NACA 0012 low speed validation problem

35matt215 · March 4, 2019, 15:46

Hi everyone,

I am modelling a NACA 0012 airfoil using CFX at velocity 1.15 m/s, 1.55 deg AoA in air at 20 deg C (Re = 76104), but I am having issues validating the drag coefficient at this velocity as there seems to be about a 35% error when compared to NASA validation.

The lift coefficient validates quite well.

I have been using this for validation: https://turbmodels.larc.nasa.gov/naca0012_val.html

Re for NASA validation = 6 million

Am I using the wrong validation due to these differences in Re?

Thank you!

ghorrocks · March 4, 2019, 18:09

Re = 76000 is likely to be a transitional flow, with turbulence transitional the rear half of the foil or maybe fully laminar flow. That depends on the exact airfoil, upstream conditions, surface roughness and so on.

Re = 6M is definitely fully turbulent over the whole airfoil.

They do not look like a good match to me.

35matt215 · March 4, 2019, 18:19

Thanks for your reply!

So do you think it would be best if I look for papers with a smaller Re in order to validate my model with?

ghorrocks · March 4, 2019, 19:44

Reynolds number is probably the most fundamental parameter in fluid mechanics. If you don't match the Reynolds number then you are talking about completely different flows.

So in short, yes - you need to find data at the same Re number you are modelling.

Here is a great resource for low Re airfoils - https://m-selig.ae.illinois.edu/uiuc_lsat.html

March 4, 2019, 15:46	NACA 0012 low speed validation problem	#1
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	Hi everyone, I am modelling a NACA 0012 airfoil using CFX at velocity 1.15 m/s, 1.55 deg AoA in air at 20 deg C (Re = 76104), but I am having issues validating the drag coefficient at this velocity as there seems to be about a 35% error when compared to NASA validation. The lift coefficient validates quite well. I have been using this for validation: https://turbmodels.larc.nasa.gov/naca0012_val.html Re for NASA validation = 6 million Am I using the wrong validation due to these differences in Re? Thank you! Last edited by 35matt215; March 4, 2019 at 18:24.

March 4, 2019, 18:09		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Re = 76000 is likely to be a transitional flow, with turbulence transitional the rear half of the foil or maybe fully laminar flow. That depends on the exact airfoil, upstream conditions, surface roughness and so on. Re = 6M is definitely fully turbulent over the whole airfoil. They do not look like a good match to me. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

March 4, 2019, 19:44		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Reynolds number is probably the most fundamental parameter in fluid mechanics. If you don't match the Reynolds number then you are talking about completely different flows. So in short, yes - you need to find data at the same Re number you are modelling. Here is a great resource for low Re airfoils - https://m-selig.ae.illinois.edu/uiuc_lsat.html Dronzer likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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March 4, 2019, 18:19		#3
35matt215 New Member Join Date: Mar 2019 Posts: 7 Rep Power: 7	Thanks for your reply! So do you think it would be best if I look for papers with a smaller Re in order to validate my model with?