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

Raja,

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

thank you very much!i got it clear now!