# How to set a specific pressure Value on a wall Using expression

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 July 4, 2013, 04:41 How to set a specific pressure Value on a wall Using expression #1 New Member   Hossein Join Date: Apr 2013 Posts: 20 Rep Power: 6 Sponsored Links HI I am modeling a Journal Bearing and I decide to Use SommerFeld Boundary condition on Journal. Definition of SommerFeld Boundary condition : This Boundary Condition Sets all values of negative gauge pressures to the value of the operating pressure. Thus, the negative, gauge pressures on the Surface of journal are set to zero when negative values occur. I know In The FLUENT, DEFINE_ON_DEMAND macro is used to definition this Boundary Condition. How Could I set This Boundary Condition on the Surface of Rotor (Journal)?? Thanks in advanced.

 July 4, 2013, 07:47 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 Can you describe the Sommerfeld Boundary condition more fully? Is it a pressure or velocity/flow rate boundary?

 July 4, 2013, 16:33 #3 New Member   Hossein Join Date: Apr 2013 Posts: 20 Rep Power: 6 SommerFeld as I described former, is a pressure condition. in journal bearing as the gauge pressure of domain go through negative values, by a mechanism, the operating gauge pressure is set to zero. in my simulation i decide to use this condition. i have seen an article on this subject before. this article uses FLUENT software to do this. CFD analysis of journal bearing hydrodynamic lubrication by Bingham lubricant

 July 4, 2013, 19:59 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 I cannot view the article you linked to. Putting a minimum value on the pressure is easy with if, step or max/min functions. I still have no idea what the Sommerfeld boundary condition looks like so cannot comment in detail.

 July 5, 2013, 02:32 #5 New Member   Hossein Join Date: Apr 2013 Posts: 20 Rep Power: 6 Thanks For Reply. Here is the article file: download link

 July 5, 2013, 06:29 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 I do not have time to analyse an entire journal article - sorry. Can you post what the important question is?

 July 5, 2013, 08:27 #7 New Member   Hossein Join Date: Apr 2013 Posts: 20 Rep Power: 6 hi the question is. how to change the operating pressure during solution in a way that zero pressures should be changed to zero.

 July 5, 2013, 08:40 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 Is it on a pressure boundary, or a velocity boundary, or something else? (If something else, then please define it mathematically)

 July 5, 2013, 08:42 #9 New Member   Hossein Join Date: Apr 2013 Posts: 20 Rep Power: 6 operating pressure not boundary...

 July 5, 2013, 08:53 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,738 Rep Power: 106 Then why call it a boundary condition (post #1)? If you want to clip the pressure to a value you can do this with a source term. You might also be able to do it using variable limits, you would have to look into this. But I can guarantee that you will have problems converging with either of these approaches. You *might* be able to get it to work by defining a variable density material where the density gets small below your threshold pressure. If you make the transition smooth you might be able to get this to work. If I remember correctly you also have to define the derivative of density WRT to pressure - not sure on that, you better check it.

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