Rotor effective turbulence

[MYMahfouz]

Dear all,

Can I use the output channel RtVAvgxh to calculate the rotor effective turbulence?
I know that this channel is affected by the tower top motions, and hence carries other frequencies from the structure.

What are the other channels I can use?

Best regards,
Youssef

[jjonkman]

Dear @MYMahfouz,

By “effective wind speed”, presumably you are referring to the average ambient wind speed across the rotor disk; is that correct?

AeroDyn’s output RtVAvgxh is the relative wind speed at each aerodynamic analysis node, averaged across the rotor and projected along the shaft axis, which is probably the closest output to what you want, but as you said, includes structural motion effects.

Best regards,

[MYMahfouz]

Dear @jjonkman,

Thank you for your reply. Sorry I think my question was not clear.

I want to get the average ambient turbulence on the rotor inside FAST.Farm by calculating the standard deviation of wind speed divided by the mean value.

So which output channel can I use to get the turbulence acting on the rotor?

I am thinking one of the following approaches:

• Using WindDiskVelX from inflowwind output.
• Using RtVAvgxh from aerodyn.
• Creating a 9 points grid and extracting the values from inflowwind and then use these values to calculate the average turbulence.

Which of these approaches is better?
Is there a channel where I can directly get the turbulence value in FAST.Farm?

Best regards,
Youssef

[jjonkman]

Dear @MYMahfouz,

I would think the latter would be the best. If you use the WindDiskVelX output from InflowWind or the RtVAvgxh output from AeroDyn, these outputs already include spatial averaging, and so, the turbulence calculation would be smoothed out.

FAST.Farm output TIAmbTα for turbine α is the ambient turbulence intensity calculated by FAST.Farm the given rotor, as is used in the eddy viscosity model, but I suspect this is not the turbulence output you want.

Best regards,

[MYMahfouz]

Dear @jjonkman,

I have a final question just to be sure I am setting the grid inside inflowwind properly.

What is the reference frame used to define the grid in Inflowwind? Is it a fixed reference frame or is it relevant to the rotor hub?
So if the turbine sways would this mean that some of the points defined are no longer on the rotor?

Best regards,

[jjonkman]

Dear @MYMahfouz.

The reference frame used by InflowWind is an inertial frame; it does not move with floater or turbine motions. This is why you must always define the wind domain to be larger than the dimensions of the turbine, so, that the turbine remains within the wind domain even when experiencing displacements/deflections.

Best regards