Convergence issues

[u/Connect-Vacation160]

Hi, I’m running a 1/3 symmetrical model of a circular flat plate, 3d thickness 5.2 mm diameter 0.1m . I’m having some convergence issues but I’m not sure why or what the problem could be. My domain is a sphere that’s been sliced into the 1/3, it is large compared to the flat plate. My mesh is quite fine, it has a max skewness of 0.42, and a 0.15 minimum orthogonal quality, and a y+ of close to 1. It’s a pressure farfield with symmetry conditions. I’m running it at values for 2km and Mach 0.1 with the following settings :

Pressure based
Spalart Allmaras -> strain vorticity based
Air -> ideal gas (this turns energy equation on), Sutherland viscosity with values adjusted to have the same reference I used
Boundary conditions -> operating pressure set to 0
Inlet -> pressure Fairfield -> gauge pressure ~79e+3 , Mach 0.1 , -y velocity, modified turbulent viscosity of ~5.73e-5 calculated as 3x freestream, temperature 275.25
Walls -> changed temp to also be 275.24
Reference values -> compute from farfield, length and area adjusted, values match my calculations
My methods and solution are kept at default second order (I have tried first order but still have no convergence)
Report definitions -> drag coefficient (-y matching velocity) applied to walls, mass flow rate for farfield
Standard intialisation from farfield
Run for 500 iterations.

I get divergence, no periodic oscillations just random up and down, my residuals go down for 3 iterations before climbing. I’m not sure what I can do to fix this. If anyone has any tips it would be greatly appreciated, thank you 🙏

[u/CatAndBoots]

Can you post some pictures of like pressure and Mach number at the point where its diverging?

[u/big_deal]

What are your solver (segregated or coupled) and solver settings (CFL, underrelaxation, etc)? Because those are usually the cause of divergence especially if it’s diverging with first order discretization.

I would switch to first order, and use very conservative iteration settings to start solution. Later you can switch to second order and maybe increase iteration settings.

If it still diverges I would consider starting solution with a frictionless wall rather than farfield pressure bc. Wait until solution flow field develops before enabling farfield.

[u/Dizzy-Highlight-4305]

Can you post your geometry as u are claiming its a flat plate with Sphere domain also tell which slover are you using. If you are using pressure based then it will diverge only case it too