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Old   August 14, 2007, 05:12
Default setting time step size!
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hi all! i am a learner of CFD,

could somebody please explain me if there is a standard procedure to follow to set the time step? or is it different for different types of flows such as paraboloc and hyperbolic? thanks in advance....raja
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Old   August 14, 2007, 07:11
Default Re: setting time step size!
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You could get an estimate of time step required for your model using the following simple concept-->

Approximately calculate the residence time of the fluid in the domain (eg. if you have inlet velocity of 0.1 m/s and length of domain is 1m, ~residence time = 10 s) usually can be put as (smallest length/velocity scales in the simulation)

For cold flow case --> time step could be 1/3 to 1/4 of residence time.

Reactive flow --> 1/10 of residence time. time step may need to be less than reaction timescale during start of calculation. The time step could be slowly increased as calculations proceed.

Turbo-machinery --> 1/w

Buoyancy driven flows --> max time scales which could be used

sqrt (L/g B * temp diff)

B--> coeff of thermal expansivity L--> length scales associated with vertical temp gradient g--> acceleration due to gravity

Use 1/3 to 1/5 of this value calculated

Best luck

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Old   August 14, 2007, 08:31
Default Re: setting time step size!
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thank you very much!i got it clear now!
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