[TeresaT]

Hello everyone!

First things first I am a student from Germany with just a little bit experience with openFoam. I will always be thankful if you answer with links to pages where I can find answers as i tried to find them without luck so far. Even tips on what keywords will work, or which book to read will be welcome.


The Problem:
snappyHexMesh runs and runs and runs.. way over 12 hours so far

The goal:
To set up a kwSST Case to simulate the flow around the "Berliner Fernsehturm" the tv tower at Alexanderplatz, Berlin, Germany.

The case right now:
For my first attempt with snappyHexMesh I made a imprecise stl of the tv tower and i used the motorbike example files. A few times snappyHexMesh worked well however I made mistakes and ended up with a mesh that contained only a few parts of my tvtower.stl. I was able to find a solution on my own. The tvtower is now located in the middle of my blockMesh generated block, just where I think it should be. The mesh of my block is already quite fine.

My General questions:

1. mesh of the outer block:
Does the amount of cells interfere with snappyHexMesh?
Is there a rule to identify the number of useful cells?

2. stl-File:
What can I do to improve my stl file?
What´s easier to mesh? Sharp edges or smooth roundings?
Right now the stl contains both, should I change it to one type only?

3. Further readings:
Can you recommend a book fur further readings?
One with a highly detailed chapter about snappyHexMesh?

Thanks to everyone who has read so far, every hint is welcome.
Greetings,
Teresa
Ps.: Prior cases using only blockMesh and laminar flow worked well so far.

[Tim_Simon]

Hey Teresa

I will try to answer your questions with my experiences in meshing with snappy. I can not guarantee you that everything is 100% true so everybody is welcome to add/improve

1. The total amount of cells should NOT interfere with snappy. There is however a cell size and followed by this an amount of cells for your domain that would be somewhat reasonable. You should decide for a maximum cell length in your domain and make that the length in your initial mesh. Keep in mind that snappy works best if the initial cells are cubic.

2. From what I experienced smooth curves are easier to handle than sharp edges. That said, there are several values you can play with in your snappyHexMeshDict to improve the outcome. Improve your stl by changing too sharp angles to curves.

What cell length does the final mesh on the surface of the tower have? Where did you get the stl data from?

Greetings from Berlin,
Tim

[TeresaT]

Hello Tim,

Thank you for your reply and greetings to my home town.
I made the stl file on my own and I just created a version without sharp edges. So today I will let snappyHexMesh taste that file. The stl file consists of 8598 triangles and is a five minute, visual judgement sort oh thing. This version has close to none details. I will create more accurate stl files later on. The goal was to learn to set up this sort of cases. My initial cells are cubic, how ever they might be somewhat small.

What cell length does the final mesh on the surface of the tower have?

Sorry I don´t think i understood this question properly. The cell length of the tvtower.stl is highly diverse. The sphere consist of many small cells and the cylindric parts of big ones. SnappyHexMesh stopped due an cat on the socket switch. (I can´t believe it!)

I just gonna start the new meshing.

Greetings,
Teresa

[akidess]

Teresa, see if http://www.sourceflux.de/ satisfies your needs on a book with some information on snappyHexMesh. They have some nice blog posts too.

[TeresaT]

Hello akidess,

thank you for the link. That looks great!

Greetings,
Teresa

[TeresaT]

Hey Tim,

your advices helped a lot. SnappyHexMesh works and I am now able to change parameters in snappyHexMeshDict and to check the results.

Thanks a lot!

Greetings,
Teresa