steady or unsteady is better?

thanos · February 22, 2010, 07:01

Hello to all!

To simulate stall in an airfoil with great accuracy what would give better results? A steady or unsteady solver?

Thanks in advance.

Bernhard · February 22, 2010, 07:39

If you perform a steady simulation, you will not be able to find any periodic solutions. If this periodicity occurs in your situation, it may become difficult to get a convergent solution with certain discretization schemes. Transient simulations come with a price of course.

thanos · February 22, 2010, 16:06

The problem i have with steady solution is that there is no stall, the Cl continues to rise! So, i was wandering if an unsteady solver could correctly calculate it.

Goldsstean · February 23, 2010, 01:55

Solve with unsteady first and then move to steady, it will give better convergence.

thanos · February 23, 2010, 11:55

what time step should i use and how may iterations per time step?

Goldsstean · February 24, 2010, 00:12

del t = del x/v

del t= time step
del x= smallest mesh size
v= velocity

or just put time step 0.001 and no. of time step (maximum) and 20 iteration per time step

thanos · February 24, 2010, 05:32

the smallest mesh size is the distance between the wall and the first cell adjacent to the wall or i understood wrong?

Goldsstean · February 24, 2010, 05:42

Smallest cell inside the domain.

Converge please! · February 24, 2010, 05:51

Quote:

Originally Posted by thanos

what time step should i use and how may iterations per time step?

A general rule for time stepping is:

Timestep = (1/10) * ((average cell size) / (average velocity))

February 22, 2010, 07:01	steady or unsteady is better?	#1
thanos Member thanos Join Date: Oct 2009 Posts: 30 Rep Power: 16	Hello to all! To simulate stall in an airfoil with great accuracy what would give better results? A steady or unsteady solver? Thanks in advance.

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February 22, 2010, 07:39		#2
Bernhard Senior Member Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 21	If you perform a steady simulation, you will not be able to find any periodic solutions. If this periodicity occurs in your situation, it may become difficult to get a convergent solution with certain discretization schemes. Transient simulations come with a price of course.

February 22, 2010, 16:06		#3
thanos Member thanos Join Date: Oct 2009 Posts: 30 Rep Power: 16	The problem i have with steady solution is that there is no stall, the Cl continues to rise! So, i was wandering if an unsteady solver could correctly calculate it.

February 23, 2010, 01:55		#4
Goldsstean Member Amol Ramteke Join Date: May 2009 Location: Kolkata Posts: 30 Rep Power: 16	Solve with unsteady first and then move to steady, it will give better convergence.

February 23, 2010, 11:55		#5
thanos Member thanos Join Date: Oct 2009 Posts: 30 Rep Power: 16	what time step should i use and how may iterations per time step?

February 24, 2010, 00:12		#6
Goldsstean Member Amol Ramteke Join Date: May 2009 Location: Kolkata Posts: 30 Rep Power: 16	del t = del x/v del t= time step del x= smallest mesh size v= velocity or just put time step 0.001 and no. of time step (maximum) and 20 iteration per time step

February 24, 2010, 05:32		#7
thanos Member thanos Join Date: Oct 2009 Posts: 30 Rep Power: 16	the smallest mesh size is the distance between the wall and the first cell adjacent to the wall or i understood wrong?

February 24, 2010, 05:42		#8
Goldsstean Member Amol Ramteke Join Date: May 2009 Location: Kolkata Posts: 30 Rep Power: 16	Smallest cell inside the domain.