HI everybody

I posted this problem 2 days ago in CFX forum

Simply, when I mesh my 3d fluid-structure interaction with ANSYS, I get the following error

"Hexahedral element has negative or zero volume. Nodes must be numbered according to the right hand rule"

I tried many meshing techniques such as changing the mesh type to tetrahedral but the same error poped up

Has any one an idea how to solve it???? I really need it this weekend!!!!

Best regards

p.s thanks to Aplus for responding to my last message in CFX forum ( he is the only one, I need more and more)

If you export it as a NASTRAN BDF file, I could take a look at it and try to find the offending element(s).

regards

DSS

www.TITANAlgorithms.com

in the /prep7 (preprocessor menu) go to "numbering ctrls" branch and use the "merge items" (it's advisable to merge all entities), after that, go to the "compress numbers" and also compress all entities. During the compression operation Ansys will renumber all entities in your model (geometry and mesh entities) and check for some numbering problems.

Good luck

Renato.

Thank you Renato

I did what you said already, but no sign of success

the same problem with one more error

"Restart time # is not in rfl file"

regards abdulaziz

Thank you TITAN

but how can I export it in this format

regards abdulaziz

Some doubts regarding your problem:

- Have you built this mesh "by hand" (inserting nodes and mounting the elements)?

- Have you checked this mesh in "preprocessor\meshing\check mesh\connectivity\Ck connectvty" before trying to solve something? It'll not solve the problem but should point you where the problem is (element number, etc...)

- Do you have any bad keypoint (assuming you're using geometry entities to guide the mesher)? A bad keypoint (or node) is easily found by listing the keypoint coordinates and looking for some weird coordinates (something like 1.321938217984961e-16)

Your problem with RST file is not a problem due your mesh. I guess it's due your model that is not well-posed yet. Thus, try to work out your preprocessor problems before trying to solve.

Cheers

Renato.

DEAR RENATO

regarding your first 2 points, I did that and I did the mesh up-bottom approch (by meshing the volumes) and I checked the connectivity, It is alright

For the third point, I do have bad keypoints and nodes, but NO WAY to avoid them becuase of geometry requirements

I think this problme " Hexahedral elements have zero or negative volume ... " because of fluid element fluid142 as I read about that in a recent paper, but the paper does not discuss the solution

best regards abdulaziz

Can I send you the Command file (*.txt) because I can not export it to the format you asked for

I don't know much about the ansys hex mesher. Most codes support neutral file export. BDF or bulk data file would allow you export and a CHEXA, 8 node element and then you could read this into another viewer to see where it hangs up. You original error makes me think that you might of turned an element inside out.

regards

DSS

strange but understandable and I'm sure you'll solve this problem soon...

I've already worked with FLUID142 as hexahedra without this kind of problem and I got curious after you said that read about a problem with hexa FLUID142. Could you send me the article where you read it?! (rnelias@gmail.com)

Regarding the bad keypoints, you can avoid this problem by simply doing the following procedure before meshing your model:

- After building your model, glue and merge all entities (I'm sure you already did that);

- Move or copy all model (volume and attached entities) to a coordinate far from your original position (let's say 5000, 5000, 5000). It'll multiply the keypoints coordinates and remove the round-off truncation errors in the keypoints coordinates;

- Delete the model in its original position;

- Move (or copy) back the copied model to the original position.

Do you know how to work with APDL (the Ansys macro language)? I'm asking because you can easily implement a macro to check and fix your bad elements - I guess I have it's already implemented but I'll have to search it on my messed computer ;-). Try to find some macro in the following link http://ansys.net/ansys.

Cheers

Renato.

Very Nice Comments RENATO

BUT I am now busy with writing my paper ( Unfortunately, that will not include the model causes this problem although it is a huge part of my thesis and paper)

Regardignt the article you asked for, FOR SURE I will send it to you but give me couple of days to finish my paper

regards abdulaziz

TITAN

THIS IS EXACTELY THE PROBLEM (I think) because FLUID142 elements should have +ve volume (as in the ANSYS help)

How to fix it, UNTIL NOW I am looking for the guys answers because I do not know

regards abdualziz

You can fix the negative volume problem by changing the local nodal conectivity of the element.

According the Ansys users guide for a FLUID142 element, a hexahedron should be numbered in the following manner to have a positive volume mapping:

1-2-3-4-5-6-7-8 (node 5 above node 1) or i-j-k-l-m-n-o-p for a general case

if you find an element like:

1-4-3-2-5-8-7-6 or i-l-k-j-m-p-o-n

you can surely assert that this element has a negative volume.

Note that you can freely change this ordering to have bad elements fixed.

Regards

Renato.

From my own experience in developing the hex smoother for Hyperion-Mesh3D, the Laplacian smoother can melt meshes too deeply and punch vertices into the element when smoothing. If the Ansys mesher is doing something similar, I would suggest trying to generate a hex mesh that has no smoothing or vertex relocation -- basically the raw un-modified output, which if carved from a structured voxex space, should not create any problems. Then work systematic smoothing back into the problem to see the moment where you punch and invert your element.

regards

DSS

www.TITANAlgorithms.com

DEAR RENATO

I am back again

I want to ask you if you have (or any guy visiting this forum) a macro file that does this check for the negative fluid elements and fixes them

I really need it

yours abdulaziz

[mechatronicstudent]

hello everybody!
I'm new in ANSYS. I have some problems in modeling & meshing.
I'm trying with : ANSYS15-x64 and in Mechanical APDL.
my system is : Intel core(i5) 2.27 with 4G RAM.
O.S : Windows 8-x64.
I create this rectangle :
wp x = 0
wp y = 0
width = 2
height = 1
I meshed it in default mode. with PLANE55 element.
Then I could Extrude it in " Cylindrical " Active C.S & with SOLID278 with " Extrude Area by XYZ" tool.
It meshed & Extruded exactly.
MY PROBLEM IS :
But when I'm trying to Extrude it in " Cylindrical-Y " Active C.S it happens this error :

*** FATAL *** CP = 4.094 TIME= 12:30:54
An allocation was made with a negative length requested: -8
Filename:..\src\FEM_advFront.cpp Linenum:3032.

I'll be so grateful if any one can help me.