[j_st1]

Hi all,

I am trying to simulate a CD nozzle exhausting into an open space to see the mach speeds and pressures (such as perfectly or over-expanded flow), however I keep getting the following console output which culminates in an error:

temperature limited to 5.000000e+03 in 1 cells on zone 3
->1.->2.->3.->4.<<<<
Stabilizing k to enhance linear solver robustness.
Stabilizing k using GMRES to enhance linear solver robustness.

Divergence detected in AMG solver: k Stabilizing epsilon to enhance linear solver robustness.
Stabilizing epsilon using GMRES to enhance linear solver robustness.

Divergence detected in AMG solver: epsilon
Divergence detected in AMG solver: k
Divergence detected in AMG solver: epsilon
Error at host: floating point exception

===============Message from the Cortex Process================================

Compute processes interrupted. Processing can be resumed.

================================================== ============================

Error: floating point exception
Error Object: ()

Error at Node 0: floating point exception

Error at Node 1: floating point exception

Error: floating point exception
Error Object: #f
Writing "| gzip -2cf > SolutionMonitor.gz"...
Writing temporary file C:\Users\Jake\AppData\Local\Temp\flntgz-102369 ...
Done.

For context, I am using walls around the CD nozzle, pressure_inlet, pressure_outlet and wall BCs with gauge total pressure = 1MPa, supersonic/initial gauge pressure = 0Pa and temperature = 300K at the inlet, and gauge pressure = 0Pa at the outlet. Operating Conditions operating pressure = 0Pa.

As I am trying to model supersonic flow I have set the solver type to density-based, and the time to steady. I am using standard k-e model with compressibility effects, energy on, ideal gas, sutherland viscosity (three coefficient method), reference values as computing from the inlet, standard initialisation from the inlet.

Method is implicit with the following:
Roe-FDS, gradient: least-squares cell based, flow: second order upwind, turbulent kinetic energy: first order upwind, turbulent dissipation rate: first order upwind.

All residuals set to 1e-05.

I have turned on solution steering to between 0.2 and 0.9 courant number, set flow type to supersonic and set high speed numerics to ON. This results in the error above.

If I turn the courant number to 5, I get the following error:

Reversed flow on 371 faces (7.6% area) of pressure-outlet 8.

Divergence detected - temporarily reducing Courant number to 0.5
and trying again...

Divergence detected - temporarily reducing Courant number to 0.05
and trying again...

Divergence detected - temporarily reducing Courant number to 0.005
and trying again...

Divergence detected - temporarily reducing Courant number to 0.0005
and trying again...
Stabilizing Coupled to enhance linear solver robustness.
Stabilizing Coupled using GMRES to enhance linear solver robustness.

Divergence detected - temporarily reducing Courant number to 5e-05
and trying again...
Stabilizing Coupled to enhance linear solver robustness.
Stabilizing Coupled using GMRES to enhance linear solver robustness.

Error at Node 0: Divergence detected in AMG solver

Error at Node 1: Divergence detected in AMG solver

I have attached a picture of my domain in Fluent as well as the mesh close up in the mesh editor.
[removed]

The mesh has ~122k cells and the mesh quality is the following:

Mesh Quality:
Minimum Orthogonal Quality = 7.04440e-01 cell 1274 on zone 3 (ID: 38689 on partition: 1) at location (-1.77623e-02, 9.39591e-03)

Maximum Aspect Ratio = 2.54219e+01 cell 48109 on zone 3 (ID: 84684 on partition: 1) at location ( 9.95108e-02, 1.05207e-02)

The main things I have played around with are the courant number (increasing to 5 and trying solution steering between 0.2 and 0.9 (based on a study I read)), and I've tried explicit and implicit solution methods, as well as first order upwind instead of second order upwind. I have tried adjusting the under-relaxation factors which didn't help (to be around 0.2) but honestly I'm not 100% sure how these work anyway.
I have also run a sim of just the CD nozzle (so no sudden expansion region) using the inviscid model and this converged successfully - I used the same BCs, so this indicates that maybe it's an issue with the sudden expansion region? Not sure how to fix this though or why this would cause an error in and of itself. When changing the model to inviscid on this current mesh it produces all the errors I'm seeing above.

I am fairly new to ANSYS modelling but I've tried to base my settings on an existing study and changing the BCs and general geometry to fit my use case. The study is called "Supersonic nozzle test using high pressure compressor" by B Gautam, if that is of any use.

Thank you for taking the time to read, and please let me know any suggestions you have that could help this sim converge rather than diverge, as it keeps on doing!