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Posted:
1 decade ago
8 nov. 2010, 16:36 UTC−5
Possible reasons: #
- insufficient mesh, i.e. use boundary mesh and finer meshes
- low tolerance
- artificial diffusion used
if you upload your example file we can help maybe better.
--
Comsol 4.0a
Ubuntu 10.04.1
Possible reasons: #
- insufficient mesh, i.e. use boundary mesh and finer meshes
- low tolerance
- artificial diffusion used
if you upload your example file we can help maybe better.
--
Comsol 4.0a
Ubuntu 10.04.1
Please login with a confirmed email address before reporting spam
Posted:
1 decade ago
9 nov. 2010, 15:14 UTC−5
Thank you for the advice. I've already tried these but cannot eliminate the error. Besides, the velocity result looks fine in 2D example (flow between two infinite large parallel plates).
The example file is attached. I treat -pz*(r^2)/4/viscosity as analytical velocity at center line of the pipe.
Many thanks.
Thank you for the advice. I've already tried these but cannot eliminate the error. Besides, the velocity result looks fine in 2D example (flow between two infinite large parallel plates).
The example file is attached. I treat -pz*(r^2)/4/viscosity as analytical velocity at center line of the pipe.
Many thanks.
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Posted:
1 decade ago
14 nov. 2010, 16:41 UTC−5
Hi, I haven't used the 3D CFD modules, but I've run the 2D models a fair bit. Looking at your pipe mesh, it is too coarse near the tube walls. Also, I would be tempted to start off with a radial mesh and then sweep that around azimuthally, and then sweep along pipe length.
Regards, John
Hi, I haven't used the 3D CFD modules, but I've run the 2D models a fair bit. Looking at your pipe mesh, it is too coarse near the tube walls. Also, I would be tempted to start off with a radial mesh and then sweep that around azimuthally, and then sweep along pipe length.
Regards, John
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Posted:
1 decade ago
15 nov. 2010, 10:30 UTC−5
Thank you John. The mesh was truely too coarse near the wall. I tried to adopt boundary layer mesh for the 2D cross section, and extrude it to a 3D pipe, and that works. The problem is that, to achieve a accurate result for such a simple structure, I've adopted 800,000 DOFs and must use a server machine with 96GB memory. That's absolutely not a wise way to build complicated models. I'm considering use linear elements to reduce the # of DOF, but that might result in larger error.
About the radial mesh and sweep azimuthally, I couldn't find it in the mesh lists. Are you also using Comsol 3.5a?
Best regards
Cheng
Thank you John. The mesh was truely too coarse near the wall. I tried to adopt boundary layer mesh for the 2D cross section, and extrude it to a 3D pipe, and that works. The problem is that, to achieve a accurate result for such a simple structure, I've adopted 800,000 DOFs and must use a server machine with 96GB memory. That's absolutely not a wise way to build complicated models. I'm considering use linear elements to reduce the # of DOF, but that might result in larger error.
About the radial mesh and sweep azimuthally, I couldn't find it in the mesh lists. Are you also using Comsol 3.5a?
Best regards
Cheng
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Posted:
1 decade ago
16 nov. 2010, 10:40 UTC−5
Thank you John. The mesh was truely too coarse near the wall. I tried to adopt boundary layer mesh for the 2D cross section, and extrude it to a 3D pipe, and that works. The problem is that, to achieve a accurate result for such a simple structure, I've adopted 800,000 DOFs and must use a server machine with 96GB memory. That's absolutely not a wise way to build complicated models. I'm considering use linear elements to reduce the # of DOF, but that might result in larger error.
About the radial mesh and sweep azimuthally, I couldn't find it in the mesh lists. Are you also using Comsol 3.5a?
Best regards
Cheng
That was my first impression with the model too. Mesh was too coarse. Nevertheless, if you use streamline diffusion you can use first order elements theoretically. Also, use symmetry in your model, this way you will have 1/2 of your elements.
--
Comsol 4.0a
Ubuntu 10.04.1
[QUOTE]
Thank you John. The mesh was truely too coarse near the wall. I tried to adopt boundary layer mesh for the 2D cross section, and extrude it to a 3D pipe, and that works. The problem is that, to achieve a accurate result for such a simple structure, I've adopted 800,000 DOFs and must use a server machine with 96GB memory. That's absolutely not a wise way to build complicated models. I'm considering use linear elements to reduce the # of DOF, but that might result in larger error.
About the radial mesh and sweep azimuthally, I couldn't find it in the mesh lists. Are you also using Comsol 3.5a?
Best regards
Cheng
[/QUOTE]
That was my first impression with the model too. Mesh was too coarse. Nevertheless, if you use streamline diffusion you can use first order elements theoretically. Also, use symmetry in your model, this way you will have 1/2 of your elements.
--
Comsol 4.0a
Ubuntu 10.04.1
Please login with a confirmed email address before reporting spam
Posted:
1 decade ago
17 nov. 2010, 11:12 UTC−5
Since streamline diffusion is defaultly applied, do you mean I can acquire accurate result with linear elements?
Anyway, I think it's rare to solve more than 500,000 DOFs on most PCs and error exists for many cases. I want to know how is the estimated error for common fluid problems that you have dealt with?
Many thanks.
Since streamline diffusion is defaultly applied, do you mean I can acquire accurate result with linear elements?
Anyway, I think it's rare to solve more than 500,000 DOFs on most PCs and error exists for many cases. I want to know how is the estimated error for common fluid problems that you have dealt with?
Many thanks.
