# Fluctuations in lift and pitching moment coefficient values at higher angle of attack

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 December 20, 2018, 11:51 Fluctuations in lift and pitching moment coefficient values at higher angle of attack #1 New Member   Aamir Sultan Join Date: May 2018 Posts: 12 Rep Power: 7 Hello Everyone , Hope you are doing fine . I am running a 2D analysis of flow over an airfoil at Re Number=10^5 I am running a steady state , density based solver simulation of the case . At higher angle of attacks >= 10 degrees , I am getting continuous fluctuations (in the form of sinusoidal waves) for lift and pitching moment coefficient values . Any idea where am I lagging . Thanks

December 20, 2018, 12:12
#2
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,768
Rep Power: 71
Quote:
 Originally Posted by AamirSultan Hello Everyone , Hope you are doing fine . I am running a 2D analysis of flow over an airfoil at Re Number=10^5 I am running a steady state , density based solver simulation of the case . At higher angle of attacks >= 10 degrees , I am getting continuous fluctuations (in the form of sinusoidal waves) for lift and pitching moment coefficient values . Any idea where am I lagging . Thanks

Simply, this flow problem does not have a steady state in deterministic sense. Use a time-dependent solver and perform a time averaging.
In a more rigorous approach, you 2D flow problem cna have a statistically steady state that you can try to reach using the RANS equations.

December 20, 2018, 13:11
#3
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Aamir Sultan
Join Date: May 2018
Posts: 12
Rep Power: 7
Quote:
 Originally Posted by FMDenaro Simply, this flow problem does not have a steady state in deterministic sense. Use a time-dependent solver and perform a time averaging. In a more rigorous approach, you 2D flow problem cna have a statistically steady state that you can try to reach using the RANS equations.
Thank you so much for a concise and clear answer.
Actually I have to make Cl vs Alpha plot , so I am confuse that which value of lift coefficient should I use , as the values will be changing with time and I will have multiple values of lift coefficients at the end .

Another Question: I can calculate the time step size using Courant number and value of smallest cell size .
but How many iterations per time step should I choose .

I am a beginner and my questions may seem very basic to you so I am apologizing in advance .

Thanks again.