# Why is spanwise flow more prevalent in the boundary layer?

Spanwise flow is stronger in the boundary layer, but to me it's kind of counterintuitive because the BL flow is slowed down by friction, experiencing less of a pressure gradient over the wing, therefore being decelerated less in region of pressure recovery. Wouldn't this make it so there is less spanwise flow in the boundary layer? (Especially considering that the BL thickens as it passes over the wing)

Answer that might help convey what I am trying to ask.

• Does this answer answer your question? Nov 29, 2023 at 20:33
• @ROIMaison thanks for your reply. I have seen that answer before, but I still wanted some clarification. (If you click on the link in my question it's actually the same one you provided) Nov 29, 2023 at 20:38
• You should draw the velocity triangles on two locations, at the outer edge of the boundary layer and one close to the surface. Outside of the boundary layer there is a large longitudinal component, so the spanwise component leads to a smaller deflection. Closer to the surface, the longitudinal velocity is lower, so the same spanwise component leads to a large deflection. Nov 29, 2023 at 21:35
• @ROIMaison ah okay, thanks. On this answer, it says that there is an upward speed component from decelerating air on a wing (along with an outward) and that it is more in the BL. Does the same principle apply that you mentioned for that as well? Nov 29, 2023 at 23:28