Turbsim AnalysisTime

Wind & Water Inflow Turbulence
1

Hello, I’m trying to use Turbsim with AnalysisTime set to 600 and UsableTime set to “ALL” to generate a wind field that I can use for a 4200s OpenFAST simulation (1h+600s transient to be discarded). I noticed that the resulting FFT amplitude of the wind velocity signal seems to differ significantly from the one generated with AnalysisTime set to 630, all other inputs the same, even though the time signals look alike:

image
image1511×722 143 KB

image
image1514×681 150 KB

The statistics of the two signals are also not identical:

image

I’m using OpenFAST to precompute the aerodynamic loading needed by my frequency-domain model of dynamics, and this discrete-like behaviour of the FFT amplitudes impacts my motion results down the path. Could you help me understand where the difference comes from, and whether this is something that should be avoided, please?

2

Dear @Kasia.Patryniak,

By changing AnalysisTime between separate TurbSim runs, you are effectively changing the frequency step (df) of the internal FFTs used by TurbSim, which will result in different time series.

The “discrete-like” behavior of the FFT is likely the result of resampling the time series, considering that it appears that you are extracting the wind time series not from the TurbSim output directly, but from the InflowWind module of OpenFAST, which has a a different time step (fmax) and simulation length (df).

I’m not sure I understand your point about a 3600-s simulation + 600-s start-up transient and why you are setting AnalysisTime = 600 s. This implies to me that you should set the AnalysisTime in TurbSim to 3600 s (with UsableTime = “ALL”).

Best regards,