Problem with inviscid and laminar viscous models

- April 13, 2024 at 2:13 pm
  
  N. M.
  Subscriber
  
  Hello everyone,
  
  I am attempting to simulate a scenario involving H2 injection into an environment filled with air. Unfortunately, I am unable to upload the geometry of the case.
  
  The H2 is injected at a very high velocity (950 m/s) through an 8 mm diameter hole. At the halfway point of the simulation, the H2 injection ceases. I have modeled this behavior using a User-Defined Function (UDF), which sets the inlet velocity (the inlet being the hole through which H2 enters the domain) at 950 m/s from the start until the midpoint of the simulation, and abruptly at 0 m/s from the midpoint until the end of the simulation.
  
  The simulation concludes successfully with either the SST k-omega model or the standard k-epsilon model.
  
  However, when attempting to use the laminar and inviscid models, errors occur during the simulation.
  
  Specifically, when using the inviscid model, errors occur in the early stages of the simulation. The error message is as follows:
  
  temperature limited to 1.000000e+00 in 99 cells on zone 2
  
  temperature limited to 1.000000e+00 in 5 cells on zone 2
  
  temperature limited to 1.000000e+00 in 36 cells on zone 2
  
  temperature limited to 1.000000e+00 in 41 cells on zone 2
  
  temperature limited to 5.000000e+03 in 17554 cells on zone 2
  
  temperature limited to 1.000000e+00 in 23717 cells on zone 2
  
  temperature limited to 5.000000e+03 in 2373 cells on zone 2
  
  temperature limited to 1.000000e+00 in 15 cells on zone 2
  
  temperature limited to 5.000000e+03 in 276168 cells on zone 2
  Stabilizing enthalpy to enhance linear solver robustness.
  
  temperature limited to 1.000000e+00 in 251430 cells on zone 2
  
  temperature limited to 5.000000e+03 in 35697 cells on zone 2
  Stabilizing species-0 to enhance linear solver robustness.
  Stabilizing enthalpy to enhance linear solver robustness.
  
  temperature limited to 1.000000e+00 in 22197 cells on zone 2
  
  temperature limited to 5.000000e+03 in 480596 cells on zone 2
  Stabilizing enthalpy to enhance linear solver robustness.
  
  temperature limited to 1.000000e+00 in 21931 cells on zone 2
  
  temperature limited to 5.000000e+03 in 487291 cells on zone 2
  Stabilizing species-0 to enhance linear solver robustness.
  Stabilizing enthalpy to enhance linear solver robustness.
  Stabilizing enthalpy using GMRES to enhance linear solver robustness.
  
  Divergence detected in AMG solver: enthalpy
  temperature limited to 1.000000e+00 in 21931 cells on zone 2
  
  temperature limited to 5.000000e+03 in 487291 cells on zone 2
  
  Divergence detected in AMG solver: enthalpy
  Divergence detected in AMG solver: enthalpy
  Divergence detected in AMG solver: enthalpy
  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 2: floating point exception
  
  Error at Node 3: floating point exception
  
  Error at Node 1: floating point exception
  
  Error at Node 0: floating point exception
  
  Error: floating point exception
  Error Object: #f
  
  When utilizing the laminar model instead, the simulation progresses until halfway (until the exit of H2 ceases). When the exit velocity of H2 reaches 0 m/s, the residuals start to diverge, as depicted in the figure, ultimately resulting in an error:
  
  Reversed flow on 151 faces (38.1% area) of pressure-outlet 5.
  temperature limited to 5.000000e+03 in 91 cells on zone 2 in domain 1
  temperature limited to 5.000000e+03 on 17 faces
  temperature limited to 1.000000e+00 in 43358 cells on zone 2 in domain 1
  temperature limited to 5.000000e+03 in 12982 cells on zone 2 in domain 1
  temperature limited to 1.000000e+00 on 3665 faces
  temperature limited to 5.000000e+03 on 1191 faces
  Stabilizing species-0 to enhance linear solver robustness.
  Stabilizing temperature to enhance linear solver robustness.
  temperature limited to 1.000000e+00 in 368355 cells on zone 2 in domain 1
  temperature limited to 5.000000e+03 in 141988 cells on zone 2 in domain 1
  temperature limited to 1.000000e+00 on 12614 faces
  temperature limited to 5.000000e+03 on 12268 faces
  Stabilizing x-momentum to enhance linear solver robustness.
  Stabilizing y-momentum to enhance linear solver robustness.
  Stabilizing z-momentum to enhance linear solver robustness.
  Stabilizing species-0 to enhance linear solver robustness.
  Stabilizing temperature to enhance linear solver robustness.
  Stabilizing temperature using GMRES to enhance linear solver robustness.
  
  Divergence detected in AMG solver: temperature temperature limited to 1.000000e+00 on 16893 faces
  temperature limited to 5.000000e+03 on 20929 faces
  
  Divergence detected in AMG solver: temperature
  Divergence detected in AMG solver: temperature
  Divergence detected in AMG solver: temperature
  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 3: floating point exception
  
  Error at Node 1: floating point exception
  
  Error at Node 2: floating point exception
  
  Error: floating point exception
  Error Object: #f
  The time step is set to 0.0001. When attempting a time step of 0.00001 with the inviscid model, the simulation advances for a few additional steps. However, the error persists despite this adjustment.
  
  What could be the issue in a case like this?
  
  Thank you ????.
- April 15, 2024 at 10:30 am
  
  Rob
  Forum Moderator
  
  At 950m/s I suspect it's turbulent! The various turbulence models add additional terms into some (all?) of the equations, and these can be more stable than invisid or laminar flows as gradients tend to be marginally smoother. If you've got reactions on, how would the chemistry differ?