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[ANSYS Meshing] Help with element size

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Old   May 22, 2014, 15:02
Question Help with element size
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Hi everybody! I'm Sandra, new cfd forum and ansys fluent user! I'm glad to became part of this new world and hope to learn a lot with this incredible web page. I'm Spanish, so if I commit a grammar error please tell me .

I've started a new module in my BEng. Degree called CFD. We use fluent and make some easy tasks at first. Then they propose us to make a free 3D shape and make a study with this software. I love Formula 1 so I decided to test my CAD skills and make a good study in a front wing. But when I could finish my first version of a front wing I have really a huge problem. I can't mesh the geometry... The mesher tries to do it and spend a lot of time, and in the end it says me two errors:

"A mesh could not be generated using the current meshing options and settings. Hint: The problem geometry areas might have been highlighted. Switching to wireframe mode may make them more visible."

"The following surfaces cannot be meshed with acceptable quality. Try using a different element size or virtual topology."


Here you have a screenshot, and if you detect an error please tell me. I've tried using "sizing" only in the F. wing, inflation, and combination of both.. But still the same error or just insufficient RAM . Is it any way to solve this problem?

Thanks for reading this post. Have a nice day

S.

- Running Fluent 14.5 in an acer aspire 5771G (default components) -
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Last edited by sandri_92; June 4, 2014 at 05:00.
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Old   May 23, 2014, 03:23
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Just to check. You have a geometry of the fluid volume around the F1 wing and not a geometry of the F1 wing in ANSYS Meshing? Your picture does not make it clear and it is common mistake often made by new CFD people.

I would expect ANSYS Meshing capable to make a fairly good hybrid mesh automatically - with just a changes to the default settings.

But as a quick try. What happens when you change the relevance center to Medium, again keeping things simple?
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Old   May 23, 2014, 05:11
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Quote:
Originally Posted by siw View Post
Just to check. You have a geometry of the fluid volume around the F1 wing and not a geometry of the F1 wing in ANSYS Meshing? Your picture does not make it clear and it is common mistake often made by new CFD people.

I would expect ANSYS Meshing capable to make a fairly good hybrid mesh automatically - with just a changes to the default settings.

But as a quick try. What happens when you change the relevance center to Medium, again keeping things simple?
Hi Siw. I follow the next steps: First I design the 3D model, in this case the front wing. Then I make a "huge" volume that will contain the shape. After, I extract the shape to this huge volume (using solidworks), so there is a solid volume without this front wing (as a cavity). I set up the inlet/outlet and walls (as symmetry) and put the F.W. as a non slip wall. So the air is flowing in the solid part (interior) of the volume. It is something I've learn in the NACA 0012 workshop of the ANSYS CFX, the procedure is more or less the same.

I tried to make a default mesh, but it says the same error. I know it is a complex shape, but I shouldn't get so many errors... I'll try to change the relevance center, and then post an answer. If you see I don't follow the right steps please tell me
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Old   May 23, 2014, 06:33
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Again, a new error appeared..

"The following do not have an orientable surface mesh suitable for tetrahedron meshing. Please check CAD model for errors."

And still the same "A mesh could not be generated using the current meshing options and settings. Hint: The problem geometry areas might have been highlighted. Switching to wireframe mode may make them more visible."

Both of them with default mesh and relevance center in medium. I'm getting tired of combining different options :/
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Old   May 23, 2014, 06:57
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Let's try this in stages. Instead of generating the volume mesh. What happens if you preview the surface mesh first? Do this by right-clicking on Mesh on the GUI Outline. Next if that is OK is to preview the inflation layer (I guess you are using pre-inflation - see the advanced inflation options). This is because ANSYS Meshing builds the mesh upwards from the surface to the volume.

You could also post the SolidWorks geometry file (or IGES etc) or the Workbench Archive so that others can give it a try.

But looking at the error messages it could be that you need to fix the issues at the CAD stage and not the mesh stage in the simulation procedure.
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Old   May 23, 2014, 10:18
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Quote:
Originally Posted by siw View Post
Let's try this in stages. Instead of generating the volume mesh. What happens if you preview the surface mesh first? Do this by right-clicking on Mesh on the GUI Outline. Next if that is OK is to preview the inflation layer (I guess you are using pre-inflation - see the advanced inflation options). This is because ANSYS Meshing builds the mesh upwards from the surface to the volume.

You could also post the SolidWorks geometry file (or IGES etc) or the Workbench Archive so that others can give it a try.

But looking at the error messages it could be that you need to fix the issues at the CAD stage and not the mesh stage in the simulation procedure.
I've tried the preview but got the same error Maybe it is the cad, and also I tried to change some values/parameters and tried not to use tiny elements... Nothing changed.



Thanks again

S.

