# SimpleFoam : LowRe vs Wall Functions

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 February 3, 2010, 15:41 SimpleFoam : LowRe vs Wall Functions #1 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 7 Hello, I'm currently trying to run a simpleFoam case with & without the use of wall functions. The problem is : I fix the pressure drop to match a certain mass flow. Here I fix it to 30 so : p_inlet = fixedvalue 30, p_outlet= fixedvalue 0 The simulation seems to run fine on one geometry but not on the other and I don't understand why... The two geometries are : One with "short legs" (see pictures below) and one with "long legs" to have a developed flow. The geometry with short legs give the same results with the LowRe model (without wallfunction) and the HighRe model (with wallfunctions) : Pfield without wallfunction (LowRe) http://yfrog.com/iypressurefieldp Ufield without wallfunction (LowRe) http://yfrog.com/i3velocityfieldp Pfield with wallfunction http://yfrog.com/jvpressurefieldp Ufield with wallfunction http://yfrog.com/i3velocityfieldcp Whereas the geometry with long leg gives weird results (numerical values & flow geometry) for the pressure with the two models : Pfield (LowRe) http://yfrog.com/3mpressurefieldp Ufield (LoweRe) http://yfrog.com/58velocityfieldp Pfield(HighRe) http://yfrog.com/58pressurefieldp Ufield(HighRe) Can someone give me a hint where I could go wrong with my simulation and why do I get such changes from just changing the geometry? Thank you very much.

 February 4, 2010, 06:02 #2 Member   Join Date: Apr 2009 Location: Karlsruhe, Germany Posts: 96 Rep Power: 8 Hi GearB0x, could you upload your boundary conditions? Espacially what are your velocity bc's at the inlet and outlet. Regards Thomas

 February 4, 2010, 09:55 #3 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 7 Here are my Boundary Conditions for the LowRe model (without wall functions) : For p : Code: ```dimensions [0 2 -2 0 0 0 0]; internalField uniform 0; boundaryField { inlet { type fixedValue; value uniform 30; } outlet { type fixedValue; value uniform 0; } fixedWalls { type zeroGradient; } frontAndBack { type empty; } }``` For u : Code: ```dimensions [0 1 -1 0 0 0 0]; internalField uniform (0 0 0); boundaryField { inlet { type zeroGradient; } outlet { type zeroGradient; } fixedWalls { type fixedValue; value uniform (0 0 0); } frontAndBack { type empty; } }``` For nut : Code: ```dimensions [0 2 -1 0 0 0 0]; internalField uniform 0; boundaryField { inlet { type calculated; value uniform 0; } outlet { type calculated; value uniform 0; } fixedWalls { type zeroGradient; } frontAndBack { type empty; } }``` For k : Code: ```dimensions [0 2 -2 0 0 0 0]; internalField uniform 0.3328; boundaryField { inlet { type fixedValue; value uniform 0.3328; } outlet { type zeroGradient; } fixedWalls { type fixedValue; value 0.00000001; } frontAndBack { type empty; } }``` for epsilon : Code: ```dimensions [0 2 -3 0 0 0 0]; internalField uniform 0.3676; boundaryField { inlet { type fixedValue; value uniform 0.3676; } outlet { type zeroGradient; } fixedWalls { type fixedValue; value 0.00000001; } frontAndBack { type empty; } }``` Thank for your reply and your help

 February 5, 2010, 11:57 #4 Member   Join Date: Apr 2009 Location: Karlsruhe, Germany Posts: 96 Rep Power: 8 Hi Gearb0x, I think for the nut at the wall you should use calculated, too. But that shouldn't be the problem. How good are the residuals?

 February 6, 2010, 03:56 #5 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 7 Hello, I'll try with calculated and see if that changes anything, I'll let you know. Residuals were around 10^-5 if I remember. How can I check/plot them? I tried to run the simulation for 3000, 6000, 10000 and it doesn't seem to reach a steady state for the Low-Re model. The High Re achieve steady state.

 February 6, 2010, 05:05 #6 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 7 I've tried with nut = calculated @wall it still gives weird results, even wost because now my pressure field is totally symmetrical ... :s Is it possible that the mesh is too/not enough fine? Could that give wrong results?

 February 10, 2010, 11:58 #7 Member   Join Date: Apr 2009 Location: Karlsruhe, Germany Posts: 96 Rep Power: 8 Hi Gearbox, start a simulation with standard bc's for turbulent incompressible flow. Inlet: U,k,epsilon field fixedValues, p zeroGradient Outlet: U,k,epsilon zeroGradient, p fixedValue nut everywhere calculated for low-reynolds-models and look if everything works here fine...to be sure your mesh is okay. best regards

 February 10, 2010, 19:03 #8 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 7 Thanks for the hint! Apparently the mesh was the problem since now I get "nice" results In fact, the grading was wrong : it switched between "fine" and "coarse" part too abruptly. Best regards

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