IEC Turbulence Modeling

Standard 61400-1 of the International Electrotechnical Commission forms the basis of Windographer's treatment of turbulence intensity (TI) data. Windographer incorporates aspects of 4th edition of that standard, released in 2019, the 3rd edition (2005), and the 2nd edition (1999).

IEC 61400-1 4th Edition (2019)

The 4th edition of IEC 61400-1 defines four turbulence categories based on the reference turbulence intensity I_ref, which is the mean TI at a wind speed of 15 m/s:

Turbulence Categories in IEC 61400-1 4th ed.

Category	I_ref	Description
A+	0.18	Very high turbulence
A	0.16	High turbulence
B	0.14	Moderate turbulence
C	0.12	Low turbulence

It further defines the Normal Turbulence Model (NTM) as the following equation for representative TI versus wind speed:

where:
	I_P90	is the representative TI [unitless]
	I_ref	is the reference TI for one of the above turbulence categories [unitless]
	U	is the wind speed [m/s]

IEC 61400-1 3rd Edition (2005)

The 3rd edition of IEC 61400-1 defined four turbulence categories, also based on representative TI:

Turbulence Categories in IEC 61400-1 3rd ed.

Category	I_ref
S	> 0.16
A	0.16
B	0.14
C	0.12

The 3rd edition use the same definition of Normal Turbulence Model (NTM) as the 4th edition.

IEC 61400-1 2nd Edition (1999)

The 2nd edition of IEC 61400-1 defined three turbulence categories based on the characteristic TI at 15 m/s wind speed:

Turbulence Categories in IEC 61400-1 2nd ed.

Category	Characteristic TI at 15 m/s
S	> 0.18
A	0.18
B	0.16