Ashes provides a graphical user interface to two external turbulence generator in the  Turbulent wind tool . This section explains the conceptual diferences between both approaches but does not detail the theoretical background behind them as this can be found in the litterature.

The TurbSim generator was developed by NREL. The details of the theory behind this generator can be found in the TurbSim User's Manual here  https://www.nrel.gov/docs/fy09osti/46198.pdf .

When using TurbSim, you can either set the turbulence intensity manually or select a Turbulence c ategory , which will define the turbulence intensity according to the  IEC-61400-1 (2019) . 

The turbulence intensity for different wind speeds will vary as shown in the figure below:

The turbulence intensity is defined as the standard deviation divided by the wind speed. The standard deviation is plotted in the figure below

The theory behind this generator can be found in  Mann (1994e)