1. DYNAMIC STALL OCCURRENCE ON A HORIZONTAL AXIS WIND TURBINE BLADE
P M V Subbarao
Professor
Mechanical Engineering Department
Diagnostics of Most Probable Off-design Conditions

2. DS is not A Design Condition for HAWT

3. Extraneous Wind Conditions
Wind Shear
Wind Shifts
Wind Gusts

4. Unsteady Wind Distribution

5. Extraneous Loads
Tower Shadow
Gravity

6. OCCURRENCE OF DYNAMIC STALL in HAWT
A dynamic stall event occurs on a blade section when the local angle of attack rapidly increases through the static stall point.
Turbulence, shifts in wind direction or magnitude.
Wind shear, or upstream flow disturbances can alter the local velocity.
These can create blade angle of attack changes sufficient to drive dynamic stall.
Dynamic stall can be characterized by an extremely large suction peak value at or near the leading edge.
Dynamic stall is expected to occur if the peak pressure coefficient (Cp) was less than -10.

7. Importance of Dynamics Stall in HAWT
The lifetime of major wind turbine components is typically far less than their year design lifetime.
Components, such as generators and blades, are frequently subjected to dynamic loading far in excess of their design loads.
A primary source of the excessive fatigue and failure is speculated to be derived from the unsteady aerodynamics.
Unsteady aerodynamics is due to dynamic inflow, turbulence, and dynamic stall.
Unsteady detached flows are responsible for the fatigue cycles with the highest peak-to-peak loading for both blade and rotor shaft bending, reducing turbine lifetime.
Therefore, an understanding of the flow physics which dictate these forces would be beneficial in designing more reliable wind turbines.

8. The Effect of Gusts on Angle of Attack
Under steady operating conditions, the turbine blade is designed to maintain a constant circulation profile over the span of the blade.
However, when a gust impinges on a blade the change in angle of attack across the blade Δα is highly non-uniform.
Δα(r) is the change in angle of attack as a function of radius r,
ΔU is the change in the free stream velocity,
α0 is the initial angle of attack.

9. Effect of Gust on Spanwise Variation of AOA
Both the change in magnitude and the spanwise gradient of angle of attack are largest in the near-hub region.
Creates a scope for Dynamic stall behavior.

10. A Section of A Turbine Blade Experiencing Gust
A rotational accelerations must act to stabilize the vortex, with three critical rotational accelerations affecting vortex attachment :
Angular acceleration (aang),
Centripetal acceleration (acen)
Coriolis acceleration (aCor)
The rate of spanwise circulation redistribution generated as:
For stable operation of rotor

11. Three dimensional Nature of Flow
In transient flow conditions wind turbines develop a gradient in angle of attack along the blade span.
This generates a spanwise vorticity gradient.
It is postulated that, in combination with the spanwise flow induced by rotational accelerations, this spanwise vorticity gradient acts to redistribute circulation along the span of the blade towards the root.
For locations near hub this redistribution would result in a greater magnitude of local circulation.
This increases the lift experienced near hub.

12. The Distribution of Inflow Conditions Over 20,577 Blade Rotational Cycles

13. Experimental Test Rigs

14. Frequency of Dynamic Stall Occurrence

15. Distribution of Frequency of Occurrence of Stall

16. The Frequency of Dynamic Stall Occurrence at Two or More Span Locations During A Cycle

17. Dynamic Controls Systems
Typical HAWT blades have a twist distribution such that the entire blade is at the same angle of attack for a given wind speed.
Therefore, when dynamic stall occurs, it will affect extremely large portions of the blade at the same time.
By adjusting blade pitch, the entire blade can be put into a operational regime in which dynamic stall is not likely to occur.
Doing so should yield a significant reduction in rotor loads.