Boundary layer thickness in Xfoil

Nixmich · January 25, 2011, 09:50

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, 11:32

Quote:

Originally Posted by Nixmich

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, 03:00

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, 08:52

i don't get it. the dotted red line on the foil should be the BL.

Nixmich · January 26, 2011, 10:49

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, 03:59

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.

January 25, 2011, 09:50	Boundary layer thickness in Xfoil	#1
Nixmich New Member Lukas Join Date: Nov 2010 Posts: 11 Rep Power: 15	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?

January 26, 2011, 03:00		#3
Nixmich New Member Lukas Join Date: Nov 2010 Posts: 11 Rep Power: 15	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... Last edited by Nixmich; January 26, 2011 at 03:39.*

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January 26, 2011, 10:49		#5
Nixmich New Member Lukas Join Date: Nov 2010 Posts: 11 Rep Power: 15	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.