Non positive volume exist - How to find where ??!!
 

I've just finished my mesh, and fluent tells me that "non positive volume exists".

Houw can I do to find where, and correct it ???

Many thanks !

Re: Non positive volume exist - How to find where
 

When I got the same problem, I always make smoothing and then read the mesh file into fluent again. I use ICEM CFD. That can work.

zafer

Re: Non positive volume exist - How to find where
 

How do you smoth ??

(the part of my geometry which causes the problem is a single parallelogram, meshed in tri...! there is no curve!)

Re: Non positive volume exist - How to find where
 

I use ICEM CFD to generate mesh. Smoothing is under "edit mesh" menu. I have structured + unstructured hybrid mesh around a bluff body. You can there smooth you mesh.

Grrrr !
 

The problem, is that I'm running a 2D case... I don't understand why I've got a volume (negative or positive...)

I'don't have icem cfd...

Re: Grrrr !
 

Is it axisymmetric? If so, then you're problem is that you're below the X-Axis. In Fluent 2Daxi model, you must use the X-Axis as your axis of rotation, and your geometry must all lie in the +Y direction (above the X Axis).

Hope this helps, and good luck, Jason

Re: Grrrr !
 

Thanks, but the face involved is far from the X axis. when I export the face alone (no mesh) everything's ok.

And when I mesh it -> I get a neg volume...

find out !
 

"Gambit error, not found in gambit, but detected by fluent" the hotline said !

Re: find out !
 

I had the same problem in FLUENT, here is the way I solve it:

In FLUENT, you type(!) the following;

1. g 2. mz 3. rfh

This converts the negative volumes to positive for 2D case.

NOTE: "g" means "grid", "mz" means "modify zones" and "rfh" means "right face handedness"

Dear Tayfun,

I tried to follow the steps of Grid, modify zones, rfh. It says the negative volume have been converted but after checking the grid, it still says negative volume exists. Can you please help me in this.

I think maybe u can try to round/fillet all those edges.
sharp edges can give many problem while meshing.

Anyhow, just a suggestion.

If you have negative volume, the best way is to correct your mesh.
If you work with Gambit go back over there, and see where are those cells
Examine Mesh / Volume ... set the upper limit to 0
It will show you all the negative cells
Then correct your topology for avoiding this issue

Actually when I create the mesh, it shows positive volume. but as soon as I start the iteration in Fluent, it shows negative volume detected. I even checked the mesh parameters. The Grid info before iteration shows, Xmin = 0, Xmax=0.084 and Ymin=0, Ymax=0.09. After starting iterations, when I check the grid values, the values of Xmin and Ymin are changed. I even tried translation of the coordinates in Fluent, but it doesnt help:confused:.

*moving mesh?
*2d-axi?

Ya.. I am trying a moving mesh. Actually I am trying to compress fluid using fluent. I am using a simple rectangular geometry. The top wall, the side walls which are deforming and the bottom wall is the moving wall. I have drawn the geometry in a positive XY co-ordinate system.

ok, then your initial grid is ok, and your issue remains in the MDM-set up.
you have to set the moving wall as rigid body (here you have to give the motion with a profile or a UDF)
The side walls (adjacent to moving wall) as deforming
and the last one as stationnary.
Pay attention to meshing options panel, bad values will generate bad mesh

Thanks a lot max.

I will try these steps.
If I have any trouble, I will ask you again :)

Tutorial 12 from Help may help you (Using Dynamic Meshes)

Max,

I have one more query, I am defining the zones as you explained. While,defining the meshing options, we have to describe the cell height. Is there any way to find the optimum value as per the geometry. My current geometry is 84mm in length and 90 mm in height.

Thanks for your help.

I don't know anymore (I switched to OpenFOAM, I am not using Fluent anymore)
But I think there is a Panel "Geometry Definition" or something like that, which gives you some info about cell height.
But basically, if you have meshd your stuff you can remember the cell height.

Error : Update dynamic mesh failed. Negative cell volume detected.
 

hi dear all...
i have same problem as u mayur, when i check mesh in fluent, fluent does not show any negative volume ,but when i start iteration this error shows up,i changed time step from 0.0001 to 0.00001 and cell height is 0.002 and my case is Dynamic mesh (smoothing and remeshing) could u help me out plz?
thanks a lot in advance for any help provided...:o

maybe is your body's displacement to big.
Try to reduce the time-step again

negative cell volume
 

i changed time step from 0.001 to 10e-7 even smaller but again error message showed up!i think we should try something else and change another parameter such that fluent remeshes the pipe by itself. i think when the small cylinder moves fluent can not remesh that zone so this error message shoes up!any idea???:o
tnx in advance dear max....

you need to localize where is the problem. (which cells has negative volume)

how can i do that?

display/contour/
there is someting about mesh or cell (then volume)

Divergence detected in AMG solver: x-momentum
 

hi dear MAX...
i reran fluent and this i used following command text
define/models/dynamic-mesh-controls/spring-on-all-shapes?...> yes

fluent did not show negative volume message this time and iteration progressed 35 time step:eek:
but another error message this time:Divergence detected in AMG solver: x-momentum
i decreased under relaxation factor for momentum from 0.7 to 0.1 but no luck:confused:
could u plz guid me and help me out
tnx

prior to calculations did you check mesh motion (solve/mesh motion...)
it will help you debugging your case (if the divergence comes from your mesh or not)

negative volume
 

i tried ur suggestion and this is the result

Error: eval: unbound variable
Error Object: motion
Invalid time
time step size (s) [9.9999997e-05]

i dont know what does it mean!by the way every time that i run fluent and change some parameters different error messages shows up,negative cell volume,Divergence detected in AMG solver: x-momentum,etc...sometimes it shows up soon and sometime late,what's the matter??

how can i solve this two annoying problem???
dear max it's very nice of you to point me in the right direction:o

the divergence should come from negative cells or high skewed cells.
But you are not suppose to get such warning in mesh motion since no physics equation is solved.
Are you giving a motion, or is the motion computed through an UDF.
Do you have the possibilty to use laering method instead of smoothing/remeshing?

i have to move center of a small circle on the edge of a bigger circle and for this i wrote an udf;small circle rotates at 4 rad/sec;i changed different parameters and iteration proceed 3700 iteration,but everytime error messages that i mentioned showed up!
you think for this motion i should set spring factor=1.0 ?

hi
i have same problem, plz help me.
i read abovebut not useful for me.

I am not very experienced wit smoothing/remeshing, but more with layering.
you can try changing parameters...
Since I don't have Fluent, I cannot help you more than I did.
But once again: before starting to iterate, you need to be sure that your motion don't give you bad mesh.

Dynamic mesh deformation
 

Hello,

I am facing the exactly same problem of negative mesh volume. My case is moving of a cyinder in the y direction only.I created the mesh in gambit. Initially before running the mesh in CFX, when I checked it in ICEM all the vol were positive but after about 5 iterations the calculations is terminated and error says negative vol detected.

I would appreciate if you could tell me how to go about. Can I repair the mesh or smooth it in ICEM.

Please help is needed. I would really appreciate any type of help.

I had the same doubt. Your advice helped. Thanks a ton :)