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).

2 Model

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 
, all nodes are constrained to move at a given velocity and the constraint is then removed for 
. 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 
$$1\text{ m}\cdot{s}^{-1}$$

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.

3 Benchmarks

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

4 Results

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.

ModeMedian [Hz] Standard Deviation [Hz]
First Tower Fore-Aft0.27850.004854
First Tower Side-to-Side0.27730.004582
First Blade Collective Flap0.69550.01434
First Blade Asymmetric Flapwise Pitch0.66430.01431
First Blade Asymmetric Flapwise Yaw0.62880.02937
First Blade Asymmetric Edgewise Pitch1.07690.01186
First Blade Asymmetric Edgewise Yaw1.08700.01018
Second Tower Fore-Aft2.4140.1393
Second Tower Side-to-Side2.34530.0536
Second Blade Collective Flap1.9590.05151
Second Blade Asymmetric Flapwise Pitch1.81180.0531
Second Blade Asymmetric Flapwise Yaw1.6850.1517

The report with the results can be downloaded from here: