Low Reynolds NACA 0012 validation

35matt215 · March 5, 2019, 16:37

Hi everyone,

I am modelling a NACA 0012 at 1.15 m/s, chord length = 1 m, 3 deg AoA, through air at 20 deg c and Re = 7000.

Using the SST turbulence model, I have obtained values for lift and drag coefficient being approx Cl = 0.13 and Cd = 0.015.

I am struggling to find validation for such a low Reynolds number, so I was wondering: would anyone be able to inform me if these values are accurate for this low Reynolds number?

Thank you in advance!

ghorrocks · March 5, 2019, 18:19

Didn't I post a link to a massive database of results for low Re airfoils? Why are you asking again?

If you can't find your exact airfoil in the database then just choose one which is close and model that as a validation.

By the way - I suspect that the airfoil at this Re is not turbulent. It will be either laminar, or transitions to turbulence mid-chord. That means you should be using a laminar flow model, or maybe SST with the turbulence transition model.

35matt215 · March 5, 2019, 18:40

Thank you for your reply.

Yes you did post that but I could not find the exact one and was not sure but now it is clear, so thank you for clarification.

I did try to run it as laminar but the Cl and Cd were not stable and RMS residuals did not converge and it seemed to converge quite well with SST. I have also tried the Gamma Theta SST model but it also did not converge too well however, I will play around with that further.

Thank you again.

ghorrocks · March 5, 2019, 18:43

You select the physics models based on what the physics actually are, not what converges easily.

35matt215 · March 17, 2019, 08:58

I ran transitional turbulence model and seem to be getting quite good results when compared to published data.

I noticed there seems to be a laminar separation bubble that occurs near the trailing edge of the lower surface of the airfoil. However from reading papers, it seems that the separation bubble normally occurs at the top surface of the airfoil.

Can anyone maybe offer an explanation as to why I am getting this separation bubble on the bottom surface of the airfoil?

Thanks.

sheaker · March 17, 2019, 09:17

Quote:

Originally Posted by 35matt215

I ran transitional turbulence model and seem to be getting quite good results when compared to published data.

I noticed there seems to be a laminar separation bubble that occurs near the trailing edge of the lower surface of the airfoil. However from reading papers, it seems that the separation bubble normally occurs at the top surface of the airfoil.

Can anyone maybe offer an explanation as to why I am getting this separation bubble on the bottom surface of the airfoil?

Thanks.

Dear Matt
Could You possibly share the case?

EDIT:
I am sorry. I thought it was an OPENFoam sub-forum.

35matt215 · March 17, 2019, 09:30

Please see attached the images of this.

ghorrocks · March 17, 2019, 17:55

It is because you are using a low angle of attack. Increase the AOA and the separation bubble will move to the top face.

35matt215 · March 17, 2019, 18:31

Thanks for replying.

I increased the AOA and it occurred on the top surface as you said.

Is this definitely a separation bubble and not recirculation? Is it okay if this 'bubble' occurs on the bottom surface?

Thank you.

ghorrocks · March 17, 2019, 23:12

Separation bubbles and recirculations are the same thing. Just laminar separation bubbles are small, linked to the transition from laminar to turbulent flow and the flow generally reattaches afterwards. Recirculations are typically larger features and often do not re-attach but stay separated for the entire chord.

But there is no clear distinction between the two, large separation bubbles and small recirculations are the same thing.

March 5, 2019, 16:37	Low Reynolds NACA 0012 validation	#1
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	Hi everyone, I am modelling a NACA 0012 at 1.15 m/s, chord length = 1 m, 3 deg AoA, through air at 20 deg c and Re = 7000. Using the SST turbulence model, I have obtained values for lift and drag coefficient being approx Cl = 0.13 and Cd = 0.015. I am struggling to find validation for such a low Reynolds number, so I was wondering: would anyone be able to inform me if these values are accurate for this low Reynolds number? Thank you in advance! Last edited by 35matt215; March 25, 2019 at 17:56.

March 5, 2019, 18:19		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Didn't I post a link to a massive database of results for low Re airfoils? Why are you asking again? If you can't find your exact airfoil in the database then just choose one which is close and model that as a validation. By the way - I suspect that the airfoil at this Re is not turbulent. It will be either laminar, or transitions to turbulence mid-chord. That means you should be using a laminar flow model, or maybe SST with the turbulence transition model. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 5, 2019, 18:43		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	You select the physics models based on what the physics actually are, not what converges easily. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 17, 2019, 09:30		#7
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	Please see attached the images of this. Last edited by 35matt215; March 25, 2019 at 17:57.

March 17, 2019, 17:55		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	It is because you are using a low angle of attack. Increase the AOA and the separation bubble will move to the top face. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 5, 2019, 18:40		#3
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	Thank you for your reply. Yes you did post that but I could not find the exact one and was not sure but now it is clear, so thank you for clarification. I did try to run it as laminar but the Cl and Cd were not stable and RMS residuals did not converge and it seemed to converge quite well with SST. I have also tried the Gamma Theta SST model but it also did not converge too well however, I will play around with that further. Thank you again.

March 17, 2019, 08:58		#5
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	I ran transitional turbulence model and seem to be getting quite good results when compared to published data. I noticed there seems to be a laminar separation bubble that occurs near the trailing edge of the lower surface of the airfoil. However from reading papers, it seems that the separation bubble normally occurs at the top surface of the airfoil. Can anyone maybe offer an explanation as to why I am getting this separation bubble on the bottom surface of the airfoil? Thanks.

March 17, 2019, 18:31		#9
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	Thanks for replying. I increased the AOA and it occurred on the top surface as you said. Is this definitely a separation bubble and not recirculation? Is it okay if this 'bubble' occurs on the bottom surface? Thank you.

March 17, 2019, 23:12		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Separation bubbles and recirculations are the same thing. Just laminar separation bubbles are small, linked to the transition from laminar to turbulent flow and the flow generally reattaches afterwards. Recirculations are typically larger features and often do not re-attach but stay separated for the entire chord. But there is no clear distinction between the two, large separation bubbles and small recirculations are the same thing. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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