Wall functions

Matteo_Kamenjasevic · March 19, 2022, 05:00

Hi guys,
I have a question about wall functions in Openfoam. If I create a mesh in Salome with viscous layer 2D that average yPlus is about 3 (y+ ~ 3), do I need wall function in my scripts (omega script). I found that if you set up very fine mesh than wall functions will automatically recognize it and not be activated in your simulation. Is that true? I hope I was enough clear...

piu58 · March 19, 2022, 10:09

Most probably, you don't need a wall function. Or more accurate: A wall function may introduce errors. The wall function increases the wall shearing stress, which is too low if a large y+ is used. If y+ is around unity, the simulation takes this into account automaticly. If you use it in such a case, you get a too high value of wall shear stress.

Matteo_Kamenjasevic · March 19, 2022, 12:14

Ok thank you for explanation but I have one more question. If there is no need for wall functions, how I can disable them. For example: there is script "nut" and on default it says "type nutLowReFunction". What I have to set, " type fixedValue; value uniform 0"?

piu58 · March 19, 2022, 12:26

OF does not reduce velocity near walls, but increases viscosity. If you have y+ around unity you may set nut to zero. Otherwise, zeroGadient may be used too.

March 19, 2022, 05:00	Wall functions	#1
Matteo_Kamenjasevic New Member Matteo Join Date: Mar 2022 Posts: 4 Rep Power: 4	Hi guys, I have a question about wall functions in Openfoam. If I create a mesh in Salome with viscous layer 2D that average yPlus is about 3 (y+ ~ 3), do I need wall function in my scripts (omega script). I found that if you set up very fine mesh than wall functions will automatically recognize it and not be activated in your simulation. Is that true? I hope I was enough clear...

March 19, 2022, 10:09		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	Most probably, you don't need a wall function. Or more accurate: A wall function may introduce errors. The wall function increases the wall shearing stress, which is too low if a large y+ is used. If y+ is around unity, the simulation takes this into account automaticly. If you use it in such a case, you get a too high value of wall shear stress. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

March 19, 2022, 12:26		#4
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	OF does not reduce velocity near walls, but increases viscosity. If you have y+ around unity you may set nut to zero. Otherwise, zeroGadient may be used too. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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March 19, 2022, 12:14		#3
Matteo_Kamenjasevic New Member Matteo Join Date: Mar 2022 Posts: 4 Rep Power: 4	Ok thank you for explanation but I have one more question. If there is no need for wall functions, how I can disable them. For example: there is script "nut" and on default it says "type nutLowReFunction". What I have to set, " type fixedValue; value uniform 0"?