meshing problem

  • Last Post 02 May 2019
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usama posted this 28 April 2019

hello dear all,

I am doing meshing between two concentric circles. the distance between two concentric circles 5 micrometers. I have previous studies meshing I want to make mesh like that. 

I tried the inflation layer between two concentric circles but I want inflation between two circles. I have attached two two mesh images. and

also attached

the meshing archive file please check and correct me.

Thanks in Advance.



Attached Files

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peteroznewman posted this 28 April 2019

Hello Usama,

I was interested to read about Kundsen number, which rwoolhou recommended you lookup in a previous post.
Please reply with the Kundsen number for this problem.


usama posted this 28 April 2019

Dear peter i know about the Knudsen number K n = λ / L, where λ is the mean free path and L is the characteristic length. But i do not know how to calculate it. The pressure is atmospheric fluid has viscosity 0.45 and density 1220 The gap between two cylinder is 5 micrometer. And the fluid has height of 100 micrometer from inlet. And semi circle diamter is 5.40 mm and big circle diamteter is 12 mm. Actually i just want to check the flow in microcavities and some backflow(fluid accumilation or fluid backflow). Can you please explain how to find the knudsen no. Actually i know about knudsen number but for this problem how to calculate this. Regards Usama

usama posted this 29 April 2019

Dear peter I know about knudsen number but i do not know how to find for this problem. A dimensionless parameter known as Knudsen number K n = λ / L where λ is the mean free path and L is the characteristic length. I have the fluid moving with velocity 0.001 having viscosity and density 0.45 and 1220. The thickness height of the fluid on small roller(r=2.6mm) is 100 micrometer. And the gap between two circles is 5 micrometer. The big circle has the radius 6mm. The cavity dim. H/W is 30/40.

Actually i want to show the filling process before nip forces thatswhy i keep the distance two small. The out put is to show the optimal filling in the cavities at different speeds. I just want to know how the inflation will set in the middle of the circles as the pervious study did. Please help me regarding this this whole thing wasted my 9 days. Second thing if we increase circles diameters then the area on the left side will increase and the programe generated element order will also increase but the distance between two circles is two small thats why we need to keep element order for meshing in the range of micrometer. If i keep the element order in this range for big cylinder(@see latest post) i am getting error again and again after 1 day "Software Excecution error in the mesher". Please guide me for this also. Regards Usama

rwoolhou posted this 29 April 2019

The Knudsen Number is fluid & domain dependent.  The characteristic dimension here is 5 microns; you need to find/calculate the mean free path. 

I would also suggest building the model at a larger scale: CAD faceting will give you problems as the gaps close to 5 microns. I'd build the whole device in mm (eg have a 5mm gap) and then scale when reading into Fluent. 

Are you trying to show cavity filling with a free surface? Eg oil into air? 

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usama posted this 29 April 2019

Yes, I am trying for cavity filling actually the fluid is placed on a small circle of certain height 100 micrometers. And i want to show optimal filling situation.

Can you please explain how to scale in fluent. Can we reduce the size between two circles in fluent up to microns?

On a larger scale, how can I show the filling of the system? If it creates the problem then how the previous studies have done this meshing between two circles having microns distance between the circles.

Here is the schematic of the imprint system.


peteroznewman posted this 29 April 2019

Is this an Intaglio printing system?  The fluid is the ink.  The green line is the paper or media to which the ink will transfer?  The cavities in the plate will hold the ink to transfer the image to the paper. Your schematic doesn't show where the ink is being added and you don't show where the ink is being wiped off the surface of the plate, leaving the ink in the cavities only.

The suggestion from rwoolhou is to draw the geometry 1000 times larger by creating all the features in mm as if it were microns, and mesh 1000 times larger, pretending 1 mm is 1 micron.  When the mesh is finished, a simple scale factor is used when it is imported into Fluent that divides the geometry by a factor of 1000. Fluent has a better ability to handle very small dimensions than the Mechanical program.  This is a trick to work around the limitations of Mechanical.

