This question already has an answer here:
A lot of times I see small tube like streams of visible moisture training from the middle of an airliners wing. I assume this is induced drag created by wingtip vortices, when the aircraft is heavy, clean, and slow. However, I figured this would only happen at the “wingtips” and not from the middle section of a wing. I have seen them at the wingtips but a lot of the time it’s coming from the middle. If this is from the wingtip vortices how come they would come from the middle?