Frictional drag comes from friction between the fluid and the surfaces over which it is flowing.
This friction is associated with the development of boundary layers, and it scales with Reynolds number as we have seen above. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to Reynolds number than the frictional drag. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body. At small angles of attack , the boundary layers on the top and bottom surface experience only mild pressure gradients, and they remain attached along almost the entire chord length.