Starting vortex


The starting vortex is a vortex which forms in the air adjacent to the trailing edge of an airfoil as it is accelerated from rest in a fluid. It leaves the airfoil, and remains stationary in the flow. It rapidly decays through the action of viscosity.
The initial presence of a starting vortex as an airfoil begins to move was predicted by early aerodynamicists, and eventually photographed.
Whenever the speed or angle of attack of an airfoil changes there is a corresponding amount of vorticity deposited in the wake behind the airfoil, joining the two trailing vortices. This vorticity is a continuum of mini-starting-vortexes. The wake behind an aircraft is a continuous sheet of weak vorticity, between the two trailing vortices, and this accounts for the changes in strength of the trailing vortices as the airspeed of the aircraft and angle of attack on the wing change during flight.
The starting vortex is significant to an understanding of the Kutta condition and its role in the circulation around any airfoil generating lift.
The starting vortex has certain similarities with the "starting plume" which forms at the leading edge of a slug of fluid, when one fluid is injected into another at rest. See plume.