Environmental Engineering Reference
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have to become lighter, meaning that their lowest eigenfrequencies approach a
certain limit. This limit is usually given by the excitation frequency of the rotor
multiplied by the number of the blades. Therefore a model has to be constructed
which includes both aerodynamics and structural dynamics.
Different stages of sophistication have to be identifi ed
I Aerodynamic
1.
Blade Element Momentum Code (BEM)
2.
Wake Codes
3.
Full 3D-CFD including turbulence modeling
II Structural Mechanics
4.
Beam (1D) model
5.
Shell (2D) model
6.
Solid (3D) model
In principle 3 · 3 = 9 possibilities for coupling the various methods can occur. At
the present time (1 with 4) coupling is one of the most commonly used in industry.
Several industrial codes are available, some of which are:
FLEX, from Stig
Øye, DTU
BLADED, by Garrad Hassan
GAROS, by Arne Vollan, FEM
Phatas, by ECN
and many more.
There are several diffi culties in improving the accuracy of this approach. Beam-
like parts of the turbine, like the tower and blades are easy to fi t into the BEM Code.
Not so easy to include are major parts of the the drive train, gear-boxes and the elec-
tromechanical parts such as generators. Also control systems which are used more
and more to decrease loads have to be included. So in the forseeable future, coupling
between more or less standardized and specialized software systems will be seen.
Examples may be MATLAB/Simulink for electrical control, fl exible coupling of
multi-body systems like gear-boxes and shafts with standard FEM tools like ANSYS
or MSC NASTRAN, to name only a few. From fl uid mechanics it is not easy to see
how BEM can be improved. CFD is rather time consuming and not a priori better
when BEM is improved by empirical enhancement. At the present time, coupling of
types (3 with 5) or (3 with 6) is the subject of ongoing research.
5.2 Tasks of aeroelasticity
By far the most important task of aeroelasticity is certifi cation or type-approval.
Most operators of wind-farms can only fi nance, operate and insure their turbine
when appropriately certifi ed. Usually during this process many aeroelastic load
cases have to be simulated. They are documented within rules or guidelines.
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