Hi again everybody and thanks for the previous answer!
I would like to create a simulation where the wind slowly increases from the cut-in speed of the turbine to the rated speed (and over). Of course I can do that with some analytical smooth ramp, but I wish to test the control system ALSO under normal turbulence.
At the moment I am working only with the hub height speed.
What could be the right approach?
adding a smooth ramp to the values of the .hh file? Say for example that I have an .hh file of 2.5 m/sec mean, I could add a lineraly increasing speed from 0 at 0 sec to 7.5 at 600 sec. Of course this would invalidate the turbulence value…
the same but multiplying, instead of adding?
patching together a few TurbSim simulation “segments” with different wind speeds (and accepting the “discontinuities”)
Could this be a suggestion for a future improvement in the TurbSim capability?
Thanks, best regards
Introducing a ramp in the wind speed generated by TurbSim by some external means is not recommended. If the ramping time should take place over a significant part of the simulated record length the process will violate the basic assumptions upon which the turbulence generation process in TurbSim operates. The reason for this is the underlying theory assumes the process is stationary and a significant change in wind speed from cut-in to rated this assumption does not apply. Further, in real flows the turbulence characteristics are also changing as the speed increases and something that will not happen in TurbSim. The only way to do this correctly would be to use either a Large-Eddy or Direct Numerical Simulation (LES or DNS) of the desired flowfield which, of course, won’t run in a few minutes let alone the work involved. With these simulations the turbulent characteristics will change as the speed increases.
Wind Speed Step Part 2:
This did not get sent with the original post.
As far as piecing together small records with different wind speeds to form the ramp, I do not recommend it. Not only do you run the risk of introducing discontinuities in the wind speed but in the turbulence characteristics as well. The combination could easily induce unwanted transients in the turbine response that could make interpretation difficult.
While it does not reflect what will be happening in actual flow with relatively slowly ramping wind speed, the approach of removing the mean from the hh time series and then adding the ramp to the zero-mean turbulence signal is about the best you can do. I should mention that it is possible to obtain large ramps in wind speed from simulations over short amounts of time when high levels of turbulence are requested. The only way to know is to examine the simulated records and could take a lot of runs.