Help needed with 2D Cl AoA results

Avraham · December 13, 2020, 23:14

Hi everybody, I am new to this site and this is my first post here.

I have been doing the Cornell University Tutorials regarding 2D Fluent steady state turbulance analysis of a NACA airfoil and using the Ansys student workbench package.

All settings are as per tutorial; Mesh quality, Convergence quality, Velocity, input and output expressions etc...

My question is related to the Cl readings...

As my AoA is increased beyond 10degree the Cl reading keeps increasing when it should actually be decreasing... Can somebody please tell me what am I missing here?!

The Velocity_Inlet direction is fixed in X direction and I am augmenting the AoA of the airfoil via design point table input.

The output data for Lift Drag Cl Cd and moment are aligned with NASA results, however as the AoA is increased I am not seeing any falloff of the Cl?! What could be the cause of this?

Appreciate any help here, thanks....

LoGaL · December 17, 2020, 13:56

which turbulence model did you choose?

Avraham · December 18, 2020, 21:45

Thanks for the response!

I have tried a few turbulence models - K-epsilon, K-omega, Spalart Allmaras (SST)... they all converge and provide similar results; Spalart Allmaras demonstrates boundary layer separation best, and my Y+ is currently at 35... None of the turbulence models demonstrate a fall-off of Cl with increased AoA.. I am thinking to try meshing in ICEM to get the Y+ to below 1

LoGaL · December 20, 2020, 05:42

So wait, do you have tetra grid with Y+=30 around the airfoil? If you want to predict stall, that is garbage, if you ask me
Are you at least using inflation layers? If yes, why dont' you set the first layer height to get y+ = 1 and use K-omega SST? And, by the way, since you speak about NASA results, i presume this is a standard naca airfoil, so you can fit a lot of inflation layers around the airfoil (30-40)

And Spalart Almaras SST does not exist man, either it is Spalart Almaras or K-omega SST. The former is bad to predict stall, the latter requires Y+ = 1. Anyhow my call is to improve your mesh, I can confirm it if you posyt a picture of it

Avraham · December 25, 2020, 02:59

Thanks for the feedback LoGal... Yes, was K-Omega SST and Spalart Almaras... I will follow your advice and use inflation layer correctly at Y+1 and see what happens and post the results... Should I leave Energy option checked or unchecked for this kind of analysis? ��

LoGaL · December 25, 2020, 05:29

Hi Avraham, depends if your flow is compressible or not, which in turn depends on the mach number of the flow. Just leave it on anyways, it won't harm
BTw, you are not showing me a pic of the mesh

December 13, 2020, 23:14	Help needed with 2D Cl AoA results	#1
Avraham New Member Join Date: Dec 2020 Posts: 3 Rep Power: 5	Hi everybody, I am new to this site and this is my first post here. I have been doing the Cornell University Tutorials regarding 2D Fluent steady state turbulance analysis of a NACA airfoil and using the Ansys student workbench package. All settings are as per tutorial; Mesh quality, Convergence quality, Velocity, input and output expressions etc... My question is related to the Cl readings... As my AoA is increased beyond 10degree the Cl reading keeps increasing when it should actually be decreasing... Can somebody please tell me what am I missing here?! The Velocity_Inlet direction is fixed in X direction and I am augmenting the AoA of the airfoil via design point table input. The output data for Lift Drag Cl Cd and moment are aligned with NASA results, however as the AoA is increased I am not seeing any falloff of the Cl?! What could be the cause of this? Appreciate any help here, thanks....

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December 17, 2020, 13:56		#2
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12	which turbulence model did you choose?

December 18, 2020, 21:45		#3
Avraham New Member Join Date: Dec 2020 Posts: 3 Rep Power: 5	Thanks for the response! I have tried a few turbulence models - K-epsilon, K-omega, Spalart Allmaras (SST)... they all converge and provide similar results; Spalart Allmaras demonstrates boundary layer separation best, and my Y+ is currently at 35... None of the turbulence models demonstrate a fall-off of Cl with increased AoA.. I am thinking to try meshing in ICEM to get the Y+ to below 1

December 20, 2020, 05:42		#4
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12	So wait, do you have tetra grid with Y+=30 around the airfoil? If you want to predict stall, that is garbage, if you ask me Are you at least using inflation layers? If yes, why dont' you set the first layer height to get y+ = 1 and use K-omega SST? And, by the way, since you speak about NASA results, i presume this is a standard naca airfoil, so you can fit a lot of inflation layers around the airfoil (30-40) And Spalart Almaras SST does not exist man, either it is Spalart Almaras or K-omega SST. The former is bad to predict stall, the latter requires Y+ = 1. Anyhow my call is to improve your mesh, I can confirm it if you posyt a picture of it

December 25, 2020, 02:59		#5
Avraham New Member Join Date: Dec 2020 Posts: 3 Rep Power: 5	Thanks for the feedback LoGal... Yes, was K-Omega SST and Spalart Almaras... I will follow your advice and use inflation layer correctly at Y+1 and see what happens and post the results... Should I leave Energy option checked or unchecked for this kind of analysis? ��

December 25, 2020, 05:29		#6
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12	Hi Avraham, depends if your flow is compressible or not, which in turn depends on the mach number of the flow. Just leave it on anyways, it won't harm BTw, you are not showing me a pic of the mesh