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Mesh problem in StarCCM+

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Old   November 8, 2020, 14:12
Default Mesh problem in StarCCM+
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Yasin ŞAHİN
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Hi everyone,

I am kinda newbie in starccm+, cfd and cfdonline. So apologies if i write something wrong or something which is not belong in here.

I am running an external flow analysis over a vehicle which is designed to be drag efficient. The problem is when I start creating mesh its okey until high mesh cell number but after that my mesh starts strugling.

Here is the photo of normal mesh:

https://www.imageupload.net/image/WbZNt

Here is my volume shapes:

https://www.imageupload.net/image/Wbeyd
https://www.imageupload.net/image/WbTeg
https://www.imageupload.net/image/Wbafs
https://www.imageupload.net/image/WbQv5

Here is what i meant by saying strugling:

https://www.imageupload.net/image/Wb3L8

I tried to change volume shape dimensions and mesh size of volume shapes but both didnt work. For successful mesh i used:

- DOMAIN_BLOCK: [-35000.0, -15000.0, -500.0] [16000.0, 100.0, 15000.0] mm,mm,mm - 300 mm
- MID_BLOCK: [-25000.0, -10000.0, -500.0] [10000.0, 100.0, 7500.0] mm,mm,mm - 150
- BODY_BLOCK: [-15000.0, -5000.0, -500.0] [5000.0, 100.0, 3500.0] mm,mm,mm - 75
- UNDERBODY_BLOCK: [-7000.0, -1500.0, -500.0] [1000.0, 100.0, 100.0] mm,mm,mm - 25

For unsuccessful:

- DOMAIN_BLOCK: [-35000.0, -15000.0, -500.0] [16000.0, 100.0, 15000.0] mm,mm,mm - 240 mm
- MID_BLOCK: [-25000.0, -10000.0, -500.0] [10000.0, 100.0, 7500.0] mm,mm,mm - 120 mm
- BODY_BLOCK: [-15000.0, -5000.0, -500.0] [5000.0, 100.0, 3500.0] mm,mm,mm - 60 mm
- UNDERBODY_BLOCK: [-7000.0, -1500.0, -500.0] [1000.0, 100.0, 100.0] mm,mm,mm - 20 mm

I am using k-E turbulance model. I search the internet and couldnt find what to do about I am waiting for your help.

Last edited by yasinsahin; November 9, 2020 at 03:56.
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Old   November 9, 2020, 14:28
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Arjun
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So in the end. What did not work?



Meshing? Or simulation diverged??
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Old   November 9, 2020, 14:55
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Yasin ŞAHİN
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From what I understand meshing didn't work in here since there is no mesh shape in mid_block volume shape. Because normally in all mesh scenarios I saw on the internet and tried myself, there is mesh volume which is denser than the last mesh volume can be seen clearly. But in this situation I can only see 3 different mesh volume (but there is 4). So actually I am asking is that a problem and if it is how to fix that?
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Old   November 10, 2020, 00:11
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Quote:
Originally Posted by yasinsahin View Post
From what I understand meshing didn't work in here since there is no mesh shape in mid_block volume shape. Because normally in all mesh scenarios I saw on the internet and tried myself, there is mesh volume which is denser than the last mesh volume can be seen clearly. But in this situation I can only see 3 different mesh volume (but there is 4). So actually I am asking is that a problem and if it is how to fix that?



Humm, Did starccm give some error message on the console. If not then the meshing shall be done.



I believe then the problem is that the refinement/coarsening you wish to see is not visible. Which is very hard to guess why?



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Old   November 10, 2020, 06:55
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Maybe there is some rounding issuse with your finer mesh. Since you are using a catesian mesh, there are only discreet element size steps. If multiples of your given sizes do not fit into a dimension, the mesher either has to use larger, or smaller elements, wouldn't it?

I don't fully recall which route StarCCM uses.

Just play a bit with the size settings and additionally the min and maximum size settings.
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Old   December 30, 2020, 10:52
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Quote:
Originally Posted by yasinsahin View Post
From what I understand meshing didn't work in here since there is no mesh shape in mid_block volume shape. Because normally in all mesh scenarios I saw on the internet and tried myself, there is mesh volume which is denser than the last mesh volume can be seen clearly. But in this situation I can only see 3 different mesh volume (but there is 4). So actually I am asking is that a problem and if it is how to fix that?

So the 3rd volume control has the same cell sizes as the 4th volume control.

In fact, in the picture can be seen that you refine the outer volume but none of the inner volumes are refined. Based on that, I assume that if you keep going like that, your farfield will be more dense than your near field.
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Old   December 31, 2020, 06:49
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Yasin ŞAHİN
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Well I forget to wrote it in here but I solve the problem by putting a ratio between mesh sizes on volume shapes. I put the ratio manually but setting mesh size according to the base size (for example %20 of the base size for the first domain, %35 of the base size for the second domain etc.) automatically in star is also working.

Thank you all for suggestions.
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