A comprehensive review of CFD methods for dynamic stall has been published by
Ekaterinaris and Platzer (1997); for physical insight, see McCroskey (1981).

In wind turbines it is the result of atmospheric turbulence, wind shears, earth
boundary layer, etc. The aerodynamic characteristics are affected to an extend that
depends on the frequency of the changes, their amplitude and the point of operation.

Other factors affecting dynamic stall are the Reynolds and Mach numbers and the
geometrical shape. There other, maybe minor factors, like the vortex effects, blade
flapping and bending, etc…

In the following discussion we will consider the airfoil dynamic stall, which is a
particular case of rotor and wing stall. The airfoil is subject to two fundamental
periodic oscillations: plunging and pitching.

A plunging oscillation is a periodic translation of the airfoil in a direction normal
to the free stream.
A pitching motion
is a periodic variation of the angle of attack.

The most important parameters affecting the dynamic behavior of an airfoil under
periodic variations of inflow conditions are: amplitude of the oscillation, mean
angle of attack, reduced frequency, Reynolds and Mach numbers, airfoil shape
(thickness, leading edge radius, etc.), surface roughness, and free stream turbulence.

The blades of a helicopter rotor in foward flight experience unbalanced inflow
conditions. The basic azimuth positions are shown in the sketch of Fig. 1.

**Figure 1: Azimuth positions of helicopter rotor blades.**
It can be found that for some values of the advancing speed and the amplitude of the
oscillations (e.g. the rotor diameter) some sections experience *back flow*
conditions, Fig. 2.

**Figure 2: Dynamic stall on helicopter rotor.**
The blade sections more affected by dynamic inflow conditions are at the hub.
The problem is governed by the ratio of the flight speed to the tip speed (tip
speed ratio).

Too large tip speed ratios affect more radically the characteristics of the
rotor, and represent a concrete limit to the flight speed of a helicopter.

If the oscillation occurs around a mean angle of attack close to CLmax (static stall)
the viscous effect become predominant. The description of the physical events taking
place is far more difficult. Fig. 3 shows an example of lift hysteresis for an
airfoil oscillating around CLmax (these effects change with the reduced frequency).

**Figure 3a: Lift oscillations around Clmax.**

**Figure 3b: pitching moment oscillation around Clmax.**

**Figure 3c: drag oscillation around Clmax.**

Starting from the point of minimum incidence, the dynamic lift follows the static
lift, until the static lift curve deflects, due to increasing trailing edge
separation. The dynamic lift, instead, keeps growing almost linearly until a
breakdown occurs.

At the breakdown point there is massive flow separation and the lift drops to levels
far below those typical of the static curve. It will take some time to recover more
regular behavior, but the lift will remain below the static lift for most of the
remaining loop.

Understanding the initial stages of unsteady separation is extremely important.
It is known now that large scale separation is largely an inviscid problem. The
problem is more complicated in three-dimensions, because it requires the analysis
of different time and length scales.

CFD has made its entry in dynamic stall simulation in a wide range of speeds (up to
transonic) and various frequency ranges. Presently the available computational
methods are limited to two-dimensional stall. Among the main problems encountered
there is the turbulence modeling (all available models have been tried), … The
field is however in a fast expansion.