We are often asked how big turbulence grids should be to ensure that part of the turbine does not move outside the turbulence volume during a FAST simulation. If your wind-field volume is not large enough, FAST will generate a message similar to: “FF wind array boundaries violated. Grid too small in Y Direction.”
I usually make the grid width and height about 10% bigger than the rotor for land-based or non-floating, sea-based turbines. That accommodates elastic behavior of the turbine and support structure.
For a floating turbine—especially one with catenary lines—you need to consider how far it will move vertically and laterally. To determine how tall the grid needs to be, I run FAST with the aerodynamics disabled and throw the largest waves at it that it is likely to see. Add twice the largest heave to 1.1 times the rotor diameter to determine the height of the wind grid. To determine the maximum lateral motion, I run a maximum thrust case to determine the maximum surge. The maximum thrust case can be modeled using a hub-height wind file with the wind speed near rated. Calm seas are probably OK to use. Although the surge is a downwind motion, it gives you an idea of the maximum sway the turbine will experience, which could be caused by a current that is aligned to be 90 degrees from the nominal wind direction. Add twice the maximum surge to 1.1 times the rotor diameter to determine the width of the wind grid.
Another consideration in determining the wind grid width for floating turbines is platform roll. If your platform tends to heel easily, you may want to add some extra size to the grid width and maybe even the grid height to accommodate the turbine motion. Because it takes so long to generate long, high-resolution wind files, I recommend you generate all the FF wind files using low-resolution, 10-minute wind files and only half the usual number of seeds for each design load case (DLC). Run all the DLCs to see if the rotor ever violates the wind-field boundaries. If it does, FAST will abort and tell you that it had tried to index outside the grid. If you get “FF wind array was exhausted at xxxx.xx seconds,” it means the combination of surge and pitch has pushed one of the blades past the beginning or end of the wind volume. If you get “FF wind array boundaries violated. Grid too small in Y Direction,” it means a combination of sway and roll caused the problem. If the error is in the Z Direction, it means a combination of heave and heel (roll and/or pitch) caused the problem. Heel can cause one of the blades to get too close to the mean sea level. You are more likely to violate the grid at the bottom boundary.
Because the platform motion is likely to be largest in the lateral direction than the vertical direction, you will probably end up with a rectangular grid.
Hi! im a little confused with grid sizes:
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Im trying to model: NRELOffshrBsline5MW_Floating_TLP
1.1)I did not change any data with input files downloaded from your site. I get the error, FF wind array boundaries violated. what would be the recommended grid size for this?
1.2.)You mentioned that gridsize should be 1.1xrotor diameter, does it mean that it is my minimum grid size? Like for example for the 5MW WT, rotor diameter is 126m, so my grid size shouldn’t be less than 140???
2.) i tried making turbsim files with grid sizes greater than 100 but i get the error: error allocating file size 0MB, is there a maximum grid size that you can create, or do i need to use faster computer to produce the file?
3.) i also tried to model a 1.5MW WT. why is that 7x7 gridsize is sufficient to simulate the WT? the 1.1x rotor diameter is not applicable?
Jerome,
You get the “FF wind array boundaries violated” error when part of the turbine falls outside the predetermined wind volume. This is especially problematic for floating turbines. Please reread the original post in this thread about how to deal with floating turbines. The 1.1 factor applies only to land-based turbines. You can also get this error if you request tower aerodynamics without asking TurbSim to generate winds for the tower.
TurbSim’s memory requirements do not depend on the size of the grids but the number of grid points and time steps. See our FAQ on this subject ([url]National Wind Technology Center's Information Portal | Wind Research | NREL). If the program does not have sufficient memory, you will either have to reduce the number of grids points or use the 64-bit version of the program. I think TurbSim64.exe is included in the archive.
The sample input files in the CertTest folder are there for two reasons. They allow us to do a regression test to see if we broke anything when modifying the program and they give users sample input files. Because we need to keep the size of the archive file small, we do not use high-resolution wind files. Our rule of thumb for grids is to make the distance between grid points approximately equal to the chord length at the 70% radius. See my FAQ on field sizes for more help ([url]National Wind Technology Center's Information Portal | Wind Research | NREL). For a typical turbine of moderate solidity, this comes to about 41x41 (for a land-based turbine). For one-hour wind files, such a high-resolution grid will require 64-bit versions of both TurbSim and FAST. You may have to scale back from that for practical reasons.
Hi, Jerome.
I think you are getting confused about what Marshall refers to as “size”. For clarification, the “1.1 x rotor diameter” refers to TurbSim’s GridWidth and GridHeight input parameters, and will be measured in meters. (The grid in your question 1.2. shouldn’t be smaller than 140 meters.)
The grid density is determined by TurbSim’s NumGrid_Z, NumGrid_Y, and TimeStep inputs, which give the number of points in each direction of the grid (time determines the X direction).
hello everyone
When I use the TurSim.exe,it produce the .sum and .wnd,when I use the 17and17 rigd sizes ,FAST abort with error:
but when I set the rigd sizes 8/8,FAST run normally.
I have checked the output channel and found no error. Is the result file(.out) normal?I hope the rigd size will not affect the output result.
BEST REGARD
Ruiliang.Wang
Hi.
I am not sure what you mean by “rigd sizes”; I assume you mean the “number of grid points in the y and z directions”. If the number of grid points is the only thing that you changed in the TurbSim simulation, I would think you would get the same error with both wind files.
It looks like your wind file was generated with about 40 seconds of usable time, but you are trying to simulate 630 seconds in FAST.
Set UsableTime to 630 seconds in TurbSim and select an appropriate number of grid points for the turbine you are modeling, then rerun TurbSim and rerun FAST with the new wind file. I suggest you read the “Turbine/Model Specifications” section of the TurbSim User’s Guide to understand the TurbSim inputs that determine the size of the turbulence grid.
Hello,Bonnie
Thank you for your help.I think I understand it.I appreciate it very much.
Best
Ruiliang.Wang