Wind speed changes under steady wind type

[abdullah-74]

Dear @jjonkman,

I am using Fast.Farm ver 3.5.3 to conduct simulations for 1x3 wind farm with NREL 5MW offshore wind turbine constructed on a semi-submersible platform. I have rotated the wind farm and all offshore support structures by 180 deg, to simulate the case for wind along the negative x-axis (The wind in simulation is coming from the positive x-axis as PropagationDir = 0).

When I set the wind type as steady wind at 12 m/s, I see sudden sharp changes in wind speed at certain points (see below). This also occurs for Turbine 1 which should be receiving free stream wind conditions.

Even if I do not rotate the wind farm and offshore support structures T2 and T3 still experience this sudden change in wind speed.

I am not quite sure why this is the case.

Thank you in advance.

[jjonkman]

Dear @abdullah-74,

Can you clarify what you changed when you refer to rotating the wind farm and support structures by 180 deg?

Best regards,

[abdullah-74]

Dear @jjonkman,

I followed guidance from previous questions on how to rotate wind farm from the NREL forum, so what I did was:

• Keep ‘PropagationDir’ equal to 0deg;
• Set ‘PtfmRefzRot’ in ‘HydroDyn’ input module to 180
• Rotate the positions of the fairlead/anchors in the ‘MoorDyn’ input module by 180 deg
• Rotate the joints in the Morison elements, which can be found in the ‘HydroDyn’ input module under ‘MEMBER JOINTS’ by 180 deg

Many thanks,

[abdullah-74]

Here are my input files, in case you would like to have a look
Input files.zip

[jjonkman]

Dear @abdullah-74,

I haven't reviewed your input files in great detail, but how you describe the wind farm rotation makes sense to me.

I'm not fully sure I know what is going on, but is there a reason why the wind point you are outputting wind at is located 210-m downstream of each turbine?

Does the flow visualization you are generating (e.g., via NOutDisWindXY = 1 with OutDisWindZ = 90 m) offer a clue as to what is happening?

Best regards,

[abdullah-74]

Dear @jjonkman,

I have realised the 'InflowWind' module I included in my input zip file is of the 2nd case (when I do not rotate the wind farm). The reason I am outputting wind 210-m downstream of each turbine is that that is the PtfmSurge of T2 and I am interested in the wind speed just before that. The outputting wind in case 1 (where I rotate the wind farm) is:

0 WindVxiList - List of coordinates in the inertial X direction (m)
0 WindVyiList - List of coordinates in the inertial Y direction (m)
90 WindVziList - List of coordinates in the inertial Z direction (m)

The flow visualization for case 1 is below taken at 1998 s and 2997 s, respectively:

It is clear from this that incoming freestream wind is disturbed but I am not sure why this would be the case.

Many thanks,

[jjonkman]

Dear @abdullah-74,

Can you share the input files for the case you are showing the visualization of?

Best regards,

[abdullah-74]

Dear @jjonkman,

My input file for the case is below:
Input files case 1.zip

Many thanks,

[jjonkman]

Dear @abdullah-74,

Based on your input files, I'm not sure where the flow disturbance upstream of the first turbine is coming, but I haven't tried to run your simulation myself. Perhaps this is from some start-up transient involving large floater motion? Do the visualizations earlier in the simulation offer a clue?

Best regards,

[abdullah-74]

Dear @jjonkman,

Thank you for you advice, I think I will have a look at this and have a play around trying to diagnose the issue and will let you know of any updates.

Many thanks,