jjonkman
commented
2 months ago Thanks, @verlivkra, for reporting this issue.
The reason for these steps is because the wake deficits in FAST.Farm are updated at a rate of DT_Low, but the disturbed wind is processed by OpenFAST at a faster rate of DT. This results in steps in the disturbed wind because the wake deficit data is not interpolated between time steps when the wake deficit varies over time. Reducing f_c reduces the time variation of the wake deficit, reducing the steps. The step changes in disturbed wind could lead to step changes in loads on the downstream wind turbine that would be unphysical.
The proposed workaround is a reasonable compromise until a permanent solution can be found and implemented.
Bug description I am working with two IEA 15 MW reference turbines in FAST.Farm. The upstream turbine is fixed, and the downstream turbine is floating. When plotting the disturbed wind that the downstream turbine sees, there are discontinuities/steps in the wind field, occurring at every DT_Low time step. The discontinuities appear only after the wake comes into play. They are seen for both the polar wake and the curled wake. I have used f_c=0.077 Hz in accordance with the FAST.Farm documentation (equation 4.212 in the main version of readthedocs).
Plotting the axial wake deficit at hub-height 0.5D downstream of the upstream turbine, I see that the wake deficit is noisy when using the recommended cut-off frequency, whereas when f_c is reduced, the wake deficits become smoother.
When reducing f_c, the discontinuities are still present, but they seem to become smooth with time. I have only run 400 s simulations here. With NumPlanes = 400, t_transient = 358 s. See plots of the hub height wind speed at increasing time slots.
Suggested workaround Until this issue is solved in FAST.Farm, a suggested workaround is to use a compromise value of f_c, which captures the core wake deficit over time, while minimizing steps in the wake deficit (f_c = 0.007 Hz seems reasonable for this particular case).