Minimum Size Constraints on Simulations Query

Leddsworth · November 28, 2016, 14:29

Ahoy hoy,

Been running into bother meshing and running Fluent simulations involving exceptionally small geometry dimensions, around 1mm to 5mm wide fluid passages, and was wondering if there is some lower limit on the size Ansys can reasonably simulate?

From my analysis of the problem messages after trying a wealth of meshing methods, the automation seems to struggle with surfaces of the pipe walls and fluid boundaries and is creating interfacing at these dimensions that shouldn't be there. I have tried a few work around's already, including manually setting all the contact regions but I am still plagued by failing meshes, conflicting zone names and unresolvable contact regions.

https://i.imgsafe.org/c7512309b4.png
The image shows a sample of the heat exchanger concept, outer Fluid region, Central Pipe Wall Region and Inner Fluid Region.

Can anyone please advise a method to successfully mesh and discretize this model?

LuckyTran · November 28, 2016, 16:08

There is a lower limit because Fluent works natively in meters, but it's around 1e-06 m. Fluent shouldn't have issues with mm scale features.

Leddsworth · December 1, 2016, 06:02

You are correct, Ansys is capable of computing simulations at dimensions this small.

The errors seemed to stem from automated face pairing for connections on a series of layer 3D curved faces. I managed to resolve the conflict by better defining the contacting parts by grouping parts together in the Geometry editor to minimise the number of contact regions in the Mesh editor.

VincentVega · September 20, 2021, 05:06

Quote:

Originally Posted by LuckyTran

There is a lower limit because Fluent works natively in meters, but it's around 1e-06 m. Fluent shouldn't have issues with mm scale features.

I'm sorry for reviving an old thread, but what exactly is the lower limit, and where can I find more about it? Couldn't find anything via Google and the manual.

I'm asking because it try to simulate a microjet (diameter of 1,3e-6m), and the 2D one turned out good. Now I wanted to draw a mesh for a 3D validation but ANSYS DesignModeler and Spaceclaim both don't let me draw simple circles smaller than 1e-5m in diameter - which let me in turn doubt my 2D solutions because there must be some reason for a restriction like that. My minimum volume (in 2D) is ca. 3e-24m^2.

Somebody in another thread mentioned round-off errors in small size domains and I guess this is connected. Is there anywhere where I can read up on that to make sure I'm not calculating nonesense?

LuckyTran · September 20, 2021, 10:02

Well, it gets a little more complicated when other software are involved but basically...

Eventually your stuff gets saved into a .cas file which can be binary but is usually an ascii text file with limited precision. The fastest way to appreciate this is to open your .cas file with a text editor, scroll all the way down so you can all the coordinates of the mesh points. Last time I checked it was around 6 digits.

November 28, 2016, 14:29	Minimum Size Constraints on Simulations Query	#1
Leddsworth New Member Craig Leddicoat Join Date: Nov 2016 Posts: 2 Rep Power: 0	Ahoy hoy, Been running into bother meshing and running Fluent simulations involving exceptionally small geometry dimensions, around 1mm to 5mm wide fluid passages, and was wondering if there is some lower limit on the size Ansys can reasonably simulate? From my analysis of the problem messages after trying a wealth of meshing methods, the automation seems to struggle with surfaces of the pipe walls and fluid boundaries and is creating interfacing at these dimensions that shouldn't be there. I have tried a few work around's already, including manually setting all the contact regions but I am still plagued by failing meshes, conflicting zone names and unresolvable contact regions. https://i.imgsafe.org/c7512309b4.png The image shows a sample of the heat exchanger concept, outer Fluid region, Central Pipe Wall Region and Inner Fluid Region. Can anyone please advise a method to successfully mesh and discretize this model?

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November 28, 2016, 16:08		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	There is a lower limit because Fluent works natively in meters, but it's around 1e-06 m. Fluent shouldn't have issues with mm scale features.

December 1, 2016, 06:02		#3
Leddsworth New Member Craig Leddicoat Join Date: Nov 2016 Posts: 2 Rep Power: 0	You are correct, Ansys is capable of computing simulations at dimensions this small. The errors seemed to stem from automated face pairing for connections on a series of layer 3D curved faces. I managed to resolve the conflict by better defining the contacting parts by grouping parts together in the Geometry editor to minimise the number of contact regions in the Mesh editor.

September 20, 2021, 10:02		#5
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Well, it gets a little more complicated when other software are involved but basically... Eventually your stuff gets saved into a .cas file which can be binary but is usually an ascii text file with limited precision. The fastest way to appreciate this is to open your .cas file with a text editor, scroll all the way down so you can all the coordinates of the mesh points. Last time I checked it was around 6 digits.