3D Meshing

[Andres29]

To whom it may concern,

Hello... my name is Andres Bernal and I am a London South Bank University student carrying out a thesis about the influence of the buildings shape onto the atmospheric pollutants concentration due to the innovative forms of them nowadays. I am working on a building known as "The Walkie-Talkie Building" and the first stage of the project was to create a geometric form of the building and the surrounding area using the Gmsh grid generator. In order to move along with the project we need to mesh the whole area which is enclosed into a whole domain (a bigger cube). When I mesh the geometry in 1D and 2D, It runs perfectly, but when I try with 3D it shows me a lot of errors and I do not know why. I would like you to help me in this issue as I have been trying to figure out the problem but I have not been able to and I got stuck for this matter. I would be really grateful, thanks.

Any advice would be much appreciated.

Kind Regards,

Andres Bernal

[mzenker]

Hi,

I had a look at your geometry (looks impressing, wow!), but did not wait for the error to occur (takes too long). I have seen that not all surfaces are part of your volume, and would suspect that there is the reason for the problems you have. You can examine that using tools > visibility, switch to "elementary entities" and click on "volume 2", then "apply". You will see some parts disappear. If you click on the asterisk and "apply", they appear again. I would recommend to fix that.
You might also use a larger mesh width far away from the building to speed things up. Just change the characteristic length of the corners of your cube - you also need to change the maximal mesh width under tools > options > mesh > general.
BTW there is also a gmsh mailing list where the real gmsh pros are...

HTH,

Matthias

[Andres29]

Hello Matthias,


Thank you very much beforehand for your attention. I had a look about what you said regarding the surfaces that were not part of the volume, and you were right. There were 3 surfaces missing. I already fixed that. I also used a coarser mesh. I used "6" in the Element Size Factor, much faster. However the only thing that I got was to reduce the errors from 2722 to 408, I was not able to mesh the volume. Any other advice may be? or perhaps I am not applying what you are telling me properly?

Once again, thank you very much and I hope I can sort this out with your help.

Kind Regards,

Andres Bernal

[mzenker]

Hi Andres,

if you post your fixed geometry, I can have a look...

Matthias

[Andres29]

Oh sorry...please find attached the new file with the adjustments.

Kind Regards,

Andres Bernal

[mzenker]

Hi Andres,

there are still lines which are not part of your volume. You see them disappear whan you hit "Apply" in tools > visibility with "Elementary entities" and only volume 2 selected.
The lines are on the ground between the buildings. What are they supposed to be - sidewalks? In that case, you would need to model them in more detail.

HTH,

Matthias

[Andres29]

Hello Matthias,

Yes, those lines are supposed to be sidewalks or somerhing like that. They are defining streets and lanes, that is why they are not included in the Volume command, because the mesh is supposed to touch every single surface inside the cube and they are just lines ... or Do I have to include them?. Are those lines an error? If I delete those lines, am not I going to get any errors? and...how can I model them in more detail as you say? ... or what did you mean by that?

Kind Regards,

Andres

[mzenker]

Hi Andres,

if you don't include those lines in the volume, they will get a separate 1D mesh which will then overlap with the 2D mesh on the ground, and this gives you those errors. So either you include the lines in the volume, e.g. by segmenting the ground surface along those lines, or you even model them as real sidewalks with a height above the street level, or you simply delete them.
AFAIR there is a possibility in gmsh to include a point in a surface so that there will be a mesh node at that location. This might be possible also for lines, but I have never done that. You may find more info in the archives of the gmsh mailing list (search for "point in surface" or "line in volume" or the like).
If the lines have no function physically, I would delete them.

HTH,

Matthias

[Andres29]

Hello Matthias,

The lines did not have any function phisically, so I have deleted the lines and their points as well but nothing changed, I still have the errors. I am unable to mesh the volume. Please find attached the new file.

Kind Regards,

Andres

[mzenker]

Hi,

there are still things not included in your volume. Please check yourself, I told you how...

Matthias