Starting Vortex
As we know from Bernoulli's equation, we know that there is lower
pressure above the wing, creating lift on the wing. We can consider
that because of this, the air velocity is greater above the wing than
than below the wing. The greater velocity above the wing creates a
relative circular motion around the wing, in a clockwise direction
like that in the figure below, causing a counterclockwise response:
image from http://web.mit.edu/2.972/www/reports/airfoil/
After a foil accelerates through a fluid, it may leave a starting
vortex behind of it, which can remain suspended in the fluid while
the foil continues to move away. If an airplane leaves a starting
vortex while landing or taking off, any planes that happen to be
flying behind of it may collide with the wake turbulance left behind.