Covergence with trasnient but not with steady in ansys fluent

sanjaykummar2810 · November 6, 2018, 04:22

Hello all
I am doing heat transfer probelm in ANSYS Fluent, where im facing problem with convergence in steady state, but with transient the simulation is converging. I have orthogonal quality 0.20 , skewness 0.75.

Probelm statement :
Hot air is flowing in a duct of 10m long and thickness of 10mm. I know the thermal conductivity of the duct. I need to find the temperature distribution on the inner side of the duct.

1.why the simulation is converging with transient not with steady state?
2. Can i take the last time step results in transient as final result or should i take avg of all the timesteps for final result?

Can anyone please help me

Thanks in advance

RaiderDoctor · November 8, 2018, 21:37

Hey Sanja,
It's a little difficult to pinpoint what about your setup isn't working right. Could you please provide more info and pictures? Basically, take all of the guesswork out of it on our part, and we'll be able to help you out more efficiently! Until then, I don't think I can provide a good answer for question 1.
Question 2 on the other hand, that's kind of simple. If your solution is is independent of time, or has effectively reached steady-state where no quantities change between time steps, then you can take the last time step. Otherwise, it's not an example of a steady-state solution. Taking the average of all time steps is not the same as reaching steady-state. Remember, steady-state means that values do not change with respect to time.

Hope this helps! Good luck.
RD

Time4Tea · November 9, 2018, 10:16

To add to what RaiderDoctor said, if the transient solution is changing between time steps and is not 'settling down', then that may be an indication that there actually is a transient aspect to the underlying fluid behavior, which may be why the steady solution is having trouble converging. Possibly some sort of instability that is being triggered by the flow?

November 6, 2018, 04:22	Covergence with trasnient but not with steady in ansys fluent	#1
sanjaykummar2810 New Member sanjay Join Date: Jun 2018 Posts: 12 Rep Power: 8	Hello all I am doing heat transfer probelm in ANSYS Fluent, where im facing problem with convergence in steady state, but with transient the simulation is converging. I have orthogonal quality 0.20 , skewness 0.75. Probelm statement : Hot air is flowing in a duct of 10m long and thickness of 10mm. I know the thermal conductivity of the duct. I need to find the temperature distribution on the inner side of the duct. 1.why the simulation is converging with transient not with steady state? 2. Can i take the last time step results in transient as final result or should i take avg of all the timesteps for final result? Can anyone please help me Thanks in advance

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November 8, 2018, 21:37		#2
RaiderDoctor Senior Member Join Date: Dec 2016 Posts: 152 Rep Power: 10	Hey Sanja, It's a little difficult to pinpoint what about your setup isn't working right. Could you please provide more info and pictures? Basically, take all of the guesswork out of it on our part, and we'll be able to help you out more efficiently! Until then, I don't think I can provide a good answer for question 1. Question 2 on the other hand, that's kind of simple. If your solution is is independent of time, or has effectively reached steady-state where no quantities change between time steps, then you can take the last time step. Otherwise, it's not an example of a steady-state solution. Taking the average of all time steps is not the same as reaching steady-state. Remember, steady-state means that values do not change with respect to time. Hope this helps! Good luck. RD

November 9, 2018, 10:16		#3
Time4Tea Senior Member Lee Strobel Join Date: Jun 2016 Posts: 133 Rep Power: 10	To add to what RaiderDoctor said, if the transient solution is changing between time steps and is not 'settling down', then that may be an indication that there actually is a transient aspect to the underlying fluid behavior, which may be why the steady solution is having trouble converging. Possibly some sort of instability that is being triggered by the flow?