Jason Jonkman of NREL recently upgraded the FAST code to include furling effects . The upgrades include a lateral thrust offset and skew angle of the rotor shaft from the yaw axis, rotor-fUrling and tail-furling degrees-of-freedom (DOFs), up- and down-furl stops, and tail fin aerodynamics and inertia. The location and orientation of the new furling DOFs are completely user-specified, making the simulator flexible enough to model many furling wind turbine configurations.
For example, to model a wind turbine with tail-furling like the SWRT, the new tail-furl DOF should be enabled and the rotor-furl DOF locked. The angular motion of the tail boom and fin through the tail-furl DOF then takes place about a tail-furl axis defined by input parameters TFrlPntxn, TFrlPntyn, TFrlPntzn, TFrlSkew, and TFrlTilt. Inputs TFrlPntxn, TFrlPntyn, and TFrlPntzn locate an arbitrary point on the tail-furl axis relative to the tower-top. Inputs TFrlSkew and TFrlTilt then define the angular orientation of the tail-furl axis passing through this point. Note that the tail is modeled as a rigid body with lumped tail-boom and tail-fin point masses and a moment of inertia specified about the tail-furl axis.
The structural dynamic equations of the motions of the full system now include all of the inertia forces (gyroscopic, Coriolis, etc.) that result from furl motion. These equations do not require that the furl motions remain small because small angle assumptions were not used in their derivations. A FAST user does not need to worry about the complexity of the equations of motion because they are built within the code; a user need only be concerned with defining the configuration of the system properly.
The furling hinges can be ideal with no friction. A standard model is also available that includes a linear spring, linear damper and Coulomb damper, as well as up- and down-stop springs, and up- and down-stop dampers. FAST models the stop springs with a linear function of furl deflection. The furl stops start at a specified angle and work as a linear spring based on the deflection past the stop angle. The furl dampers are linear functions of the furl rate and start at the specified up-stop and down-stop angles. These dampers are bidirectional, resisting motion equally in both directions once past the stop angle. Hooks for interfacing user-defined furl springs and dampers are also available.
A simple tail fin aerodynamics model has been implemented in FAST. Hooks for interfacing user-defined models are also available. By accessing information from AeroDyn, the simple model computes the relative velocity of the wind-inflow and its angle of attack relative to the tail-fin chordline and uses an AeroDyn airfoil table chosen by the user to determine the lift and drag forces acting at the tail-fin center-of-pressure. To account for the velocity deficit in the rotor wake, the wind velocity at the tail-fin center-of-pressure is decreased by the average rotor induced velocity in the direction of the rotor shaft. The chordline and plane of the tail fin may be skewed, tilted, and banked relative to the tail boom as shown in Figure 58.
To verify the correct implementation of the newly added furling dynamics, response predictions from FAST were compared to those of ADAMS using models of the SWRT. Data on the validation of the FAST model may be found it Jonkman and Hansen . It should be noted that, because blade torsional stiffness cannot currently be modeled in FAST, the torsional stiffness inputs to the ADAMs model assumed an infinitely stiff blade and the variances between the FAST and ADAMS models from live blade twist did not show up in the comparison of furling dynamics between the models.
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