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September 16, 2011, 00:43 |
Measuring wall shear stress in bend pipe
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#1 |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
Posts: 96
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My geometry is a bifurcation and on of the vessel is bent. I want to measure the wall shear stress along the bent vessel. The wall shear stress is already calculated. I only find a way to measure the wall shear stress along the straight pipe but no idea how to do it in bent pipe.
Any hind would be helpful. Cheers! |
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September 16, 2011, 01:40 |
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#2 |
Senior Member
Bernhard
Join Date: Sep 2009
Location: Delft
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You can use the utility wallShearStress.
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September 16, 2011, 01:46 |
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#3 |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
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Hi Bernhard,
I think wallShearStress utility is for turbulent flow. My case is laminar flow and I want to plot the wall shear stress along the pipe. My code is to find laminar flow but don't know how to measure it. Thanks for the hind anyway. |
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September 16, 2011, 02:06 |
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#4 |
Senior Member
Bernhard
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Of course you can still use that utility. I suppose it reads turbulenceProperties or RASProperties. You set it to laminar there. Otherwise you make your own version of the wallShearStress utility and update it for laminar flow. I think the first method will work, since the RASModels have a dummy laminar model included.
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September 16, 2011, 02:38 |
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#5 |
Senior Member
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Dear Bernhard,
I think this utility needs some modifications because it computes wall traction instead of wall shear stress, right?
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Amir |
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September 16, 2011, 03:55 |
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#6 |
Member
Guifan Li
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Location: New York City, U.S.
Posts: 96
Rep Power: 15 |
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September 16, 2011, 03:59 |
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#7 | |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
Posts: 96
Rep Power: 15 |
Quote:
I will try it later today. However, i still don't understand how to plot wall shear stress along a bent pipe. Have you got any idea for that? Thanks |
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September 16, 2011, 05:08 |
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#8 | |
Senior Member
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Quote:
In many solvers such as FLUENT and OpenFOAM wall traction is considered as wall shear stress and I don't know why! If we assume T as stress tensor and n as unit normal vector of desired face we have: t= T.n ; t= traction (force vector exerted to desired face per unit area); the utility compute this, but we want: shear stress= t - (t.n)n ;which shear stress is tangential vector here. Bests,
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Amir |
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September 16, 2011, 23:30 |
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#9 |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
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Rep Power: 15 |
Can any one tell me the difference between wallShearStress and the definition?
For newtonian flow the wall shear stress is defined as mu*du/dy which is proportional to the normal velocity gradient to the wall But in wallShearStress it is defined: wallShearStress.boundaryField()[patchi] = ( -mesh.Sf().boundaryField()[patchi] /mesh.magSf().boundaryField()[patchi] ) & Reff.boundaryField()[patchi]; -- What does this code mean? why it calculated wall shear stress ? In wallGradU: The velocity gradient is defined as wallGradU.boundaryField()[patchi] = -U.boundaryField()[patchi].snGrad(); -- This makes sense to me. And mu*wallGradU is the definition one. Can anyone explain? |
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September 16, 2011, 23:36 |
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#10 | |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
Posts: 96
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Quote:
Thanks for your reply. To my knowledge, the wall shear stress is mu*velocity gradient. In the code of wallShearStress: -mesh.Sf().boundaryField()[patchi] /mesh.magSf().boundaryField()[patchi] Do you mean this is the T(stress tensor)? Why the say shear stress = t-(t.n)n ? If T is the shear tensor, isn't T's normal component T.n already the shear stress? I am confused here. |
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September 17, 2011, 07:37 |
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#11 | |
Senior Member
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Quote:
T is stress tensor and n is unit normal vector, according to openFOAM definitions: Code:
n=-mesh.Sf().boundaryField()[patchi] / mesh.magSf().boundaryField()[patchi] Code:
T=Reff.boundaryField()[patchi] Code:
wallShearStress.boundaryField()[patchi] = ( -mesh.Sf().boundaryField()[patchi] /mesh.magSf().boundaryField()[patchi] ) & Reff.boundaryField()[patchi]; Code:
shear stress= t - (t.n)n
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Amir |
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September 18, 2011, 08:21 |
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#12 | |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
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Quote:
