Learning Forum

[Simon]

Hello,

I am simulatinng a water entry problem, i.e., a sphere entering a air/water interface. Volume of fluid method is used to model the flow of air and water, and overset mesh (dynamic mesh) and 6DOF are used to model the motion of the sphere. However, the simulation stopped at the first few time steps because of negative cell volume detected or floating point exception.

Here are the residuals (Figuire 1) and the initial (Figure 2) and final (Figuire 3) contours of volume fraction. When the simulation stopped, it seems that the volume fraction of water in overset mesh is not correct, not calculated from the backgrouond mesh (Figure 3).

How can I troubleshoot this issue?

 

[Federico]

Hello, 

how did you set up your Dynamic Mesh? Negative Volume results from collapsed cells that were not allowed to accomodate mesh motion.

[Simon]

Hello Federico,

I tried different ways, like just selecting smoothing, selecting both smoothing and remeshing, or unchecking smoothing, layering and smoothing. 

Remeshing parameters were set based on the mesh scale info. If unchecking the three methods, does it mean using overset mesh, not dynamic mesh?

[Federico]

Which version of Fluent are you using?

Have you tried Unified Remeshing, instead of Methods based remeshing?

[Simon]

I used Ansys 2023R2 and 2022R1. I haven't tried unified remeshing. I will try it.

[Federico]

Additionally, negative cell volume error may be due to distance moved per time step may be greater than the edge or cell size in the dynamic region.

You can avoid this problem by giving correct time step size. 

[Simon]

Adaptive time was used, and the time step was set between 1E-8 and 1E-12 with the initial time step of 1E-10. The sphere and overset mesh have an initial -Y velocity of 2 m/s though, set by 6DOF.