Learning Forum

[Jose Amigo]

Hi,

I'm trying to model the motion of a solid object at the interface of a phase change material as it melts, as shown below.

I would assume I need the solidifcation/melting model + VOF + dynamic mesh + overset model to do this. However, the solidification model is incompatible with the overset model. Is there another way to approach this problem?

The closest problem I've seen to this is in the link below, but the difference is that they didn't need the overset model in that case.

/forum/forums/topic/dynamic-mesh-melting/

Any help is appreciated.

[Rob]

I can't see an image so am working a little blind. 

VOF is a free surface model, so may not be needed for solidification. The solidification model "freezes" flow, ie prevents it moving and also adds/removes the phase change energy. Overset is a way of adding an extra block of fixed (not changing) mesh which can also move using the various models in Fluent: a ship could be overset onto the background water. Deforming mesh would alter the shape of the mesh. 

In your case if the block of ice is moving and also melting you'd potentially need to code up phase change and then move a block of mesh (solid body motion) whilst altering the surface shape/position as the ice melted. 

 

If you can post some images I can comment. Staff are not permitted to download anything or give detailed advice. 

 

[Jose Amigo]

Hi Rob,

Thanks for the reply. Sorry, here is the picture. The sphere is solid and has the same shape during the whole simulation. I wanted to capture its motion as the PCM melts (its completely solid in the beginning).

[Rob]

No worries, the Forum has been randomly creating empty or duplicate posts for a couple of days.  Can you use 6DOF dynamic mesh & solidification & melting? Overset just avoids the remeshing step. 

[Jose Amigo]

Hi Rob,

You are right, I don't actually need overset mesh for this problem.

I tried 6DOF dynamic mesh + solidifcation & melting. To keep it simple, I'm only allowing y-translation for my cylinder. I used a udf to stop y-translation as soon as the solid wall has a cell with a liquid fraction < 1. However the mesh always crashes when the solid is getting close to the solid PCM. The picture below shows the last frame before it crashed.

I'm already using small time steps (10^-6 s), but I'm still not able to get the solid closer than this picture. Eventually, I want to add heat transfer between the solid and the PCM to see the effect on the melting process, so having them closer than this is ideal.

I've set the whole mesh of the PCM as a deforming mesh, but apparently the "solid" part of the PCM (which is still a fluid material) cannot deform. Is there a way to make it deformable to prevent the mesh crash?

Any other hint on how to approach this problem appreciated.

 

[Rob]

Please can you show the mesh? Show all edges, and overlay the contour you plotted, with node values off. The PCM isn't a solid (in the Fluent sense, it's a fluid that doesn't move) so shouldn't alter the 6DOF other than by not flowing. I suppose there is a potential issue with the 6DOF motion and the solidification model freezing the flow, but as you've told the 6DOF to stop before it hits the PCM that shouldn't be a problem: it may also not be a problem anyway. 

[Jose Amigo]

Hi Rob,

Thanks for the reply. I'm attaching 2 pictures. The first shows a time in which the mesh still has a good quality and the second one, shows the mesh just before crashing. There are 2 cell zones: one rigid zone which is next to the cylinder's wall and one deforming zone consisting in the rest of the PCM domain. I coded a udf for the 6DOF method to stop the motion of the cylinder when a cell inside the rigid zone has a liquid fraction < 1. However, this action never takes place because for some reason, the solid PCM mesh doesn't deforms and therefore the cells get squeezed as the cylinder approaches solid-liquid interface. If I run this with a completely liquid PCM, there are no issues.

Do you think there is a way to prevent this cell squeezing by tunning the remeshing setup?

[Rob]

It's the remeshing that's gone wrong - it's failed all around the sphere. Check what the methods do, and probably change from Local Cell. I know what the remeshing is capable of, but not which buttons to press.