A wind turbine blade having a longitudinal direction with a root end and a tip end as well as a chord extending in a transverse direction between a leading edge and a trailing edge is described. The blade comprises a flow control surface with a suction side and a pressure side. A number of boundary layer control structures are formed in the flow control surface. The boundary layer control structures include a channel submerged in the flow control surface with a first end facing towards the leading edge and a second end facing towards the trailing edge of the blade. The channel further comprises a bottom surface extending from the first end to the second end. The channel at the first end comprises a first channel zone comprising a first sub-channel having a first cross-sectional area and a second sub-channel having a second cross-sectional area, the first sub-channel and the second sub-channel crossing each other at a point of crossing.