Structural Dynamic Behavior of Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 631

The desire to account for all such effects within one or two comprehensive models is
driving engineers to develop computer codes of ever-increasing complexity.
Improved structural dynamic models of wind turbines appear to be developing along
two different paths. The first approach involves special purpose codes, each written to
analyze the structural dynamic behavior of a specific configuration of wind turbine. Such
codes generally have 12 to 15 degrees of freedom (DOF) tailored to the significant motions
of the “target” structural system. The second approach seeks to adapt general purpose
structural analysis codes to simulate wind turbine dynamics. These general purpose codes
are powerful enough to handle almost any turbine configuration, but they are complex and
expensive. Moreover, the structural dynamicist must have a significant level of expertise
in their use, and each new configuration requires considerable time for modeling and
verification efforts.
Typical Special-Purpose Model
The structural analysis program PHATAS-II , developed at the Netherlands Energy
Research Foundation (ECN), is an example of a special-purpose code [Lindenburg and Snel
1993]. The PHATAS computer model can be used to calculate dynamic displacements and
loads for the main structural subsystems of a HAWT. Wind inputs to this model include
these:
-- steady wind, including vertical and horizontal gradients
-- wind direction as a function of elevation
-- rotationally-sampled wind turbulence
-- turbulent wind in time-series form
A number of structural degrees of freedom are available within the code, such as the following:
-- flapwise and chordwise elastic blade deformations
-- blade pitch motion
-- blade flapping and teetering hinges
-- torsional deflection of the power train
-- variable rotor speed
-- pitch and yaw motions of the nacelle
-- bending and torsional deflections of the tower
Typical General-Purpose Model
The ADAMS code, developed at Mechanical Dynamics, Inc. , is an example of a
general-purpose, multi-body structural analysis program that has been adapted for wind
turbine applications [Malcolm and Wright 1994]. This code has virtually unlimited degrees
of freedom and places no limits on the type or magnitude of displacements. While the
ADAMS code can be run on a high-end personal computer (PC), it is exceptionally slow
when modeling systems with many degrees of freedom or when performing simulations
involving wind turbulence inputs. For example, it can take several days of PC run time to
simulate 10 minutes of real time operation of a HAWT subjected to turbulent winds.
The strengths of this type of computer model are that almost any structural system
(wind turbine or otherwise) can be modeled with it once the required skills have been
learned by the user, and a mixture of complex external loadings and operating conditions
can be applied. As PC performance increases, this type of general-purpose computer model
is expected to used more and more for the structural analysis of modern wind turbines.