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-   -   physical time scale calculation!!! (http://www.cfd-online.com/Forums/cfx/86489-physical-time-scale-calculation.html)

mohammad March 24, 2011 00:07

physical time scale calculation!!!
 
Hi every body
I'm running a NREL wind turbine with 72 rpm in a stationary domain(domain length=70m) around a rotary one which the blades are located in rotary.

I'm getting confused with to different things.
1- in "physical time scale " help text you will see this expression:
\Delta t= L/U for external flow ,L should be based upon a characteristic geometric length (e.g., chord length) .

2- in the FAQ part ,about convergence of steady solutions it is just recommended to to use " average residence time".

I'm confused because the second "tip" completely depends on the dimension of the domain while the first method is based on chord length.

Which one is true???

Regards

ghorrocks March 24, 2011 21:31

It does not really matter. For a steady state run these just guess a time step size. For most simulations anything even vaguely close will do fine. You can be off by a factor of 100 and it still converges just the same.

So, as per my previous post on this topic, just start with any guessed time scale then adjust it as the simulation progresses to make it as large as possible without causing convergence problems.


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