Learning Forum

[afroditi.chalkopoulou]

I am simulating a 2D closed cavity with a moving wall (similar to a piston) using ANSYS Fluent (2023 R2) and dynamic mesh with layering. The mesh is structured (100 × 10 cells), cavity length 51 mm (and height 3.81 mm - axisymmetric, transient and pressure-based solver). The piston is the right wall, moved using DEFINE_CG_MOTION, while the fluid cell zone is deforming. Top, bottom, and left walls are stationary (I included top and bottom in the dynamic mesh zones as deforming instead of fluid_region, which seems to have worked correctly).

Behavior:

• When viscous model = Inviscid, the simulation runs without crashing (pressure decreases as expected for expansion & temperature and density barely change due to low Mach number).

• When switching to viscous model = Laminar, the solver crashes immediately with a floating-point exception / AMG divergence, even when:

  • piston velocity = 0 (no mesh motion),

  • walls are tested with both slip and no-slip,

  • very small time steps are used

What could cause laminar + ideal gas + energy to crash immediately in a static or quasi-static case when inviscid works, and what should be checked regarding the parameters, mesh or something else? Could it be the dynamic mesh motion?

[Rob]

Possibly, what conditions did you initialise the domain at? This should be in the Fluids channel. 

[afroditi.chalkopoulou]

I initialized the domain using standard initialization.
Initial conditions are:

• Velocity = 0 m/s

• Temperature = 300 K

• Gauge pressure = 0 Pa

• Material: air (density set to ideal gas)

• Energy equation on

Operating pressure is set to 101325 Pa.

I have also tested piston velocity = 0 m/s (static case), and the laminar model still crashes immediately, while the inviscid model runs.

[Rob]

Initialise with a very low velocity, eg 0.0001 m/s The above should work, but if everything is zero you risk a numerical oddity in the equations. I assume the mesh is OK?

[afroditi.chalkopoulou]

Yes, according to mesh metrics the mesh quality is really good. 

Even with the addition of the very low velocity it keeps breaking immediately. 

[Rob]

Please can you post some images? Mesh, materials settings as a start. 

[afroditi.chalkopoulou]

Let me know if you need something more specific

[Rob]

Try turning off the MDM and see what happens. When trouble shooting we turn things off until it works and turn things back on in a different order until it breaks. Then focus on whatever that was.