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 Nixmich January 25, 2011 10:50

Boundary layer thickness in Xfoil

Hi,
does anybody know if it is possible to calculate the boundary layer thickness on an airfoil with Xfoil? I've managed to calculate delta* which is the displacement thickness but no success concerning the boundary layer thickness. Are there any alternatives if Xfoil doesn't work?

 sail January 25, 2011 12:32

Quote:
 Originally Posted by Nixmich (Post 292095) Hi, does anybody know if it is possible to calculate the boundary layer thickness on an airfoil with Xfoil? I've managed to calculate delta* which is the displacement thickness but no success concerning the boundary layer thickness. Are there any alternatives if Xfoil doesn't work?
use xflr5. it's open source, free, based on the "engine" of xfoil but it's more user friendly and can estimate the bl thickness.

 Nixmich January 26, 2011 04:00

Okay, I'll try that, thanks a lot!

Edit: ehm, like in Xfoil, I can plot the displacement thickness D*, but not the BL thickness...

 sail January 26, 2011 09:52

1 Attachment(s)
i don't get it. the dotted red line on the foil should be the BL.

 Nixmich January 26, 2011 11:49

Thanks again for your help, but I think that's not the BL, although the box you have to check is labelled "show BL". The dotted line looks exactly like the D* line in xfoil (which does certainly not represent the BL thickness, as you can verify by some velocity profiles displayed on discrete points on the airfoil). Even if it was the BL, it wouldn't help me much because I can't read any values from that line.
The only thing I'm able to plot or export is D* vs x, again exactly the same values as in xfoil.

 ChrisDanger January 27, 2011 04:59

There are no general equations for boundary layer thickness. However, from the Xfoil 6.9 User Primer:

Quote:
 The difference between the true viscous Cp distribution (solid line) and the inviscid Cp distribution (dashed line) is due to the modification of the effective airfoil shape by the boundary layers. This effective airfoil shape is shown superimposed on the actual current airfoil shape under the Cp vs x plot. The gap between these effective and actual shapes is equal to the local displacement thickness delta*, which can also be plotted in the VPAR menu. This is only about 1/3 to 1/2 as large as the overall boundary layer thickness, which can be visualized via the BL or BLC commands which display velocity profiles through the boundary layer.
That looks as accurate as it's going to get, unfortunately.

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