The most prominent example of dynamic airfoil stall
refers to helicopters under forward or maneuvering flight conditions.
Here, the incidence of the rotor blade sections
periodically changes from moderate up to fairly large values.
In contrast to steady-state airfoil stall,
the rapidly increasing incidence usually delays
the onset of stall
to regimes which may exceed the static stall
angle by a significant amount.
However, once dynamic stall occurs, the aerodynamic loads are
generally more severe compared to steady stall and may cause
significant increases in the blade stresses and the control
system loads.
Due to the complexity of the involved flow-physics
(e.g. reverse transition) an accurate numerical prediction of dynamic stall
in the frame of RANS is quite challenging.
Figure 3: Lift- and
Pitching moment coefficient in the case of dynamic stall
(Ekaterinaris [3])
Numerous experiments have revealed the sequence of flow events
depicted in Figure 1 taken from Ekaterinaris [3]. First, a vortex starts
to develop near the airfoil leading edge as the angle of attack
is rapidly increased past the static stall angle. This vortex
then is convected downstream near the airfoil surface which
causes an increase in lift due to the suction induced by the vortex.
The magnitude of the lift increase depends on the strength of the
vortex and its distance from the surface. The streamwise movement
of the vortex depends on the airfoil shape and the pitch rate.
As the vortex is convected past the trailing edge, the pitching
moment briefly attains its maximum negative value and then both
lift and pitching moment start to drop rapidly.
The flow over the airfoil remains stalled until the angle of attack
has decreased sufficiently to enable flow reattachment. As a result
of this sequence of flow events, the unsteady lift, drag, and
pitching moment coefficients show a large degree of flow hysteresis
when plotted as a function of incidence angle.
The amount of hysteresis and the shape of the hysteresis loops vary in
a highly non-linear fashion with the amplitude of oscillation, mean
angle of attack and reduced frequency of oscillation.
Dynamic Airfoil Stall 