Last edited by sandri_92; June 4, 2014 at 04:59.
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Old   May 23, 2014, 14:18
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I had problems with importing your CAD model. Can you send it as a neutral format e.g. iges or parasolid.
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Old   May 23, 2014, 15:54
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I also had problems to open the CAD file, because i haven't access to Solidworks. There is a free file viewer for Solidworks files which i used.
I saw a few cavities with a sharp angle between surfaces. The vertical wing wasn't fully joint to buttom plate. This might could be a conversion problem on my side.
Also the area of the lowest horizontal wing joint to the buttom plate has some very sharp edges. I'm not used to meshing directly in fluent. But i think, those areas can lead to troubles while meshing, especially when you apply a boundary layer mesh.

As siw suggest, could you provide the geometry in another format?
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Old   May 24, 2014, 10:22
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Quote:
Originally Posted by bluebase View Post
I also had problems to open the CAD file, because i haven't access to Solidworks. There is a free file viewer for Solidworks files which i used.
I saw a few cavities with a sharp angle between surfaces. The vertical wing wasn't fully joint to buttom plate. This might could be a conversion problem on my side.
Also the area of the lowest horizontal wing joint to the buttom plate has some very sharp edges. I'm not used to meshing directly in fluent. But i think, those areas can lead to troubles while meshing, especially when you apply a boundary layer mesh.

As siw suggest, could you provide the geometry in another format?

I was wondering how to convert to a different extension, and then realized it was file>save as.. hah I'm new in the business...

Last edited by sandri_92; June 4, 2014 at 04:59.
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Old   May 24, 2014, 13:25
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I've imported your parasolid file and previewed a surface mesh (see image which is in the attached zip file with the other images at http://www.megafileupload.com/en/fil.../Mesh-zip.html). But there are some issues that need fixing/looking at/thinking about before going any further otherwise you'll not get a good (or any) mesh.

I've some questions/observations about the geometry (I probably have the names of the wing components wrong):

  1. There is a gap between part of the rectangular plate and wing (see image 1). Is this correct?
  2. The trailing edge of the wing has a very small thickness, can you make this a sharp trailing edge instead? (see image 2). Likewise the curved wings mix both sharp and blunt trailing edges.
  3. There aren't any curves defining the wing shapes (see image 3).
  4. The front wing curves very close to the plate so the inflation layer will have problems (or just fail) - see image 4. Also the second wing TE and first wing any partially join (see image 5) - had a look at this bit in ICEM and it seems they don't connect at all. You might want to omit inflation layer there locally but you would need to segregate the wing surfaces to have areas to assign and omit the inflation layers on.

Last edited by siw; May 24, 2014 at 13:34. Reason: Extra comment
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Old   May 25, 2014, 02:27
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Wow Siw, really really thank you. I think I have a lot of work to do now. Very interesting your comments. I must admit that the gap (1st error) shouldn't exist, and so the rest of the points may be considered .

I was searching in view options but couldn't find it. I want to have the same view like the one you called surface mesh. When I make my mesh I can view the wireframe, but not the object in red and isolated (the F.wing alone with its mesh). How can I do it?

Again, my respect for sharing your time with me. Thank you and also bluebase.

S.
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Old   May 25, 2014, 07:00
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I'll explain step-by-step what I did for the surface mesh image, sorry if this seems overly simple but you said this is new to you.

After importing your parasolid into a Fluent Analysis System in Workbench (just like your Workbench project) I opened ANSYS Meshing. I assigned Named Selections to the faces of the fluid domain geometry called Inflow, Outflow, Wing (could have gone further e.g. Wing_1, Wing_2 etc) and so on. I always use Named Selections like this as it is useful for all the CFD stages (mesh generation, assigning boundary conditions, post-processing etc). Then if you select a Named Selection in the Outline an option called Visibility appears in the Details - you can see this is Yes for the Wing but I set it to No for all the other Named Selections. Then click on the Wing Named Selection to see the associated geometry and mesh. This is the best way I find in ANSYS Meshing for these external CFD tasks for getting access to the geometry of interest (car, aircraft etc) to assign specific meshing features (edge, face sizing etc). You can also colour by Named Selection, see Random Colors.

Other than that I set the relevance center to Medium and previewed the surface mesh - this was just to see what would happen. But as I said in the previous post work needs to be done to the geometry in order to make the hybrid volume mesh. The grey part of the wing without the surface mesh was due to the use of Virtual Topology - if I recall correctly.

Fix your geometry, post it here and let the CFD community give it another try if your having problems. One other thought, do you have access to ICEM CFD?

Last edited by siw; May 25, 2014 at 07:03. Reason: Typo
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Old   May 25, 2014, 11:28
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Quote:
Originally Posted by siw View Post
I'll explain step-by-step what I did for the surface mesh image, sorry if this seems overly simple but you said this is new to you.

After importing your parasolid into a Fluent Analysis System in Workbench (just like your Workbench project) I opened ANSYS Meshing. I assigned Named Selections to the faces of the fluid domain geometry called Inflow, Outflow, Wing (could have gone further e.g. Wing_1, Wing_2 etc) and so on. I always use Named Selections like this as it is useful for all the CFD stages (mesh generation, assigning boundary conditions, post-processing etc). Then if you select a Named Selection in the Outline an option called Visibility appears in the Details - you can see this is Yes for the Wing but I set it to No for all the other Named Selections. Then click on the Wing Named Selection to see the associated geometry and mesh. This is the best way I find in ANSYS Meshing for these external CFD tasks for getting access to the geometry of interest (car, aircraft etc) to assign specific meshing features (edge, face sizing etc). You can also colour by Named Selection, see Random Colors.

