Load case 1.2
1 Test description
In this load case, we compare the eigenfrequencies of the model between aeroelastic software. The results are given in figure 6 in Jonkman et al. (2010f).
The model is the Offhsore NREL 5-MW reference wind turbine, which corresponds to the offshore template in Ashes. For each comparison, the correpsponding mode shape is applied as initial conditions, as is done in Decay test : For each load case, at the beginning of the simulation
$$t=0$$, all nodes are constrained to move at a given velocity and the constraint is then removed for
$$t>0$$. The velocity applied at each node is scaled with the displacement of that node for a given mode shape, and normalized so that the maximum velocity is
Note: in this benchmark, we only compare the frequency of oscillation of the response (hub node displacement for tower modes, tip displacement for blade modes), and not the amplitude.
In the following sections, the different mode shapes are illustrated.
3.1 First Tower fore-aft
3.2 First Tower side-to-side
3.3 First Blade Collective Flap
3.4 First Blade Asymmetric Flapwise Pitch
3.5 First Blade Asymmetric Flapwise Yaw
3.6 First Blade Asymmetric Edgewise Pitch
3.7 First Blade Asymmetric Edgewise Yaw
3.8 Second Tower Fore-Aft
3.9 Second Tower Side-to-Side
3.10 Second Blade Collective Flap
3.11 Second Blade Asymmetric Flapwise Pitch
3.12 Second Blade Asymmetric Flapwise Yaw
A time simulation with the aforementioned initial conditions is run for 10 seconds. The average oscillation period during these 10 seconds is then compared to the expected values.
The expected values for the eigenfrequencies corresponding to the modes deicted in the previous section can be found in Figure 6 of Jonkman et al. (2010f). In this benchmark, we compare the results produced bt Ashes to the median of all the results submitted by the participants. The test is considered passed if the results from Ashes are within one standard deviation of the median.
The table below sumarises the median and the standard deviation for each mode.
|Mode||Median [Hz]||Standard Deviation [Hz]|
|First Tower Fore-Aft||0.2785||0.004854|
|First Tower Side-to-Side||0.2773||0.004582|
|First Blade Collective Flap||0.6955||0.01434|
|First Blade Asymmetric Flapwise Pitch||0.6643||0.01431|
|First Blade Asymmetric Flapwise Yaw||0.6288||0.02937|
|First Blade Asymmetric Edgewise Pitch||1.0769||0.01186|
|First Blade Asymmetric Edgewise Yaw||1.0870||0.01018|
|Second Tower Fore-Aft||2.414||0.1393|
|Second Tower Side-to-Side||2.3453||0.0536|
|Second Blade Collective Flap||1.959||0.05151|
|Second Blade Asymmetric Flapwise Pitch||1.8118||0.0531|
|Second Blade Asymmetric Flapwise Yaw||1.685||0.1517|
The report with the results can be downloaded from here: