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Divergence detected in amg solver

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Old   August 24, 2022, 12:37
Default Divergence detected in amg solver
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Hammad Naseem
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Hello Guys,
I am trying to run an air flow simulation of a mask assembly on FLUENT. In the simulation i am defining the input and the output as shown in the attached picture. The geometry is successfully meshing however, when I am running the simulation, I am getting an error after the Hybrid Initialization. The error is related to the "Divergence detected in AMG Solver", picture attached. The geometry is of 3 parts, the middle part is the fan enclosure which I am setting at 5600 rpm in "CELL ZONE CONDITION > MESH MOTION"

Also note that in my previous simulation, I did not have have the cylindrical enclosure as seen in the figure at the geometry bottom, there I was successfully getting the simulation done. Same parameters were used for both the cases (setup parameters are below).

Can anyone please suggest why exactly I am getting this divergence error and what can I do to rectify it?

FLUENT Parameters:
-Transient, Pressure based, Absolute
-Method = K-epsilon, std
-Boundary Conditions => inlet = 5m/s and outlet = 0 Pa
-Method = SIMPL
-Intialization = hybrid
Calculation => time step = 0.005 sec
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Old   August 24, 2022, 18:04
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Alexander
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isn't 0.005 sec is huge value for 5600 rpm?

1e-5 maybe better, may be even smaller?
in case you prepared you mesh correctly (all interfaces are defined well) mesh quality could be an issue. Based on experience 7mln mesh is too huge usually

You may try standard initialization from inlet
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Old   August 25, 2022, 03:57
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Hammad Naseem
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Quote:
Originally Posted by AlexanderZ View Post
isn't 0.005 sec is huge value for 5600 rpm?

1e-5 maybe better, may be even smaller?
in case you prepared you mesh correctly (all interfaces are defined well) mesh quality could be an issue. Based on experience 7mln mesh is too huge usually

You may try standard initialization from inlet
Thanks for the response.

However, please note that in the previous part (without the bottom cavity) I also gave the same RPM and time steps but there I was getting a simulation result.

Also now I have reduced the mesh nodes to 5 mln and have even set a lower rpm (=500) with smaller time step (1e-6) but still I am getting the same error of divergence
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