Can anyone share the udf of drag force?Thanks
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Can anyone share the udf of drag force?Thanks
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You mean, share the UDF equivalent to this equation? Regards |
Drag Force
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Yes, could you help me ?
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It represents the gradient of liquid volume fraction.
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Udf
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?
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I use the VOF model, not the mixture or Eulerian Model, can I use an udf? |
Vof
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.
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But I want to input a shear stress on the inter-surface defined by myself,such as in
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Shear Stress
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?
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Counter Gas Flow
Shear stress due to the gas flow at the free-surface is included by default. You don't need to do that.
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Are you sure? I see many articles use additional udf ? Which chapter describe in flue
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Vof
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 ima
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Drag Force
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.
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Do you have any theory?
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Theory
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.
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As you can see, A single momentum equation is solved throughout the domain, and the r
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No-Slip
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.
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