Final report for WP4.3: Enhancement of design methods ... - Upwind

In every time step the system matrices of Flex5, i.e. mass M, stiffness S, damping D, and the current loading
F is transferred to the finite element code. There the models are combined on the level of equations of
motion and the hydrodynamic loads are added. The equations are solved in the finite element code. The
results, i.e. displacements, velocities and accelerations ( x, x&,
&x
&)
are used in Flex5 to update the geometry,
the loading and the controls. Figure 2.2 shows the coupling process and data exchange.
Flex5
FE-Code
t n-1
t n-1
Newmark
Solver
x, x& , && x
13
t n
t n
M Flex , D Flex ,
S Flex, F Aero
Newmark
Solver
t n+1
t n+1
shared memory or
DLL interface
Figure 2.2: Process of the full integrated coupling between Flex5 and a finite element code [7]
Three full integrated couplings have been realized: Flex5-FECOS [6], Flex5-Poseidon and Flex5-ASAS
[7]. The first one was created within UpWind, the two other couplings have been created in the Project
OWEA - Verification of offshore wind turbines [8]. The interface and the modelling capabilities are different
between the tools, but the coupling method is the same. FECOS is a finite element code compiled as a
dynamic link library (DLL) and is included in Flex5 directly. Therefore no special interface is necessary to
handle the data exchange. Flex5-Poseidon uses a shared memory interface where both tools access the
same memory. Flex5-ASAS uses a separate DLL for the data exchange.
Advantages and disadvantages
An important advantage of the full integrated coupling is that specific simulation codes are combined without
losing their potential. The application possibility of the coupled tools has been extended. The method
considers dynamic effects between the wind turbine and the support structure. The load calculation takes
relative velocities of the entire structure into account. Non-linear problems can also be simulated using
Flex5-ASAS.
The main disadvantage is the computational time. The simulation speed is lower than the sequential approach
and much lower than a Flex5 stand-alone simulation. The reasons for this can be found in the
large number of freedoms and the data exchange between the tools in every time step. The simulation
speed can be increased if a reduction method is implemented in the finite element code, but this restricts
the simulations to linear problems.
2.2 Combined multi-body / modal approach
This section describes the GH Bladed software code as an example of an integrated design tool for calculating
wind turbine performance and dynamic response [9]. Bladed was originally developed for the modelling
of onshore fixed-bottom wind turbines, but has been extended to include hydrodynamic loading for the
modelling of offshore wind turbines. In the last year the core structural dynamic calculation in the code has
been extended to incorporate multibody dynamics.
The Bladed code uses a modal representation to model the structural dynamics of a wind turbine. This
approach has the major advantage of giving an accurate and reliable representation of the dynamics of a
wind turbine with relatively few degrees of freedom, making it a fast and efficient means of computation. In
the previous version of the Bladed code, the modal properties of the rotating and non-rotating components
of the system (i.e. the rotor and tower) were computed independently using a finite element representation
of the structure, then coupled together using the appropriate equations of motion in the dynamic response