the udf source terms？

[vinerm]

Shear stress due to the gas flow at the free-surface is included by default. You don't need to do that.

[hitzhwan]

Originally Posted by vinerm

Shear stress due to the gas flow at the free-surface is included by default. You don't need to do that.

Are you sure？ I see many articles use additional udf ？ Which chapter describe in fluent help document？

[vinerm]

The difference between Mixture and VOF is that in VOF the interface is resolved. Therefore, user does not need to use a drag model nor is there a requirement to specify diameter for secondary phases. Drag is predicted based on the velocity gradient at the interface of immiscible fluids.

To test it, set up a simple case with two inlets and one outlet. Inlets can be separated by a thin line. Use lower inlet for liquid and upper one for gas and let the gas velocity be higher than liquid. Then observe the interface evolution.

[hitzhwan]

Originally Posted by vinerm

The difference between Mixture and VOF is that in VOF the interface is resolved. Therefore, user does not need to use a drag model nor is there a requirement to specify diameter for secondary phases. Drag is predicted based on the velocity gradient at the interface of immiscible fluids.

To test it, set up a simple case with two inlets and one outlet. Inlets can be separated by a thin line. Use lower inlet for liquid and upper one for gas and let the gas velocity be higher than liquid. Then observe the interface evolution.

What is drag force equation default？But many drag force are different like in the image， how can define it differently？

[vinerm]

What you are looking at are drag models. Those are required when the free-surface is not resolved, e.g., in Mixture model or DPM. In VOF, those are not required because the free-surface is a result of the simulation and all the forces at the free-surface can be calculated from the first principles. No modeling is required. So, there is no drag model equation.

[hitzhwan]

Originally Posted by vinerm

What you are looking at are drag models. Those are required when the free-surface is not resolved, e.g., in Mixture model or DPM. In VOF, those are not required because the free-surface is a result of the simulation and all the forces at the free-surface can be calculated from the first principles. No modeling is required. So, there is no drag model equation.

Do you have any theory?

[vinerm]

There is no theory here, only statements of the model. You can read more about VOF online. In most of the codes, no-slip is maintained between immiscible phases at the interface of the fluids. So, if one fluid moves towards one side, the other fluid moves as well, provided the second fluid does not have enough inertia to stop that motion. In a way, the fluids are glued to each other at the interface.

[hitzhwan]

Originally Posted by hitzhwan

Do you have any theory?

As you can see, A single momentum equation is solved throughout the domain, and the resulting velocity field is shared among the phases(from the help document). So there exists no velocity difference between the liquid-gas interface, so there is no shear stress , so I think it can not calculate it.

[vinerm]

Yes, that's correct. And that is what I meant when I mentioned that the fluids are glued together at the interface. However, that does not mean there is no shear stress or infinite shear stress as many think. It only mean no-slip condition, similar to no-slip as solid boundary. However, does a no-slip at solid boundary imply no drag? Drag or shear stress has got nothing to do with no-slip. All it requires is velocity gradient. And that exist almost everywhere in the domain in case of turbulent motion. And same is true about the interface.