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usama posted this 30 April 2019

Dear peter its advance system. Roll to roll nanoimpeint lithography. (A simple example check your phone screen protector having small patterns on it.) The green line is web having polymer on it and web has tension on both ends. It will move from left to right by two unwheeling and wheeling rollers. The big roller having patterns on it and supported roller applies a nip force(adjustable) for the filling process. Actually before applying the nip forces there will some accumilation of fluid which will filled in the pervious cavities its a parameter which is helpful for filling. Different system has different optimal speed foe filling i need to check for my sytem which is a larger system than i showed in the archive files actually results have been published for smaller system. Some extra information: (the big roller created by photolithography process). (Actually i have published one article thin film coating for big roller by taking that thing taking into account that continum mechanics limites by some matlab coding but its the pre-topic for this system now i am doing for roll to roll nanoimprinting process So continum mechnics will not break in this case also.) The brackets things are extra for only for your understanding. 2. So you suggested me that make the system 1000 times larger this. What you say if the big roller has diameter of 300mm and supported roller 100mm and the size will be the 5 micrometer between two circles that you suggest me make it larger 1000 times. So i need to apply the same approach for them also? Regards Usama

usama posted this 30 April 2019

Dear Peter and rwoolhou Its my last project for graduation. And i have to submit it as soon as possible please help in order to fix the problem.

I have seen some tutorials for scaling. I need some calrification. If i multiple all dimension with 1000 in DM. And mesh them then export the mesh file in fluent and choose scaling 1. I need to change the dimensions in mm and reduce it by scaling factor 1000mm or 0.001m please explain this thing further. 2. If i do so the meshing will be reduce to mm scale and the gap will be 5 micrometer between two circles? And also the other dimensions will also change? Like diameter of of circle and supported circle. 3. Since i am working in 2D so i have to scale X and Y factors and axis? 4. For small system like i have given above it can be easy by taking it into mm scale and mesh the.but if i choose big system like i have told the dimension above if i make the geometry and supported roller on left side cover larger area but the distance between big circles is also micrometers this thing create problem for meshing i think . " excecution error for meshing" (you can see my other post). The above archeive file for reproducing the pervious results they use two body system one is patterns and one is outer region and mesh them seprately and use interface matching for patterns and outer region with patterns regions. But if we run this file in fluent that give warning that interfaces are overlapping. I want to create one body part system with no interfaces. Between them. Does the scaling work for them. 5. One last thing if i increase the size by 1000 times the meshing and number of elements and nodes will also increase and can not be cover in ansys student versions. What you say about this. Suggest me about this. How to overcome this thing? Regards Usama

rwoolhou posted this 30 April 2019

The problem you have is you're looking at massive differences in length scale. 5 microns adjacent to 300mm is going to push CAD tolerances even if you scale up. Similarly using VOF (if you need multiphase) at that scale is difficult as the surface tension forces (N/m) become massive. 

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usama posted this 30 April 2019

Dear rwoolhou and @peteroznewman I have dimensions for big circle 300 mm small circle 100 mm and the difference between two big circles 5 micrometer. And patterns (H/W=30/40) If i multiple these dimensions 300mm×1000 100mm×1000 5 micrometer×1000 And patterns 30 micrometer×1000 40 micrometer×1000 For the CAD And mesh them and put it into fluent and scale up Does the dimensions and meshing will change? I will use multiphase with open channel B.Cs in fluent And i set surface tension between two phases 0.03 (N/m) with wall adhesion B.Cs Does these things can simulte for what i want? Like as you said about surface tension effects? Regards Usama

rwoolhou posted this 01 May 2019

Fluent will scale whatever dimensions you tell it to. As it's accurate to many more significant figures than CAD (especially with double precision) it's been a route in for micro scale modelling for years.  Your problem is the range of scales in CAD: if you're not careful even with scaling the curve faceting will close up the channel. 

At 5 microns you're also into a flow regime that CFD codes aren't designed for. Yes it will work, but the resolution required may give time steps that are too small to solve in a reasonable time frame. You also need to read up on how your liquids interact with the material surfaces at that scale: CFD models assume a bulk surface property. 

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usama posted this 02 May 2019

Sorry for the late