aerodynamic coefficients (viscous/inviscid flow)

nicola_salin · May 4, 2011, 11:51

Hi everyone,
I've written a panel code that solves a potential flow around wings. After that in the same code I've added a viscous correction (boundary layer equations for laminar and turbulent layers). The issue that I'm struggling with is: how should the aerodynamic coefficients change from a potential solution to a potential solution that considers the viscosity of the air? Both cases consider same geometry, same angle of attack (small AOA), same flow speed. In short, is that correct that CD and CL are a bit lower with a viscous solution rather than with a inviscid solution?
Best regards,
Nicola Salin

truffaldino · May 5, 2011, 04:08

It seems that in your case decrease in induced drag due to drop in Cl at fixed aoa exceeds viscous drag.

Why do not you plot Cd vs Cl polars and compare viscous/inviscid Cd at fixed Cl? If your code is correct viscous Cd should be higher here.

Also you can compare your results with vortex lattice cods available for free on the internet: for instance, you can download XFLR5, it takes just a couple of hours to learn it.

nicola_salin · May 5, 2011, 12:30

Thanks a lot. I'm gonna try plotting the polars. I've downloaded the last version of XFLR5 and it gives me an error with the viscous option on. That's odd, I remember that in a older version it was working.

truffaldino · May 5, 2011, 18:35

Also you should be careful when comparing your code with XFLR5, XFLR uses x-foil to calculate viscous drag and it takes separation bubbles into account. While simple BL methods do not account for the bubbles.

May 4, 2011, 11:51	aerodynamic coefficients (viscous/inviscid flow)	#1
nicola_salin New Member nicola salin Join Date: May 2011 Posts: 2 Rep Power: 0	Hi everyone, I've written a panel code that solves a potential flow around wings. After that in the same code I've added a viscous correction (boundary layer equations for laminar and turbulent layers). The issue that I'm struggling with is: how should the aerodynamic coefficients change from a potential solution to a potential solution that considers the viscosity of the air? Both cases consider same geometry, same angle of attack (small AOA), same flow speed. In short, is that correct that CD and CL are a bit lower with a viscous solution rather than with a inviscid solution? Best regards, Nicola Salin

May 5, 2011, 12:30	Xflr5	#3
nicola_salin New Member nicola salin Join Date: May 2011 Posts: 2 Rep Power: 0	Thanks a lot. I'm gonna try plotting the polars. I've downloaded the last version of XFLR5 and it gives me an error with the viscous option on. That's odd, I remember that in a older version it was working.

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May 5, 2011, 04:08		#2
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	It seems that in your case decrease in induced drag due to drop in Cl at fixed aoa exceeds viscous drag. Why do not you plot Cd vs Cl polars and compare viscous/inviscid Cd at fixed Cl? If your code is correct viscous Cd should be higher here. Also you can compare your results with vortex lattice cods available for free on the internet: for instance, you can download XFLR5, it takes just a couple of hours to learn it.

May 5, 2011, 18:35		#4
truffaldino Senior Member Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17	Also you should be careful when comparing your code with XFLR5, XFLR uses x-foil to calculate viscous drag and it takes separation bubbles into account. While simple BL methods do not account for the bubbles.