Turbulent BCs: Wall Functions

lordvon · September 30, 2010, 14:56

Hello,

Should k and omega for komegasst turbulence model be fixed at walls or zerogradient? From the forums I have seen zerogradient, but here: http://repo.or.cz/w/OpenFOAM-1.6.x.g...am/motorBike/0 , I see fixed.

FelixL · October 1, 2010, 04:11

Hi, there,

when using the k-Omega-SST turbulence model in OpenFOAM, you have to use the boundary conditions kqRWallFunction and omegaWallFunction for k and omega, respective. The reason is that the implementation of k-Omega-SST in OpenFOAM is the High-Re-version, which requires a wall function approach.

When specifying a wall function BC, you also have to set a value (as you saw in the motorBike-tutorial case). Set it to the initial values you used for your internal Fields of k and omega, since - to my knowledge and experience - it won't affect your solution. The wall functions automatically take care for the value at the wall.

Greetings.

lordvon · October 1, 2010, 10:45

Is that supposed to be so in OF1.5-dev?

I have set in the /0/omega file the walls to be of type omegaWallFunction, with value $internalField, but I get this error:

Code:

generic is the wrong omega patchField type for wall-functions on patch casing
    should be zeroGradient

I made sure in constant/boundary the corresponding patch type is set to 'wall'.

If I set the wall to be of type zeroGradient, I do not get an error, but the simulation eventually diverges. Should I add an (initial) value along with the zerogradient type (Is it even legal)? It does not give a problem if I do so, but is the value being ignored?

Thanks for your help.

lordvon · October 1, 2010, 12:43

Also I am trying to ramp up the rpm using a logistic function; does anyone know how I can get the current rpm to print in the terminal so I know it is working?

Is it ok if the very first frame looks crazy in terms of pressure distribution (rotating object in external flow)?

Will sharp edges in the rotating part or small gaps between rotor and stator affect convergence?

lordvon · October 1, 2010, 14:26

Here is something that confuses me:

http://repo.or.cz/w/OpenFOAM-1.6.x.g...incompressible

The simpleSRFFoam and simplefoam/motorbike examples both use komegasst, but the srffoam has epsilon and omega both defined in the 0/ directory, but motorbike has only omega and in the omega file it says it is an 'epsilon' object. Why the difference?

September 30, 2010, 14:56	Turbulent BCs: Wall Functions	#1
lordvon Senior Member Robert Join Date: Sep 2010 Posts: 158 Rep Power: 15	Hello, Should k and omega for komegasst turbulence model be fixed at walls or zerogradient? From the forums I have seen zerogradient, but here: http://repo.or.cz/w/OpenFOAM-1.6.x.g...am/motorBike/0 , I see fixed.

October 1, 2010, 10:45		#3
lordvon Senior Member Robert Join Date: Sep 2010 Posts: 158 Rep Power: 15	Is that supposed to be so in OF1.5-dev? I have set in the /0/omega file the walls to be of type omegaWallFunction, with value $internalField, but I get this error: Code: generic is the wrong omega patchField type for wall-functions on patch casing should be zeroGradient I made sure in constant/boundary the corresponding patch type is set to 'wall'. If I set the wall to be of type zeroGradient, I do not get an error, but the simulation eventually diverges. Should I add an (initial) value along with the zerogradient type (Is it even legal)? It does not give a problem if I do so, but is the value being ignored? Thanks for your help.

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October 1, 2010, 04:11		#2
FelixL Senior Member Felix L. Join Date: Feb 2010 Location: Hamburg Posts: 165 Rep Power: 18	Hi, there, when using the k-Omega-SST turbulence model in OpenFOAM, you have to use the boundary conditions kqRWallFunction and omegaWallFunction for k and omega, respective. The reason is that the implementation of k-Omega-SST in OpenFOAM is the High-Re-version, which requires a wall function approach. When specifying a wall function BC, you also have to set a value (as you saw in the motorBike-tutorial case). Set it to the initial values you used for your internal Fields of k and omega, since - to my knowledge and experience - it won't affect your solution. The wall functions automatically take care for the value at the wall. Greetings.

October 1, 2010, 12:43		#4
lordvon Senior Member Robert Join Date: Sep 2010 Posts: 158 Rep Power: 15	Also I am trying to ramp up the rpm using a logistic function; does anyone know how I can get the current rpm to print in the terminal so I know it is working? Is it ok if the very first frame looks crazy in terms of pressure distribution (rotating object in external flow)? Will sharp edges in the rotating part or small gaps between rotor and stator affect convergence?

October 1, 2010, 14:26		#5
lordvon Senior Member Robert Join Date: Sep 2010 Posts: 158 Rep Power: 15	Here is something that confuses me: http://repo.or.cz/w/OpenFOAM-1.6.x.g...incompressible The simpleSRFFoam and simplefoam/motorbike examples both use komegasst, but the srffoam has epsilon and omega both defined in the 0/ directory, but motorbike has only omega and in the omega file it says it is an 'epsilon' object. Why the difference?