Learning Forum

[Kunal Chopra]

I am trying to simulate a piston pushing water inside a chamber without any air gaps with the wall. It has to be sealed i.e. piston should be in contact with chamber walls. How do I do it through dynamic meshing. I am first trying it in 2D with a very simple Define CG_MOTION and providing it a velocity. My interior and cylinder walls are both deforming while piston (Upper Wall) is rigid body. However, after a few time steps, I am getting Negative Cell Volume detected and dynamic mesh fails. How to solve that?

[SRP]

Hi,

Dynamic meshing requires an appropriate time step size. A time step that is too large can lead to large deformations that cause negative cell volumes.

You can refer to fluent user's guide to repair neagtive volume: 5.1. Improving the Mesh (ansys.com)

Thank you.

[Kunal Chopra]

Hi, thankyou for the reply.

My step size is of the order of 1e-6 with smallest mesh size of 1mm and velocity being 0.5m/s and still I am getting the error.

Also, I am not able to get to the link you provided as I am using a student license and do not have a customer key. Are there any other open resources for the same?

Thankyou.

 

[SRP]

Hi,

If you are not able to access the link, please refer to this forum discussion;Using Help with links (ansys.com)

Thank you.

[Kunal Chopra]

When I am modelling the piston , I need it to move in a sealed chamber. I am using a dynamic mesh, my piston is 90mm whereas the whole chamber is just 500mm, so if I model it very close to wall with just 4-5 mm gap, I am facing the issue of negative dynamic mesh volume. To resolve that I am now moving the top wall considering it as piston. This works well in 2D but in 3D, it is causing the negative cell volume problem which I think is due to the problematic area of chamber present in my geometry.

Also, even when gravity is turned off, and I have provided the velocity in downward direction of 0.001 m/s, just after 3-4 tie steps of 0.00001 sec, the piston's CoG reaches the velocities of >10,000 m/s causing the mesh update problem.

 

How do I resolve it and make the piston move only at the specified velocity without causing much damage to the mesh.