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?
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.