Optimum mesh sizing size

Project Support
1

This is a followup of: Increasing mesh sizing size - #3 by jjansman

Dear support,

A co-miller (at a traditional wind mill) also is working with SIMSCALE. He is using another scientific article to verify the results of SIMSCALE:
Ren, Xinyi et al.: The influence of wind-induced response in urban trees on the surrounding flow field. In: Atmosphere 14 (2023), issue 1010, pp. 1-23.

<just to be sure: we are not really trying to check the results of SIMSCALE and the literature, just to have some ‘nice’ examples>

His results are at: Ren trees by nl_victor_reij | SimScale
SIMULATION: 1EffectiveRenCylinder-blockfar

We have the results here sumurised: Simuate the wind from Ren 'sarticle

He looked at the effect of the mesh default size (DS) in a Cartesian box; so he reduced the mesh default size in steps of 0.1m (from 0.5 to 0.2m). The Cartasian box is around a cylinder towards the end of the external flow volume.

Interestingly a mesh default size 0.5m of the Cartesian box matches a Fineness=9 for the whole external flow voulme (without Cartesian box). This was somewhat expected as the mesh size using Fineness=9 is also close to 0.5m

It looks that a number of cells with mesh default size of 0.4 and 0.3 stays quite the same (around ~4.5M), and the results are thus also quite similar.

If we go to a mesh default size of 0.2m, the results change considerable IMHO!

So what is happening? Do we need to reduce further the mesh default size to stabilise (we were not able to that in the Community plan)?
Or is the ‘local stable’ situation around 0.3 and 0.4 the best ‘place’?
Why is this results of 0.2m so different?
Or is this due to the Community Plan properties?
So how do we determine the optimum mesh default size?

All the best for 2025,

P.S. This matches somewhat my own experiences with the Nageli fence (using perforated plate porosity),
where a smaller mesh default size is considerable different at a certain size, see the earlier messages in this thread.

P.S.S. In the above models we used a porous medium: one is a perforated plate and the other Darcy-Forthcheimer. Is it perhaps the porous medium that cases this change with a small mesh default size?
But I see a similar effect when not using a porous medium…

2

Hi @jjansman

It’s somewhat common that results change when the mesh is refined, that’s the reason a mesh sensistivity study is often advised: Mesh Sensitivity Study for CFD Simulations | Knowledge Base | SimScale

As to your questions regarding the Community account, it’s built-in restrictions make it difficult to run very complex studies. The reason is that you can only go up to 16 computing cores which are often not enough for very large meshes.

Cheers
Igor

3

That is indeed why I did this sensitivity analysis with ONE simple cylinder (so not a ‘very complex study’, I hink).
And with or without porous media (of this simple cylinder) the velocity results are non-converging for smaller mesh size. So what to do? Or how to interpret? Or is the Communtity plan already too limited for this very simple model?
But I get the feel this is still an art.