[Zigec]

Hey,

I looked through the forum and this boundary condition from fluent came a few times up, but there has never been a full answer on this topic.

First. It's clear, that for using this BC, one needs to have a pressure-based solver, the energy equation needs to be enabled and the laminar flow has to be selected. At this point you get under Options the LPBS choice.

So far, everything after the documentation. But what after this? When I go in the wall boundary tab, I have the same options, as when i don't enable LPBS: No-Slip, specify shear (free slip) and Marangoni stress, of course in addition to Stationary and Moving wall.

I tried a few simulations with different settings at this point, but I wasn't able to perform a slip.

Can someone help at this point?

Regarding my case. I have a mixture of water-vapor and air at -35C and 4Pa. The channel is around 20 mm in diameter.

Regards,
Ziga

[LuckyTran]

I see your confusion.

Originally Posted by Zigec

First. It's clear, that for using this BC, one needs to have a pressure-based solver, the energy equation needs to be enabled and the laminar flow has to be selected. At this point you get under Options the LPBS choice.

After you make it through this part. What you need to do is go to material properties and there you can specify the Leonnard-Jones parameters and the coefficients for the LPBS condition.

It only makes sense to use the LPBS with a no-slip wall, because if the wall was free-slip then there would be no need to activate the LPBS condition. When the shear is specified, the fluid does not care what is the wall velocity. This explanation unfortunately is not in the documentation.

[Zigec]

Thanks for the quick replay!

One more question. In case, that the Kn is not in the appropriate range for the slip flow, can it happen that the slip doesn't appear?

I just want to check every possible case, that i don't write on the forum 10 times

[LuckyTran]

Honestly I am not sure. As far as I know, there is no low Kn or high Kn cut-off for when the LPBS model is applied. The full-slip and no-slip conditions are asymptotes of this model (so there should always some slip). But one would have to verify it.

[zhangdongjie]

Originally Posted by LuckyTran

I see your confusion.



After you make it through this part. What you need to do is go to material properties and there you can specify the Leonnard-Jones parameters and the coefficients for the LPBS condition.

It only makes sense to use the LPBS with a no-slip wall, because if the wall was free-slip then there would be no need to activate the LPBS condition. When the shear is specified, the fluid does not care what is the wall velocity. This explanation unfortunately is not in the documentation.

2D how to open low pressure boundary slip？

[zhangdongjie]

Originally Posted by Zigec

Hey,

I looked through the forum and this boundary condition from fluent came a few times up, but there has never been a full answer on this topic.

First. It's clear, that for using this BC, one needs to have a pressure-based solver, the energy equation needs to be enabled and the laminar flow has to be selected. At this point you get under Options the LPBS choice.

So far, everything after the documentation. But what after this? When I go in the wall boundary tab, I have the same options, as when i don't enable LPBS: No-Slip, specify shear (free slip) and Marangoni stress, of course in addition to Stationary and Moving wall.

I tried a few simulations with different settings at this point, but I wasn't able to perform a slip.

Can someone help at this point?

Regarding my case. I have a mixture of water-vapor and air at -35C and 4Pa. The channel is around 20 mm in diameter.

Regards,
Ziga

Low pressure boundary slip only works in 3D. How can I open this button in 2D?