# Lift and Drag Coefficient graph is Oscilating is it Ok?!

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June 10, 2019, 04:59
Lift and Drag Coefficient graph is Oscilating is it Ok?!
#1
Member

Os
Join Date: Jun 2017
Posts: 49
Rep Power: 4
Hello Foamer ,
I am making 2D airfoil Simualtion using openFoam (pimpleFoam)
and after i run my simuation i got this graph for both drag and lift and as you can see the graph is not that smooth, is this normal in transient simulations (AOA14.04 °) ? and how can i process the graph to get just one value since it's fluctuating ?!

Many Thanks
Attached Images
 Screenshot from 2019-06-10 09-38-08.jpg (53.7 KB, 20 views)

 June 11, 2019, 03:11 #2 Senior Member   Join Date: Jan 2014 Posts: 157 Rep Power: 7 At such an high angle of attack separation appears and you reach the non-linear regime -> see lift over AoA curve. Latter one results in fluctuating forces and moments. So yes, your simulation is physical. At this regime phyiscal results can be only resolved by a proper chosen mesh and turbulence modelling. Even higher turbulence models as DES are sometimes not capable of capturing the right separation point and the wake behaviour. Anyways, two-equations models do resolve this phenomena in terms of integral vaues quite well. So choose for comparison besides the integral forces also the Strouhal Number and so on. Next, think about your transition point. I guess your setup is calculated fully turbulent. You can argue that in this stage, the flow is already deattached and the assumption of fully turbulent flow is valid. If you want to go more into detail you should read some literature and or do some simulations using transition models or fixed transition points. Hope this helps you.

June 15, 2019, 06:26
#3
Member

Os
Join Date: Jun 2017
Posts: 49
Rep Power: 4
Hello ,
Thank you for the reply ,
because i thought that my simulation is not correct since the force coefficient where like Fluctuating and i found that it is normal , so to calculate the Force Coefficient we just need to make sur that the simulation has converged ( we gave it enough time ) and than we calculate the average for the values we get ( for the last 0.5s of 1 second ) depending on the time we gave for the simulation .

But i still got one small question about transient simulation ( M beginner ) i ve change a bit the mesh and once i Plot those values for Cm, Cd ,Cl i got the following graph .

Do you think That the graph is Okk and that it's not representing any Unphysical variations since it's converged to a fixed value at the end as you can see ?

The reason for my question is because once i conduct a steday state simulation with AOA 0 degree i got the following graph (2) ,and for some reason that graphh (2 ) got stuck in my head as what the Cm , Cd,Cl graph should look like in a transient simulation (AOA=14.04)

PS:
*-My simulation is 2D airfoil simualtion

*-Re=700.000
*-turbulence model =KOmegaSST
*-resolving using wall Function

*-Y+>30

Many Thanks
Attached Images
 Screenshot from 2019-06-15 11-18-44.jpg (51.1 KB, 8 views) Screenshot from 2019-06-15 11-25-01.jpg (47.6 KB, 8 views)