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-   -   3D In-Cylinder Dynamic Mesh Skewness Issue (http://www.cfd-online.com/Forums/fluent/73405-3d-cylinder-dynamic-mesh-skewness-issue.html)

ChesterP March 7, 2010 15:30

3D In-Cylinder Dynamic Mesh Skewness Issue
 
Hello everybody,

I have meshed each zone as in the User's Guide and applied same Named Selections (for clarity) for the volumes.

In FLUENT I have specified the following for the Dynamic Mesh properties:
- PistonHead = Rigid Body - UDF : **piston-full** ;
- LayeringZoneAbovePiston = Rigid Body - UDF : **piston-limit** ;
- RemeshingZone = Deforming ;

I have enabled Smoothing, Layering and Remeshing.
For the moment I am only considering the movement of the piston (valves are fixed). When I am previewing the mesh motion there are two problems that arise, namely :
- cell and face skewness increase until they reach 1, some degrees after TDC ;
- when the piston head reaches the BDC there is an error concerning negative volume;

Thanks in advance,

Chester

mayur March 11, 2010 06:45

Chester,
I am trying to work on the same module as you are. I had some similar problems. I will try to answer your question.
About the dynamic zones, I am not sure if Layering zone above piston should be a rigid body. I guess removing that will solve the problem. Just mention the area (Fluid) between the Cylinder top wall and Piston as deforming and it should be ok.
Concerning the negative Volume, are you starting your iterations from TDC or BDC?? Normally negative volume is a result of incorrect meshing and incorrect zone specification. Just try what I said above. I think it should work.
All the best

ChesterP March 23, 2010 08:57

Desperate...
 
I'm starting from TDC. I tried smaller crank angle step 0.1 deg., I tried to remesh after each time step etc... It solves NOTHING

In fact the problem (skewness>0.95 --> 1) seems to come from the spring based smoothing and remeshing of the tetrahedral type volume after 2 to 3 deg. after TDC.

Any ideas please?

mayur March 23, 2010 09:03

Well, I am starting from TDC too... I would suggest you to have a look at your mesh. I think reducing the mesh size will help. First I was using a mesh with 1 mm spacing but because of it, the skewness was high at small corners so I got a error in Fluent. I reduced it to 0.5 and it works fine. Why not you also just give it a try.

Did you look into the 12th Tutorial : Cold flow Simulation Inside an SI Engine?? It might help a little too

ChesterP March 23, 2010 10:17

Yes I checked the simpler 2D case. It seemed pretty clear to me.
I applied the same rationale to my problem.

I tried to make a finer mesh of the incriminated zone (max length = 0.7 mm) but very soon after TDC the skewness is steadily increasing until it reaches 1. I selected as Dynamic Mesh Zones both the wall and the interior.

I don't know what to do...

mayur March 23, 2010 10:30

Well, it seems you have to do a little bit of trial and error. Make sure you give the correct cell ht in dynamic zones. I will tell you my dynamic zones and accordingly you can compare yours.
1) the Fluid between Piston (at TDC) and cylinder top as deforming zone
2) cylinder top - stationary
3) Piston- Rigid body with **piston-full** as the UDF/profile
4) Inlet_valve - Rigid body with the inlet Valve profile as the UDF
5) Exhaust_valve - Rigid body with the exhaust valve profile as UDF

Also did u make the interface zones correctly, Coz I took a long time to figure out the correct way of defining them.

Well this might be some very basic info but i hope it helps.

ChesterP March 23, 2010 10:46

Thank you for your help Mayur ;)

Ok in fact for the moment valves are fixed. I am only trying to make the in-cylinder fluid region deforming well when the piston is moving from TDC to BDC.

Also the region between the piston head and the cylinder head is composed of two volumes included in the same part (Chamber), namely :
1/ The upper part (just below cylinder head) : TET - Patch Conforming
2/ The lower part (just above piston head) : QUAD - Sweep

I use smoothing and remeshing for the TET and layering for the QUAD withe the **piston limit** UDF for the QUAD as stated in the FLUENT User's Guide.

