2d software for thick airfoils, Re=2-4M, Mach=.2-.3

frank999 · July 13, 2011, 00:09

What software would you recommend to get the most accurate drag estimation on a 20+% thick airfoil at 2 to 4 million Reynolds number and mach around .2? Let's say you don't want to exceed about 30 minutes run time on a descent 64 core cluster.

Would the same code be good for predicting maximum CL and the corresponding angle of attack, or would you recommend something else?

Martin Hegedus · July 13, 2011, 02:44

I don't know the details of your problem, but given that your Reynolds number puts you in the transitional area, I think you may be looking for something with an appropriate turbulence model. Also, given your cpu requirements, I assume you are looking for a RANS model. Assuming a code like XFOIL doesn't work for you. I don't know the ins and outs of the various validation work for these models, but it seems that the ones used for transition are more "researchy" than production. In my opinion, you'll need to be careful with them. IMO, there seems to be a bit of hand waving about them and the wind tunnel data used for validation for these thick airfoils may be questionable too, especially at and past stall. Again, my opinion.

frank999 · July 24, 2011, 13:35

Yes, I think I need transition to be predicted well. I know Martin said the models may be more researchy, but any one in particular that would be recommended?

I've compared XFOIL and a RANS solver with wind tunnel data for a few cases. The w.t. data does seem sketchy and it's hard to determine which solver (XFOIL or RANS) is giving better predictions. Both solvers seem to over-predict CLmax significantly, XFOIL slightly more over-prediction. Minimum CD occurs at a significantly different CL between XFOIL, RANS, and wt data, and the curves are significantly different so it's hard to say which solver is better (again the wt data itself is questionable). Any comments / suggestions?

Any research out there addressing these problems, even on the experimental/testing side, to address these issues with thick airfoils?

Thanks

Martin Hegedus · July 24, 2011, 14:04

Recently there was this conversation about CFX's gamma-theta transition model. http://www.cfd-online.com/Forums/cfx...cted-lift.html

This is for a NACA 0020 but at a much lower Reynolds number (122,000) so the physics will probably be different. The difficulty was with predicting stall angle correctly. For that case, transition could happen on the bubble. However, for your case, the model may do better.

July 13, 2011, 00:09	2d software for thick airfoils, Re=2-4M, Mach=.2-.3	#1
frank999 New Member Frank Join Date: Jul 2011 Posts: 2 Rep Power: 0	What software would you recommend to get the most accurate drag estimation on a 20+% thick airfoil at 2 to 4 million Reynolds number and mach around .2? Let's say you don't want to exceed about 30 minutes run time on a descent 64 core cluster. Would the same code be good for predicting maximum CL and the corresponding angle of attack, or would you recommend something else?

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July 13, 2011, 02:44		#2
Martin Hegedus Senior Member Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19	I don't know the details of your problem, but given that your Reynolds number puts you in the transitional area, I think you may be looking for something with an appropriate turbulence model. Also, given your cpu requirements, I assume you are looking for a RANS model. Assuming a code like XFOIL doesn't work for you. I don't know the ins and outs of the various validation work for these models, but it seems that the ones used for transition are more "researchy" than production. In my opinion, you'll need to be careful with them. IMO, there seems to be a bit of hand waving about them and the wind tunnel data used for validation for these thick airfoils may be questionable too, especially at and past stall. Again, my opinion.

July 24, 2011, 13:35		#3
frank999 New Member Frank Join Date: Jul 2011 Posts: 2 Rep Power: 0	Yes, I think I need transition to be predicted well. I know Martin said the models may be more researchy, but any one in particular that would be recommended? I've compared XFOIL and a RANS solver with wind tunnel data for a few cases. The w.t. data does seem sketchy and it's hard to determine which solver (XFOIL or RANS) is giving better predictions. Both solvers seem to over-predict CLmax significantly, XFOIL slightly more over-prediction. Minimum CD occurs at a significantly different CL between XFOIL, RANS, and wt data, and the curves are significantly different so it's hard to say which solver is better (again the wt data itself is questionable). Any comments / suggestions? Any research out there addressing these problems, even on the experimental/testing side, to address these issues with thick airfoils? Thanks

July 24, 2011, 14:04		#4
Martin Hegedus Senior Member Martin Hegedus Join Date: Feb 2011 Posts: 500 Rep Power: 19	Recently there was this conversation about CFX's gamma-theta transition model. http://www.cfd-online.com/Forums/cfx...cted-lift.html This is for a NACA 0020 but at a much lower Reynolds number (122,000) so the physics will probably be different. The difficulty was with predicting stall angle correctly. For that case, transition could happen on the bubble. However, for your case, the model may do better.