# Error from FF .wnd file when running 'FAST'

Hi All,

A couple of errors are generated when using a FF .wnd file in FAST. I am unsure why the code is attempting to access a negative time step? I assume the second error results from the first one.

In addition is there any way I can force Dynamic Inflow to remain active even if the velocity is lower than 8m/s?? I assume I would have to modify the source code, but where exactly?

Thanks,

Pascal

The InflowWind module in AeroDyn calculates a shifted time when it uses the FF wind files. At time zero, if you ask for wind behind the tower centerline, it effectively asks for a negative time. (The first grid in a wind file is GridWidth/2 meters past the tower centerline so that if the turbine is yawed 90 degrees, the wind is defined at those points. This shifted time also takes the X position into account, using Taylor’s Hypothesis.)

Here is the equation: TimeShifted = TIME + ( GridWidth/2.0 - X )/MeanWindSpeed

I used the numbers shown in your simulation output and calculated that you are asking for wind at X = -0.676152 meters at the start of your simulation. Because the grid is only 0.9 meters wide, it can only provide winds up to -0.45 m. It looks like there is a mismatch between your turbine and the wind field you generated for it. We recommend the wind file width and height be at least 10% larger than the rotor diameter.

The section in AeroDyn.f90 labeled
!----------------------------------------------------------
! Turn off dynamic inflow for wind less than 8 m/s …
! …

is what you will have comment out. I am not an expert in aerodynamics, but I will warn you to watch for numerial issues if you do choose to go this route.

Hi Bonnie,

Thanks for your detailed reply, after increasing the grid size, FAST terminated normally without any other error messages.

I will try to recompile FAST with the lines commented out as you suggest for Dynamic Inflow. Hopefully the numerical instabilities won’t arise becuase it is for a scaled tidal turbine simulation with signifiacntly slower flow speeds.

Regards,

Pascal

Hi Bonnie,

I have attempted to re-compile fast with the lines commented out as described above. Unfortunately the free compilers I have tried don’t work (g95 and a few others return errors when looking for .mod files after compiling from FAST_Prog.f90). I know effectively nothing about compiling programs since I generally work only with matlab. Could I be cheeky and ask you to email me a re-compiled version of the FAST executable with the lines commented out to enable dynamic inflow for all conditions? This would be much appreciated and save me hours of additional work, unless you can suggest an easy approach for me that I do not know about?

Thanks in anticipation,

Pascal.

Dear Pascal,

Once you find a need to change source code, you will most likely need to recompile again and again. I suggest you put in the effort to learn yourself. There have been many compiling questions on this form, so, there should be plenty of help/guidance.

Best regards,