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-   -   how to get the transient ttling time (https://www.cfd-online.com/Forums/phoenics/52012-how-get-transient-ttling-time.html)

 Anjani Kr. dubey February 17, 2006 03:20

how to get the transient ttling time

Anyone pl suggest me how to get the time taken by a trasient heat trasnfer to become steady in phoenics i.e how can i know the steady state settling time in phoenics.

regards & thanks in advance

 Mick February 21, 2006 09:50

Re: how to get the transient ttling time

Use some physical judgement at first.

The transition time to steady state conditions is of course problem dependent. If you have some physical idea of what this might be then use this as a first-stab bounding condition. I dont think this is something that you can calculate easily unless the problem is extremely simple. I suspect people generally use intuition on this or alternatively dont know, so it is often a question that the CFD is being used to answer.

What a CFD code will require is a descretisation of that time into suitable time steps. If inexperienced I guess the best way to proceed might be to make a simpler copy of your case (only if slow to run) to experiment on (keep the basic design) but have a coarser cell distribution (for turnover speed). Then try this discretisation on the original q1.

Otherwise just experiment with your proper case.

Remember (generally speaking) that there is something called the CFL criterion that you might have to keep your eye on to a more or less extent depending on the type of set-up you are using.

But I think for your ConjHeatTrans case then this is not raelly so important and you should be able to get away with quite coarse time intervals.

last word is that you may wish to expand the time intervals over time geometrically, so that the time steps grow over time. This can be done with the POWER LAW button and can help towards reaching steady-state for obvious reasons.

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