Converge CFD - Law of wall vs No slip

Alan Nyqvist · April 29, 2020, 07:21

Hey Guys,

Converge CFD offers the option to choose from slip, no slip and law of wall.

Other solvers like openfoam, ansys fluent and starccm only have the options of no slip or slip. Now, given that these software apply wall functions for the turbulence parameters, why does Converge have an option to use wall functions for velocity too?

I have tried 2 validation studies, Ahmed Body and a circular to rectangular transition duct, both of which can be found in the ercoftac database. Both of these studies gave good results when the wall was set to no slip as opposed to law of wall. y+ for the Ahmed body was around 120 and for the transition duct, it was 35 with the wall functions applied at the turbulence model instead.

I just wanted to know if there was any reason to use law of wall in the wall boundary condition since the results were not up to the mark.

ywang89 · May 1, 2020, 12:52

A wall function is needed for all the softwares unless you can resolve the near wall region, which means your yplus should be less than 1.

homay95 · June 12, 2020, 05:05

Hello
I have written a UDF code for velocity slip boundary condition, my UDF works properly for slip lenght equal to 1 micrometer, but when i apply slip lenght 20 micrometer, my calculation diverges and says "divergence in AMG" and "floating point exception". how can i get rid of them??
thanks in advance

April 29, 2020, 07:21	Converge CFD - Law of wall vs No slip	#1
Alan Nyqvist New Member Join Date: Apr 2020 Posts: 1 Rep Power: 0	Hey Guys, Converge CFD offers the option to choose from slip, no slip and law of wall. Other solvers like openfoam, ansys fluent and starccm only have the options of no slip or slip. Now, given that these software apply wall functions for the turbulence parameters, why does Converge have an option to use wall functions for velocity too? I have tried 2 validation studies, Ahmed Body and a circular to rectangular transition duct, both of which can be found in the ercoftac database. Both of these studies gave good results when the wall was set to no slip as opposed to law of wall. y+ for the Ahmed body was around 120 and for the transition duct, it was 35 with the wall functions applied at the turbulence model instead. I just wanted to know if there was any reason to use law of wall in the wall boundary condition since the results were not up to the mark. dp937 likes this.

May 1, 2020, 12:52	Converge CFD - Law of wall vs No slip	#2
ywang89 Member Yunliang Wang Join Date: Dec 2015 Location: Convergent Science, Madison WI Posts: 58 Rep Power: 10	A wall function is needed for all the softwares unless you can resolve the near wall region, which means your yplus should be less than 1. __________________ Yunliang Wang Applications Manager CONVERGECFD

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June 12, 2020, 05:05		#3
homay95 Member Homayoon sohrabi Join Date: May 2020 Posts: 56 Rep Power: 6	Hello I have written a UDF code for velocity slip boundary condition, my UDF works properly for slip lenght equal to 1 micrometer, but when i apply slip lenght 20 micrometer, my calculation diverges and says "divergence in AMG" and "floating point exception". how can i get rid of them?? thanks in advance