
[Sponsors] 
April 20, 2005, 05:04 
Hi all,
Which b.c. should b

#1 
New Member
Rasmus Gjesing
Join Date: Mar 2009
Posts: 7
Rep Power: 10 
Hi all,
Which b.c. should be used at a wall with a flow with high Renumber? I searched the forum and think the answer for my question is zeroGradient for both k and epsilon, is that right? But, when is it then best to use the wallfunctions? Thanks, Rasmus 

April 20, 2005, 05:55 
You should use zeroGradient fo

#2 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
You should use zeroGradient for k and epsilon at walls when using wallfunctions.
It is best to use wallfunctions when there is no choice, i.e. when you cannot afford the resolution required for running a lowRe model. 

April 20, 2005, 06:05 
Ok, but for HIGHRe flows, sho

#3 
New Member
Rasmus Gjesing
Join Date: Mar 2009
Posts: 7
Rep Power: 10 
Ok, but for HIGHRe flows, should both k and epsilon have zeroGradient b.c.?
And are wallfunctions valid for HIGHRe flows? /Rasmus 

April 20, 2005, 06:09 
You should use zeroGradient fo

#4 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
You should use zeroGradient for k and epsilon at walls when using wallfunctions.
Yes wallfunctions are valid for highRe flows. 

May 22, 2005, 21:31 
Hi All
I have looked for tr

#5 
Member
Wladimyr Mattos da Costa Dourado
Join Date: Mar 2009
Location: Sao Jose dos Campos, SP, Brazil
Posts: 36
Rep Power: 10 
Hi All
I have looked for trough this board message about boundary conditions for High Re flows. I have a doubt: I'm trying to use the sonicTurbFoam solver to calculate an external flow (3d NACA profile). I read that the tutorial NACA case use a lowRe turb. Model, thus is adequate to define noslip condition for U componentes. Otherwive, the prism tutorial test case uses HighRe turbulence model and the boundary conditions for U is also noslip. Now I have a question: Should be more correct, in the HighRe cases, that the boundary conditions for U componentes imposed as slip conditions? In that case, is the OpenFoam correctly implemented to define K and epsilon as function of wall stress provides by the wall function? At glance, what I must to do to define correctly the boundary conditionf for highRe turb/models with wall functions? Which conditions must I impose for all variables? Many Tanks in advance Wladimyr Dourado 

May 23, 2005, 03:39 
Why do you think the BC for U

#6 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
Why do you think the BC for U should be slip when using wallfunctions? The OpenFOAM implementtation of wallfunctions requires the U BC to be noslip with zerogradient on p, k and epsilon.


May 23, 2005, 09:54 
Hi Weller
In my investigati

#7 
Member
Wladimyr Mattos da Costa Dourado
Join Date: Mar 2009
Location: Sao Jose dos Campos, SP, Brazil
Posts: 36
Rep Power: 10 
Hi Weller
In my investigation in the literature, the wall laws requires the folowing setups: 1) The velocities componentes, Ui, are imposed in such way that is a slip boundary condition with zero flux trough the wall or imposed, for transpirant wall type. 2) The effective stress tensor (or friction velocity) is given by the wall function, by the distance of the boundary elements upto the wall and the velocity magnitude found at 1). The distance can be defined either by the user or by the code using standart y+ limits. This value is found often by an interactive procedure such as NewtonRapson. This found effective stress is imposed in the boundary patch (nodes and or faces). 3) The equilibrium condition and the friction velocity are used to calculate the k and epsilon values. I have implemented this procedure in my codes and I find right valus for U, K and epsilon. The imposition of noslip BC for U I guest that it provoques a condition completely wrong BC for k and epsilon once that this value of U is over the wall and then k and epsilon is also zero (if I'm not wrong). The problem lies the fact that HighRe turbulence models are unable to treat the viscous and buffer layers. Thus, is necessary a wall function to impose valid values outside these first two regions such that the turbulence model begin to be valid. Am I wrong to thing that is necessary a slip condition for U? In the case of OpenFoam, is it imposing U BC with the wall function in such way? If not, why and please let me know as much as possible. In this case, have some reference which can I read about? Many tanks in advance Wladimyr Mattos Dourado 

May 23, 2005, 10:51 
I do not agree with your asses

#8 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
I do not agree with your assessment of the need for a slip BC for U, in fact I think this is wrong because it's a nonslip wall thus U should have a zero value. No one has found anything wrong with the implementation or results from the wallfunctions in OpenFOAM and they have been extensively tested over very many years.
If you are unhappy with the current implementation and would prefer one based on a slip condition for U please feel free to implement it, you have complete source access to do so. 

May 23, 2005, 11:32 
Hi Weller
May be I don't un

#9 
Member
Wladimyr Mattos da Costa Dourado
Join Date: Mar 2009
Location: Sao Jose dos Campos, SP, Brazil
Posts: 36
Rep Power: 10 
Hi Weller
May be I don't understand well how wall function is implemented in OpenFoam. I agree with you completely about the noslip condition at wall where the velocity must be zero. I think that I have not written clear enought. In the implementations that I know up to now, the condition in the wall is noslip and the wall function is used to give the value of friction velocity at the first boundary point located over the wall at a given distance such that y+ is greater then a value suitable for the turbulence model. The boundary of the mesh is displaced by this distance from the actual wall. But seeing your position I have a question: where and how the information coming from wall function is introduced in the flowfield? In opemfoam, Is the first point really over the wall? I can't be unhappy with current implementation because I not tested it yet. May be I will intoduce in the future the Reichardt wall law but I will necesary to use as base the actual implementation. Anyway, tank you in advance for your answer 

May 23, 2005, 11:40 
Yes the BC for U does coincide

#10 
Senior Member
Join Date: Mar 2009
Posts: 854
Rep Power: 15 
Yes the BC for U does coincide with the wall and that is why it is the physical BC of noslip. I cannot see the point of moving the mesh around to correspond to a particular y+ particularly for unsteady flows where this approach would require the mesh to move every timestep just for the wallfunction!
Take a look at the implementation and you will see how it is done in OpenFOAM. 

May 7, 2006, 09:51 
when I read David Hill's thesi

#11 
New Member
Qiang Li
Join Date: Mar 2009
Location: Shenyang, LiaoNing, P. R. China
Posts: 14
Rep Power: 10 
when I read David Hill's thesis,It was writen that, Eqn. (4.36)
for y+ > 11.6, G=pow(Cmu,3/4)*rho*pow(k,3/2)*y+/yP and Eqn. (4.35) for y+ < 11.6, G=alpha*pow(Cmu,3/4)*rho*pow(k,3/2)*ln(E*y+)/kappa*yP but for wallFuctions.H in bubbleFoam solver, It was writen if (yPlus > 11.6) { G[faceCelli]+= nuw[facei}*magFaceGradU[facei]*Cmu25*::sqrt(k[faceCelli] /(kappa.value()*y[patchi][facei]); } Hill's and implementatoin of Foam are inconsistent. why? would anyone given me some hints? Best regards Q.Li 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Axisymmetric bodies wedgetype BCbs  egp  OpenFOAM Running, Solving & CFD  36  October 11, 2016 19:39 
Gmsh problem with cyclic bcbs  sripplinger  Open Source Meshers: Gmsh, Netgen, CGNS, ...  1  September 28, 2010 09:29 
CD too high  Matthew  FLUENT  1  February 21, 2009 05:42 
Implementing new bcbs  srinath  OpenFOAM  3  September 3, 2008 05:11 
Basics of setting up BCbs question  shawn  OpenFOAM Running, Solving & CFD  0  December 15, 2006 14:23 