Environmental Engineering Reference
In-Depth Information
Wind gusts and direction changes encountered during routine operations are sufficient
to drive turbine operating state out of the nominal region and into rotational augmentation or
dynamic stall. These departures are shown by the two vectors originating at the upper right
corner of the nominal region. One of these vectors points to the right and is labeled “30 deg
Operating Direction Change”, while the other points upward and is labeled “9 m/s Operating
Gust”. Wind turbine certification standards were used to establish these vector magnitudes
for operating direction change and normal operating gust [Germanischer-Lloyd 1998].
It may be noted that the UAE Phase VI machine was a stall controlled turbine and was
intentionally operated at large yaw errors. Admittedly, these two characteristics differ from
state-of-the-art utility class machines. However, wind gusts and direction changes that out-
pace turbine pitch- and yaw-actuation rates are likely lead to similar circumstances. It also
is important to remember that the current work relies on wind tunnel measurements and
computational predictions that purposely exclude turbulent inflow. Thus, current work on
modeling dynamic stall cannot quantify the extent to which turbulent inflow may compound
adverse loading effects.
Conclusions
Wind turbine aerodynamics remains a challenging and crucial research area for wind
energy, especially as turbines grow larger and aerodynamics and structural design become
more closely coupled. Clear understanding of turbine blade flow physics and model numer-
ics reveals analogies between the two and points toward attributes that yield accurate, reliable
models. Both turbine blade aerodynamics and model construction can be subdivided consis-
tent with turbine operating state and routinely occurring flow phenomena.
Under axisymmetric operating conditions and at low yaw angles, rotational augmenta-
tion delays turbine blade stall and significantly amplifies blade aerodynamic loads. This
phenomenon depends on viscous flow effects in the presence of centrifugal and Coriolis
influences, and occurs routinely during turbine operation. Current rotational augmentation
models blend theory with empiricism to achieve acceptable model accuracy and efficiency.
During yawed operation, dynamic stall takes place when blade angle of attack dynami-
cally exceeds the static stall threshold and a dynamic stall vortex initiates. As this vortex
grows in size and convects aft over the blade, it significantly amplifies blade aerodynamic
loads. Load amplification is fleeting, and ceases abruptly as the vortex departs the blade
surface. The physical complexity of dynamic stall requires a semi-empirical approach that
balances model fidelity with requirements for empirical input information.
Refinement of blade aerodynamics comprehension and modeling methodologies offers
potential benefits for future wind energy machines, by facilitating the creation and validation
of more accurate turbine aerodynamics design models. Enhanced aerodynamic design ca-
pabilities enable the development of larger, more efficient, and more cost effective turbines,
thereby ensuring the continued growth and success of wind energy generation.
Search WWH ::
Custom Search