Learning Forum

[zain.qazi]

Hello. 

I am simulating a Nitrogen High Aspect Ratio Cooling Channel using the Eulerian Model with RPI boiling Model. The model was set up gradually, starting with a Mixture Model without the Semi-Mechanistic Boiling and results used to develop the Eulerian Model. The model works well until the RPI model is enabled after which there are prompts for "reverse flow on xxx faces" appears and never goes away. Prompts for "convergence difficulties.." and "stabilising temp for robustness.." also show up until divergence and floating point exception.

Boundary Conditions: mass flow inlet 0.00425 (liquid), pressure outlet 21.4 bar, constant temp outer wall 463 K. 

Physical Properties: RGP Tables from CFX.

This is a validation of an experiment and numerical case. Using a pressure inlet at 24 bar has been tried to no avail. URFs are already at 0.1 and spacial discretizations are First Order Upwind.

What is the reason for these prompts simply due to turning on the RPI model and how to approach it?

[Rob]

Have a careful look at how the model works, and I suspect you need to further resolve the mesh. Inflation is likely counterproductive due to the way the models behave. 

[zain.qazi]

Could you elaborate why inflation layers would be counterproductive? I realize the need for a higher y+ value > 30 and has been suggested by many studies using RPI models but they make no mention of adverse effects of boundary layer cells with a growth rate.

[Rob]

Inflation meshes are great when the flow doesn't change along the cell, ie for aero meshes. For multiphase an inflation mesh is just a collection of cells with very high aspect ratio.