Thanks for the reply. I found that the tangetient component of traction tensor is t-(t.n)n as you decribed. For the code stuff: Is this the right thing: forAll(real_wall_shear_stress.boundaryField(),patc hi) { wallShearStress.boundaryField()[patchi] = ( -mesh.Sf().boundaryField()[patchi] /mesh.magSf().boundaryField()[patchi] ) & Reff.boundaryField()[patchi];[/CODE] real_wall_shear_stress.boundaryField()[patchi]= wallShearStress-(wallShearStress&(mesh.Sf().boundaryField()[patchi] /mesh.magSf().boundaryField()[patchi]))&(mesh.Sf().boundaryField()[patchi]/mesh.magSf().boundaryField()[patchi]) } the right results since mesh.Sf().boundaryField()[patchi] /mesh.magSf().boundaryField()[patchi] is the "n" Please let me know if there is something wrong. Btw, why the tangential component of t is not t&n but t-(t&n)&n ? Thanks again! |
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September 18, 2011, 08:42 |
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#13 | |
Senior Member
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Quote:
First of all: I'm not sure that inner product is overloaded for 2 variables which one of them is scalar and another is vector, so it's better to use: Code:
t-(t&n)n Code:
t-(t&n)n Code:
volTensorField gradU=fvc::grad(U); volTensorField T=mu*(gradU+gradU.T()); volVectorField nn ( IOobject ( "nn", runTime.timeName(), mesh, IOobject::NO_READ, IOobject::NO_WRITE ), mesh, vector::zero ); forAll(nn.boundaryField(), patchI) { nn.boundaryField()[patchI] = ( -mesh.Sf().boundaryField()[patchI] /mesh.magSf().boundaryField()[patchI] ); } forAll(wallShearStress.boundaryField(), patchI) { wallShearStress.boundaryField()[patchI] = nn.boundaryField()[patchI] & T.boundaryField()[patchI]; } forAll(normal.boundaryField(), patchI) { normal.boundaryField()[patchI] = (nn.boundaryField()[patchI] & wallShearStress.boundaryField()[patchI]) *nn.boundaryField()[patchI]; } forAll(shear.boundaryField(), patchI) { shear.boundaryField()[patchI] = wallShearStress.boundaryField()[patchI]-normal.boundaryField()[patchI]; } Obviously, you can change some part to use other stress tensor objects. Bests,
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Amir |
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September 19, 2011, 19:23 |
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#14 | |
Member
Guifan Li
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Quote:
Thanks for the correction. This code works however the results are a little bit different from it should be in my case. After a little bit research. I found that the definition of Shear tensor is T=2*mu*(gamma_hat)*D where D=1/2(gradU+gradU.T()). gamma_hat is shear rate. For newtonian case , mu is independent of gamma_hat, for non-Newtonian case, mu is a function of gamma_hat. gamma_hat=sqrt(2.0)*mag(symm(gradU)) for Newtonian case. In your code, I found "volTensorField T=mu*(gradU+gradU.T())" and there is a difference in gamma_hat(shear rate) term. Did you miss it or I did something wrong( please correct me if I am wrong). Cheers! Last edited by liguifan; September 19, 2011 at 20:11. |
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September 19, 2011, 22:14 |
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#15 | |
Senior Member
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Quote:
Your definition is wrong; here, viscosity is a function of shear rate, it's not a multiplication term! The differences maybe due to coarse grid near boundaries or schemes for gradients. Bests,
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Amir |
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September 19, 2011, 22:28 |
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#16 | |
Member
Guifan Li
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Quote:
Thanks for the correction. I misunderstood the definition. I try to finer my mesh and see what happened next. Regards |
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September 23, 2011, 06:06 |
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#17 |
Member
Guifan Li
Join Date: Apr 2011
Location: New York City, U.S.
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Dear Amir,
I tried and your code to get some wall shear stress plots. In published paper, they normally define wall shear stress as the way you did. After some comparison, I found that my wall shear stress is quite low than what they did on paper. There are still more than 20% difference between my plots and their plots after I multiple a factor to my plots. Have you got any experience with these kind of problem? I am stuck with it for quite a while. From the other method:The plots are totally different. Do you have any idea with this? Any idea would be appreciated Thanks |
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September 23, 2011, 09:07 |
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#18 |
New Member
lintao
Join Date: Sep 2011
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thx u very much. it helps a lot.
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September 23, 2011, 09:15 |
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#19 |
Senior Member
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Quote:
First of all, note that this relation for shear stress we discussed about is valid just for incompressible flow; for compressible cases, another term should be added. As you know, in many papers, wall traction is reported instead of wall shear stress, so I suggest you check OpenFOAM utility without changes and see what will happen. Ensure you have reached grid independence solution by examining wall shear stress of boundaries. Another suggestion; there is another utility for evaluating velocity gradient @ boundaries (wallGradU), obviously it can compute grad(U) more precisely which you can also use that in your code. Also try high order schemes for gradient and others. Bests,
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Amir |
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February 13, 2012, 09:26 |
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#20 | |
Senior Member
Join Date: Jan 2012
Posts: 197
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Quote:
Can you tell me how to measure the wall shear stress? I'm confused why my wall shear stress is zero and only a tiny part in inlet is non-zero I am modelling a 3D laminar straight pipe flow by using star ccm |
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