Maybe I can send you some pictures on your private mail adress.

mayur March 23, 2010 10:53

It would be nice if you can mail me the picture

But again did u check if you have made ur TET zone as deforming?? I think its worth a try...

ChesterP March 23, 2010 11:06

Yes I choose Deforming both for the wall and the interior of the TET zone. With smoothing and remeshing (region & local) methods, a max length scale of 1mm and a max skewness of 0.75.

I will send you pictures and results.

Wim Vandenberghe March 30, 2010 11:24

Wim
 
Hello,

i'm a student mechanical engineer, from Belgium now doing my final project in Portugal. I see you are/were working on the same thing my project is about: the flow inside a cylinder.
@ ChesterP : the user's guide you were talking about, where did you find that? Because i only found 2D tutorials about the in-cylinder option?
Is it like a tutorial you found somewhere?

Thanks

ChesterP April 1, 2010 04:01

Hi Wim,

I was talking about the FLUENT's User Guide.

Have a nice day

mayur April 1, 2010 04:35

Chester,

Did your model work??

ChesterP April 1, 2010 05:00

Quote:

Originally Posted by mayur (Post 252684)
Chester,

Did your model work??

Hi Mayur,

I am going forward :) But there are still problems when my piston is moving from BDC to TDC at around 280 I am getting several warnings :

Warning : multiple subdivision required. Subdivide using Constant Ratio.

Warning : left handed faces detected!

Warning : negative volume detected --> repeat smoothing!

Warning : non-positive volumes exist.


:(

mayur April 1, 2010 05:07

Hello chester... Nice to know its progressing...

Hope you solve the errors soon.. All the best.

Wim Vandenberghe April 7, 2010 09:36

hello guys,

is there like a chapter in the user's guide about the in-cylinder model?
Can you use that in-cylinder model to make the valve move? Or do you need an UDF for the movement of the valve?

Wim

mayur April 7, 2010 09:44

Hello Wim,

There is a tutorial from Fluent which is on in-cylinder model. Check out tutorial 12 - Cold flow simulation inside an SI engine. To make the valve move, you just need a profile and you dont need a UDF for the movement of the valve.

Mayur

Wim Vandenberghe April 7, 2010 09:47

Thanks, but i've already tried that tutorial and the first thing I did was check the mesh, and there were negative cell volumes... Do you think that might have something to do with the mesh being made in an older version than the one I have?

mayur April 7, 2010 09:52

I worked with the same tutorial and it worked. Try to follow the exact steps and values as mentioned in the tutorial. I feel that the version should not be a problem for Negative volume.

Wim Vandenberghe April 7, 2010 10:15

weird... i just checked and I can open the results that were with the tutorial, but I can't make it myself...

another question : do you know if this will work in 3D as well? Because that's the main problem here, especially for the valve... another student did a work like mine but he didn't manage to make the valve move. It was or closed or opened but not moving...and now we are thinking of new ways like for example with an UDF... but if you know that it has to be possible then at least I know that I'm not wasting my time trying to make it work...

bye

Wim

mayur April 7, 2010 10:23

Quote:

Originally Posted by Wim Vandenberghe (Post 253569)
weird... i just checked and I can open the results that were with the tutorial, but I can't make it myself...

another question : do you know if this will work in 3D as well? Because that's the main problem here, especially for the valve... another student did a work like mine but he didn't manage to make the valve move. It was or closed or opened but not moving...and now we are thinking of new ways like for example with an UDF... but if you know that it has to be possible then at least I know that I'm not wasting my time trying to make it work...

bye

Wim

Wim,

I am not yet sure about the 3D model. I will be starting my work in it. And the valves can be moved very easily. I even made my own profiles and read in Fluent. There is an example of profile in the tutorial. To make the valve move, make sure specify the valve motion and zones correctly. just follow the procedure in tutorial.


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