Error in condition CEL function

karachun · June 22, 2018, 05:21

Hello!
I make incompressible analysis and have negative values of Pressure even with included operating pressure value. It will be cavitation in model irl. At first assumption I decide to set negative pressure to zero to obtain lift force.
I create two Expressions for Y force at surface, named "wall":

Code:

P1 = if ((Total Pressure + 1 [atm]) < 0 [Pa], 0 [Pa], Total Pressure + 1 [atm]) 
Liftforce_y = areaInt_y(P1) @ wall + areaInt_y(Wall Shear) @ wall

Is I`m right or doing something wrong? I get no errors, there are resultant force but maybe there are some methodological errors?
P.S. the theme title is not valid - I have some problems with CEL expression - but solved it when I write this post)

ghorrocks · June 22, 2018, 06:49

This will mean your calculation of the forces on the body will not include the negative pressure regions, but the solver will still include the negative pressure regions. This means the force on the body will not be consistent with the simulation. This does not sound like a good approach to me. I would recommend you fix the problem at its source rather than a hack like this.

If you really do have significant regions of negative pressure then you need a cavitation model. If these negative pressures are not real then you better fix your simulation to fix it.

Also: You integrate pressure over a surface to get force, not total pressure.

karachun · June 22, 2018, 07:25

Thanks!
I have pressure range from +1,5 to - 3 MPa. The problem is flow in wery thin gap - in journal bearing. The flow is laminar. It will require expensive computational effort to resolve cavitation - mesh will become too large.
Also I dont need wery precise results.

ghorrocks · June 22, 2018, 07:29

It is your choice what to do, but that sounds like a pretty big negative pressure so I suspect cavitation will have a big effect. If that is the case then you ignore cavitation at your peril.

June 22, 2018, 05:21	Error in condition CEL function	#1
karachun Senior Member Join Date: Nov 2015 Posts: 244 Rep Power: 11	Hello! I make incompressible analysis and have negative values of Pressure even with included operating pressure value. It will be cavitation in model irl. At first assumption I decide to set negative pressure to zero to obtain lift force. I create two Expressions for Y force at surface, named "wall": Code: P1 = if ((Total Pressure + 1 [atm]) < 0 [Pa], 0 [Pa], Total Pressure + 1 [atm]) Liftforce_y = areaInt_y(P1) @ wall + areaInt_y(Wall Shear) @ wall Is I`m right or doing something wrong? I get no errors, there are resultant force but maybe there are some methodological errors? P.S. the theme title is not valid - I have some problems with CEL expression - but solved it when I write this post)

June 22, 2018, 06:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	This will mean your calculation of the forces on the body will not include the negative pressure regions, but the solver will still include the negative pressure regions. This means the force on the body will not be consistent with the simulation. This does not sound like a good approach to me. I would recommend you fix the problem at its source rather than a hack like this. If you really do have significant regions of negative pressure then you need a cavitation model. If these negative pressures are not real then you better fix your simulation to fix it. Also: You integrate pressure over a surface to get force, not total pressure. karachun likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

June 22, 2018, 07:29		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	It is your choice what to do, but that sounds like a pretty big negative pressure so I suspect cavitation will have a big effect. If that is the case then you ignore cavitation at your peril. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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June 22, 2018, 07:25		#3
karachun Senior Member Join Date: Nov 2015 Posts: 244 Rep Power: 11	Thanks! I have pressure range from +1,5 to - 3 MPa. The problem is flow in wery thin gap - in journal bearing. The flow is laminar. It will require expensive computational effort to resolve cavitation - mesh will become too large. Also I dont need wery precise results.