physical time scale calculation!!!

mohammad · March 24, 2011, 00:07

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.

March 24, 2011, 00:07	physical time scale calculation!!!	#1
mohammad Senior Member mohammad Join Date: Dec 2010 Location: UK Posts: 246 Rep Power: 17	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

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March 24, 2011, 21:31		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	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.