I want to know how the top and bottom boundary layer interact at the trailing edge of an aerofoil (zero angle of attack) and what happens to the boundary layer after a small distance from the trailing edge. Does the region at the back of aerofoil have lesser velocity due to the boundary layer? How is it carried by freestream?

  • $\begingroup$ This sounds a lot like a question that would appear on a homework assignment or something... What do you think would happen when two boundary layers meet? Is there a particular concept that you are unclear about in viscous flows that could narrow down where to focus an answer? $\endgroup$ – tpg2114 Mar 21 '15 at 19:01
  • $\begingroup$ I was confused whether the boundary layer from top and bottom surface interact such that it always form wakes at trailing edge even though the aerofoil is perfectly stream lined and having zero angle of attack or do the boundary layers just join up and form something like a bigger patch beyond the trailing edge.What happens to it as we move farther from trailing edge of the aerofoil? $\endgroup$ – Sam_92 Mar 21 '15 at 19:25
  • $\begingroup$ Not sure what you mean by "form a bigger patch" but it sounds like you already know how to find the answer to your question. I'll give you a little hint: When the boundary layers join up, there exists a velocity gradient again between the top and bottom layers. Think about how viscosity works and what it does in a fluid and you'll be able to answer your own question. $\endgroup$ – tpg2114 Mar 21 '15 at 19:27
  • $\begingroup$ Appreciate your help.By bigger patch I meant the joining of top and bottom boundary layer into one single patch where velocity gradients are signifificant. How does this velocity gradient vary as we move farther downstream from trailing edge? $\endgroup$ – Sam_92 Mar 21 '15 at 20:50
  • $\begingroup$ Back to my original question -- how do you think the flow behind an airfoil would look? I have no problem helping to clear up any misconceptions about what you think it should look like, but I would rather not just tell you what it looks like. Sketching a picture (and heck, finding a picture online) should be pretty simple; coming up with some math for it is a little more complex but something who is taking classes in viscous fluid dynamics should be able to do. $\endgroup$ – tpg2114 Mar 21 '15 at 21:25

In every boundary layer (except for exotic hypersonic cases), the speed at the wall is zero. At the trailing edge, the upper and lower layers meet, and if you imagine a plane which extends from the trailing edge backwards and follows the streamlines, the speed at the trailing edge is equally zero. The more you now move away from the trailing edge along this plane, the more the speed increases, as now the inhibiting effect of wall shear is missing, and only the shear of the layers above and below the plane acts upon the air in this plane. If you measure the speed orthogonally to this plane, you will see a speed drop near the plane which gets wider and more shallow the more you move away from the trailing edge.

This speed drop can be measured and gives a very precise value for airfoil drag. See the picture below for a rake of pitot tubes which is used for this kind of measurement.

enter image description here

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