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April 17, 2006, 20:31 
Hello
I have a question on

#1 
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kumar
Join Date: Mar 2009
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Hello
I have a question on the turbForce term in the liftDrag.C (../src/postprocessing/incompressible/liftDrag/liftDrag.C ). Laminar Flow  In the laminar case the drag is the pressure drag + x_component of the wall shear stress . although i am not an expert in C++ , i could make out that mu.value()*U.boundaryField()[patchLabel].snGrad()*mesh.magSf().boundaryField()[ patchLabel] is approximately meu*velocityGradient*area . Am i correct ? 2. turbulent drag  In turbulent flow the wall shear stress is (according to ferziger and peric) rho*(u_tau)^2. which equals rho*(C_meu)^0.25*k*sqrt(k)*vel_parallel_wall/(ln(n+E)) But the formulation given in liftDrag.C is totally different . first the laminar drag is found out ( even in turbulent case ) . then the turbulent drag is found ( this does not at all resemble the expression given by say ferziger , peric). then these 2 are added. could someone tell me why this procedure is adopted and where i am wrong ? i am stuck with my comparison. thanks a lot kumar 

April 18, 2006, 14:42 
Hello all,
Could someone co

#2 
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kumar
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Hello all,
Could someone comment on this post ? i am stuck with my results thanks in advance kumar 

April 25, 2006, 06:00 
Hi kumar,
I'm very sorry th

#3 
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Anja Stretz
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Hi kumar,
I'm very sorry that I can't help you, but maybe you can answer some of my questions?  where do I get the mu field from?  how do I have to change the code for a compressible fluid?  where do I have to write the code?  how can I actually use it?  where do I find the results thanks Anja 

April 25, 2006, 08:04 
Hi Kumar,
the difference be

#4 
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Markus Hartinger
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Hi Kumar,
the difference between laminar and turbulent navierstokes equations are the reynoldsstresses. So you can simply take the laminar forces and add the forces due to the reynoldsstresses. laminar: mu.value()*U.boundaryField()[patchLabel].snGrad()* mesh.magSf().boundaryField()[patchLabel] means: viscosity * velocity gradient normal to surface * area turbulent:  mesh.Sf().boundaryField()[patchLabel] & turbulence>R()().boundaryField()[patchLabel] means: surface vector(length equal to area) &(dotproduct) reynolds stress tensor the calculation is general, given a stress tensor which is calculated from your choice of turbulence model regards markus 

April 25, 2006, 16:24 
Hi Anja,
Let me try to answ

#5 
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kumar
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Hi Anja,
Let me try to answer your questions . 1. mu field  the liftDrag utility uses nu ( mu/density ). this is set in the transportProperties in your case directory. 2. Compressible fluid since liftDrag utitlity uses nu , you may have to modify the utility itself. 3. writing the code. The liftDrag.C ( & associated codes ) code resides in /OpenFoam1.2/application/utilities/postProcessing/miscellaneous/liftDrag/ . This program while executing calls another program also called liftDrag.C ( & associated programs ) in ../src/postProcessing/incompressible/liftDrag/ the liftDrag.C in /miscellaneous/../ calls createNu.H . createNu.H then opens transportProperties file ( this resides in the case directory )gets nu ( mu/rho ) . You can give a different nu by either modifying the transportProperties file . however you can also modify the later part of createNu.H to say for example read a certain different value , but here again , we are still talking about nu ( NOT mu) to modify the code . copy the code to your /applications directory and follow the instructions in user manual . also refer other wonderful posts ( search with liftDrag in openFoam ) Once you compile your program you are ready to go! hope this helps regards kumar 

April 25, 2006, 17:24 
Hi Markus
Thanks a lot for

#6 
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kumar
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Hi Markus
Thanks a lot for your reply. i hope i can trouble you with one more question. In the case of ke models the reynolds shear stress are not calculated . this means that the wall shear stress given in liftDrag utility is an approximate formulation because in the turbulent case  while using ke model  the wall shear stress is (according to ferziger and peric) rho*(u_tau)^2. which equals rho*(C_meu)^0.25*k*sqrt(k)*vel_parallel_wall/(ln(n+E)) , but the utility does not calculate this and approximates with the laminar formulation. Am i correct ? regards kumar 

April 26, 2006, 07:07 
Hi Kumar,
if you look for t

#7 
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Markus Hartinger
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Hi Kumar,
if you look for the keps model in src/turbulenceModels/incompressible/kEpsilon the reynolds stresses are calculated according to the boussinesq approximation common to all keps models. ((2.0/3.0)*I)*k_  nut_*2*symm(fvc::grad(U_)) the turbulence viscosity nut is calculated like nut_ = Cmu*sqr(k_)/(epsilon_ + epsilonSmall_) with standard wall function approach to get the wall values for nut (wallViscosityI.H) with yPlusLam = 11.63 if (yPlus > yPlusLam_) { nutw[facei] = nuw[facei] *(yPlus*kappa_/log(E_*yPlus)  1); } else { nutw[facei] = 0.0; } regards markus and pierre 

April 26, 2006, 20:56 
Hi Markus & Pierre
Thanks a

#8 
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kumar
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Hi Markus & Pierre
Thanks a lot for the replies regards kumar 

April 27, 2006, 09:52 
Hi all,
also thanks from my s

#9 
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Anja Stretz
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Hi all,
also thanks from my side. Anja 

August 25, 2006, 02:35 
Hi, I want to implement the vi

#10 
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Hi, I want to implement the viscous dissipation rate in my application. the term which im hopeing to get implemented is 1/2*1/Cp*(R()() & R ()())
on acount that R()() = tau/rho. However the implementation of the dot product of R()() is not accepted. Does anyone know how to do this? Thanks /erik 

August 3, 2010, 10:41 

#11 
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Alessandro
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Hi to everybody, does anybody know where this equation
nuw[facei] *(yPlus*kappa_/log(E_*yPlus)  1); is coming from?? Thanks in advance
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August 11, 2010, 03:52 

#12  
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Alessandro
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Quote:
Thanks again
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