I’d like your feedback on whether or not you think it is OK to change the way we use turbulence files in AeroDyn. I propose that we no longer abort the program if an analysis point lies outside the wind field and that we use the outer-most point instead. I also propose that we fix the wind field to the non-inertial hub instead of the inertial ground system.
The way TurbSim and AeroDyn work now is that TurbSim adds enough time to the requested length of the simulation to account for the fact that analysis points can be downwind of the turbine at the beginning of the simulation or upwind of the tower at the end of the simulation. The amount of time it adds is the grid width divided by the mean wind speed. For instance, if the turbine has a 90-degree yaw error at the start of the run, the tips may be one rotor radius downwind of the tower. At the end of the simulation, a 90-degree yaw error can cause a blade tip to be one rotor radius upwind of the tower.
I did it this way when I first put the full-field wind-file capability into AeroDyn; I found that the program was trying to index out of the array bounds before then. We run into a similar situation when excessive tower motion causes an analysis point to leave the grid in the lateral direction. We figured the worst-case scenario would be when a teetering turbine had a 90-degree teeter angle and and a 90-degree yaw error such that the blade tip could be the rotor radius plus the overhang to the side of the tower. In TurbSim, we currently recommend that you set the grid size to be twice that sum.
At the time I did this (which was nearly 15 years ago), we decided it was better to make the turbulence field a little bigger to avoid these problems instead of extrapolating outside the grid or simply using the outermost values.
Enter the offshore turbine.
Some turbines may be mounted on floating platforms that are moored using catenary lines that allow the platform to move around a bit. The motion may be tens of meters. To accommodate that much motion would require that we make the turbulence fields significantly larger and we would lose spatial resolution.
One possibility is to use the outermost value when an analysis point leaves the turbulence field. Doing so would allow us to use smaller grids. We won’t even have to accommodate the entire rotor diameter just enough to get to the outermost analysis points, which are usually not at the tip. However, with the possibility of so much motion for floating platforms, significant portions of the rotor may lie outside of the wind field.
My proposal today is to do things a little differently. I’d like to pin the center line of the wind field to the apex of the cone of rotation. That means that the wind field will move around in inertial space. If we ignore this issue, the only problems we would have remaining are at the start and end of the simulation where we could have analysis points outside the wind field. That’s no big deal at the start of the simulation because we usually throw away the first 10-30 seconds anyway. Only if we had a large yaw error would the amount of time of using constant wind be significant anyway. A large yaw error will cause constants winds at the end of the simulation, but in TurbSim we can automatically add a few seconds (R/u) to the wind field to accommodate the extra time we may need at the end of the simulation when we are recording the data. We are currently adding twice that much.
One thing I like about doing it this way is that the winds at T=0 are the ones that are actually affecting the rotor. Our current scheme has it so that (for an upwind turbine) the wind-file time affecting the rotor at the start of the simulation is the (overhang+0.5*gridwidth)/u. For a 100 m rotor with a 2 m overhang in a 5 m/s wind, that’s 10.4 s.
Addressing the issue of allowing the wind field to move around in inertial space, it seems to me this whole business of using stochastic winds is just a way to excite the rotor in a semi-realistic way. I don’t think anyone believes our simulated winds look anything like real winds.
So, I have two questions:
1) Do you thing it’s OK to use tighter grids and instead of stopping the program if you have a point outside the wind field, just use the outermost value?
2) Is it OK to pin the wind field to the rotor instead of holding it fixed in inertial space?