# The boundary layer thickness on an airfoil

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 September 13, 2012, 02:18 The boundary layer thickness on an airfoil #1 Senior Member   Nick Join Date: Nov 2010 Posts: 126 Rep Power: 9 Hey guys, I know that on a flat plate the boundary layer thickness is the thickness where streamwise velocity reaches 99% of the freestream speed. How does this work on an airfoil since the flow speeds up above the foil and slows down beneath it? Do we still use 99% of the free-stream as the thickness ? cheers, Nick R

September 13, 2012, 09:37
#2
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mhp
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Quote:
 Originally Posted by Nick R Hey guys, I know that on a flat plate the boundary layer thickness is the thickness where streamwise velocity reaches 99% of the freestream speed. How does this work on an airfoil since the flow speeds up above the foil and slows down beneath it? Do we still use 99% of the free-stream as the thickness ? cheers, Nick R
Yes,of course.
No matter how much is your speed, whenever your speed in the perpendicular direction relative to the surface reaches 99% of the free stream,you can calculate BL thickness at that point.

September 13, 2012, 09:56
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Quote:
 Originally Posted by Nick R Hey guys, I know that on a flat plate the boundary layer thickness is the thickness where streamwise velocity reaches 99% of the freestream speed. How does this work on an airfoil since the flow speeds up above the foil and slows down beneath it? Do we still use 99% of the free-stream as the thickness ? cheers, Nick R
Even on a flat plate one uses different notions of thikness for different purpoces. More meaningful (than 99%) notions of thikness are displacement thikness and momentum thikness. This depends on what for you need this thikness. If one wants to join inviscid solution far from airfoil with viscous bl solution it is necessary modify airfoil profile by displacement thikness. If one wants to find drag force, then it is defined by momentum thikness.

 September 13, 2012, 20:22 #4 Senior Member   Nick Join Date: Nov 2010 Posts: 126 Rep Power: 9 thanks it makes sense

September 14, 2012, 03:50
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mhp
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Quote:
 Originally Posted by MHP Yes,of course. No matter how much is your speed, whenever your speed in the perpendicular direction relative to the surface reaches 99% of the free stream,you can calculate BL thickness at that point.
As an additional note,please pay attention that in advanced BL references the pressure gradient over the geometry is another issue that enter in BL thickness calculations, we use 0.99% of free stream velocity for flat plate,because in that case the free velocity is constant over the plate and there is no pressure gradient, but for airfoil depending on the geometry profiles there is pressure gradients.
good luck.

October 23, 2013, 10:25
A nagging thought on thickness notions
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Daniel
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Quote:
 Originally Posted by truffaldino Even on a flat plate one uses different notions of thikness for different purpoces. More meaningful (than 99%) notions of thikness are displacement thikness and momentum thikness. This depends on what for you need this thikness. If one wants to join inviscid solution far from airfoil with viscous bl solution it is necessary modify airfoil profile by displacement thikness. If one wants to find drag force, then it is defined by momentum thikness.
I have to say, this "..there are more meaningful notions of thicknss.." talk is rather annoying. I have seen countless posts, also on other forums where this is stated!

IMO - Whichever notion you decide to go for, you need to define some sort of cut-off between your inner and outer flow, since even for the displacement and momentum thicknesses, you have to decide how much wall effects to incorporate...

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