Other than that I set the relevance center to Medium and previewed the surface mesh - this was just to see what would happen. But as I said in the previous post work needs to be done to the geometry in order to make the hybrid volume mesh. The grey part of the wing without the surface mesh was due to the use of Virtual Topology - if I recall correctly.

Fix your geometry, post it here and let the CFD community give it another try if your having problems. One other thought, do you have access to ICEM CFD?
Thank you very much. I've just changed the geometry a bit and make a default medium coarse mesh. I've used sizing on curvature (curvature and proximity makes the same error). I think as you said, the sharp edges and the complexity of the wing could lead the program to crash. Anyway it became to a 2.5 million elements. It's a huge amount of data required, and maybe I'll leave it as it is now. Refining the closer zone of the wing and using inflation are my common path to the "perfect" mesh, but in this case it will take a long time solving the fluid problem, and honestly I don't have time. I'll try to do the simulation from one night to the next day, I think it would take 6 hs (0.4 million elements takes 60-90 minutes) that would be if the courant is OK and solution doesn't oscillate.

I don't have many resources, I use ansys educational version, the only available for University students. I only have Fluent and CFX licenses :/ .

Thanks again, and I'll try to keep this community informed :P

PS: Of course, if there is something I can do to help this community please tell me! I'm really happy to be here .

S.
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Old   May 26, 2014, 10:05
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Hi again!!

I've tried to make the simulation in fluent, but it crashed T_T . I don't know exactly what happened, but as I can see in the log window, it seems to be an error while calculating the 2nd iteration. It says it diverges... I've done some simulations in 2D without problems, taking the same configuration Mr. Scipy recommends (see http://www.youtube.com/user/eoescipy/videos). He uses COPLED solution, and First Order methods in each equation. Also I tried to put a courant of 0.5, but the same error appears.

Here it is what I've copied from the hybrid initialization and so on:

__________________________________________________ ____
Initialize using the hybrid initialization method.

Checking case topology...
-This case has both inlets & outlets
-Pressure information is not available at the boundaries.
Case will be initialized with constant pressure

iter scalar-0

1 1.000000e+00
2 6.340792e-04
3 1.091976e-04
4 4.234901e-05
5 1.604134e-05
6 2.895635e-05
7 1.425875e-04
8 7.180308e-04
9 4.092010e-03
10 2.044245e-02
hybrid initialization is done.
Warning: convergence tolerance of 1.000000e-06 not reached
during Hybrid Initialization.
Writing Settings file...
writing rp variables ... Done.
writing domain variables ... Done.
writing part_1 (type fluid) (mixture) ... Done.
writing interior-part_1 (type interior) (mixture) ... Done.
writing inlet (type velocity-inlet) (mixture) ... Done.
writing outlet (type pressure-outlet) (mixture) ... Done.
writing symmetry (type symmetry) (mixture) ... Done.
writing fw (type wall) (mixture) ... Done.
writing zones map name-id ... Done.

iter continuity x-velocity y-velocity z-velocity k epsilon Cl-1 Cd-1 time/iter

turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 10744 cells
1 1.0000e+00 1.4046e-01 1.2943e-01 5.1349e-01 6.3336e+06 1.2761e+09 -3.5089e+04 1.0025e+10 42:43:26 999
# Divergence detected in AMG solver: pressure coupled -> Decreasing coarsening group size!
# Divergence detected in AMG solver: pressure coupled -> Increasing relaxation sweeps!
# Divergence detected in AMG solver: pressure coupled -> Changing to W-cycle!

Divergence detected - temporarily relaxing and trying again...

Divergence detected - temporarily relaxing and trying again...

Divergence detected - temporarily relaxing and trying again...

Divergence detected - temporarily relaxing and trying again...

Divergence detected - temporarily relaxing and trying again...
# Divergence detected in AMG solver: k -> Increasing relaxation sweeps!

Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: pressure coupled
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Primitive Error at Node 1: floating point exception

Primitive Error at Node 0: floating point exception

Primitive Error at Node 2: floating point exception

Primitive Error at Node 3: floating point exception

Error: floating point exception
Error Object: #f
_________________________________________________

Also I've checked the RAM and Processor status while making the simulation just in case temperatures want to burn my laptop :P . And yes, both at 100% working.

I should upload the workbench case, but it is now +200mb and none solution!

Should I continue this crazy excercise? I'm speechless...

Thanks for your advice .

S.

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Old   November 14, 2018, 08:54
Default Plate fin Heat Exchanger
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Can anybody help me in getting efficient meshSolid body.JPG

side view.JPG

Front view.JPG
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