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-   -   Best CFX settings for supersonic free expansion [CFX] ? (https://www.cfd-online.com/Forums/cfx/106486-best-cfx-settings-supersonic-free-expansion-cfx.html)

 nav_5 August 30, 2012 06:17

Best CFX settings for supersonic free expansion [CFX] ?

Hello there!
The objective of my simulation is to reproduce a free expansion in a vacuum vessel. The flow reaches, before 1s, supersonic velocity. I used adaptative timestep and I set the solver control as follows:

TIME STEPS:
First Update Time = 0.0 [s]
Initial Timestep = 1e-06 [s]
Timestep Update Frequency = 1
Courant Number = 5
Decreasing Relaxation Factor = 1
Increasing Relaxation Factor = 0.75
Maximum Timestep = 0.0001 [s]
Minimum Timestep = 1e-06 [s]
Option = RMS Courant Number
SOLVER CONTROL:
Turbulence Numerics = High Resolution
Blend Factor = 0
Option = Specified Blend Factor
END
COMPRESSIBILITY CONTROL:
High Speed Numerics = On
END
CONVERGENCE CONTROL:
Maximum Number of Coefficient Loops = 20
Minimum Number of Coefficient Loops = 1
Timescale Control = Coefficient Loops
END
CONVERGENCE CRITERIA:
Conservation Target = 1e-06
Residual Target = 1e-06
Residual Type = RMS
END
TRANSIENT SCHEME:
Option = Second Order Backward Euler
TIMESTEP INITIALISATION:
Option = Automatic
END
END
VELOCITY PRESSURE COUPLING:
Rhie Chow Option = High Resolution
END

The simulation seems to be fine but after a certain time the residual begin to high. I don't undestand why there are this oscillations and why the convergerce criteria is not achieved. Is there any setting most suitable for my case? Why if I load another finer mesh residuals do not fall?

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