Normal Turbulence Model

The Normal Turbulence Model (NTM) is an equation for representative TI versus wind speed given in the IEC 61400-1 standards. The 3rd and 4th editions of the standard define the NTM with the following equation:

where:

IP90  is the representative TI [unitless]
Iref  is the reference TI for the relevant turbulence category [unitless]
U  is the wind speed [m/s]

IEC 61400-1 4th Edition (2019)

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

Turbulence Categories in IEC 61400-1 4th ed.
Category Iref Description
A+ 0.18 Very high turbulence
A 0.16 High turbulence
B 0.14 Moderate turbulence
C 0.12 Low turbulence

The graph below plots this equation for the four IEC 4th edition turbulence categories:

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 Iref
S > 0.16
A 0.16
B 0.14
C 0.12

See also

Turbulence intensity

Representative turbulence intensity

Turbulence Analysis window

Turbine Spacing window

IEC turbulence modeling

Written by: Tom Lambert
Contact: windographer.support@ul.com
Last modified: October 28, 2020