Hello All.

I have a single 3D volume, an airfoil in a box, which is meshed as 4-node tets. I want the airfoil to move by some motion which I've prescribed in a DEFINE_CG_MOTION UDF macro.

The UDF works fine, when I preview the motion, I see the airfoil moving as expected. However, nothing else moves. The rest of the mesh stays stationary. As a result, when I run the simulation, I get negative volume errors.

I have smoothing and remeshing turned on (not layering though), and "spring-on-all-shapes" set to [yes]. The airfoil zone is set to "rigid body". I've tried all kinds of combinations of setting the other boundaries to various dynamic types: stationary, rigid body, deforming... with no luck.

I assume the fluid zone must be set to deforming... what else should my boundaries be set to in order to correctly spring to the new shape?

Any help would be greatly appreciated! Thanks!

-Dave

I am still very stuck. Please help!!!

[mrestrepo30]

Hi Dave,

Did you figure out how to solve your problem? I'm having the same problem with a 3D simulation, and I've tried implementing the smoothing parameter, and several combinations for the layering split and collapse factors, but I haven't been able to run the simulation...

Thanks!

[lzgwhy]

Hi, mrestrepo. I am doing a calculation with dynamic mesh. I found the same problem you mentioned. I think the dynamic mesh technique in Fluent is not very good or not very sufficient. So, I suggest you make your own updating mesh scheme according to you problem. I am seeking such a scheme for me, though I have not found. If you like, we may have a discussion about this.

[mrestrepo30]

Hi!
I managed to make the layering work, however, I did it using a CG_MOTION macro (which does rigid body motion). For my application I need to use a GRID_MOTION for displacing every node independently, but for thin kind of macro I haven't been able to do the layering.
Which type of macro are you using?

[lzgwhy]

Hi, mrestrepo. I use the macro DEFINE_GRID_MOTION, which I think can be used to define rigid motion, or to describle deformation. Both I have done both processes with this macro. If you like, could you give me more details about your problem? Since I know the layering method has some limits, so the main features of your problem is crucial.

[mrestrepo30]

Hi lzgwhy,

I'm implementing a sinusoidal movement on a cylinder wall using GRID_MOTION, so there's no rigid body motion but more a independent node displacement. So, my problem is that I know layering works for CG_MOTION (rigid body motion), but I'm not sure if I can implement layering on the sinusoidal movement of a wall. Thanks!

[lzgwhy]

Hi, mrestrepo. I guess you may being use the layering technique in Fluent, in which the grids may be collapsed or generated based on some criteria such as minimum or maximum scale. This method is robust in 2D problem, but less in 3D problem. DEFINE_GRID_MOTION (GRID_MOTION ) can be combined with this method, i.e. can just be used to define the deformation or motion of boundary. If you use this macro to define motion of nodes of fluid zone, layering technique can not be used, I think.

Also, I guess you may being use the Layer Deformation Smoothing technique in Fluent. Yes, you can use DEFINE_GRID_MOTION (GRID_MOTION ) to define deformation of the boundary, for example, a wall, and Fluent will perform this technique (Layer Deformation Smoothing ) wiht setting other parameters. However, this method may be just fit for the problem of small deformation in boundary. Thus if your problem refer to large deformation, I suggest you abandon this method.

I had used the two methods mentioned above. However, I do not think they are robust for gerenal 3D problem. So, I suggest you design your own method with the macro DEFINE_GRID_MOTION, which can be used to define the motion of every node in fluid zone or boundary zone, allowing users to control the dynamic mesh with some other methods, for example, calculating the displacements of nodes with Laplacian equation. I am looking for a new strategy for my problem. Thanks for your attention.

[ghazanfarzahedi]

Hi all
I have the same problem too.
I want to model motion of a manikin that rotates (oscillates) arund his axis with plus/minus 30 degrees vs his original location.
I used moving mesh for this (smoothing and rememeshing). But after passing about 25 degrees in first course, the message of
"Negative volume detected" is showed. I reduced time step but in that time the same message is showed.
Would you have any idea about the reason of this phenomena.
Thanks a lot for your favor
Ghazanfar Zahedi

[mrestrepo30]

Hi,
At the end I didn't use the layering capability for my simulation, but instead I only used the smoothing parameter of the dynamic mesh. This worked fine, and now I can displace my walls without getting any negative volumes... and the mesh is decent. If you're gonna try this don't forget to set the smoothing for the structured mesh on the TUI. Hope it works for you too.

[lzgwhy]

Hi, mrestrepo. I tried the structure smoothing method, however, it can not work for my question, maybe because of the large deformation. I wonder how large the defromaion is for your problem, small or larger? Thank you!

[mrestrepo30]

Hi,

my displacement is a 30% of the width of the model
initial diameter:1cm
max diameter: 1.30cm
min diameter: 0.7 cm
I don't know if yours is bigger than that. As I said the mesh quality is not the best, but you can try boundary layers at the displacement zones to improve the quality.

[almostafa67]

hi everybody...
hi dear all...
i faced an error message,negative cell volume detected,i think if i activate spring-on-all shapes this error does show again,but i do not know hoe to activate it in ansys 12.0,
any help will be appreciated...