Gradient Turbulence Intensity (gTI)

Background

Standard IEC 61400-1 wind models assume a spatially uniform turbulence intensity across the rotor disk — the same standard deviation of wind speed applies at the blade tip as at the hub. In reality, the atmospheric boundary layer produces turbulence that varies with height. Near the ground, where wind shear and surface roughness dominate, turbulence intensity is higher. At greater heights, the flow is smoother. For large modern turbines with rotor diameters exceeding 200 m, this vertical gradient in turbulence intensity can span a significant fraction of the rotor disk and influence blade fatigue loading in ways that uniform turbulence models do not capture.

This phenomenon is well documented in the wind energy literature and in measurement campaigns at tall towers and offshore met masts. The gradient turbulence intensity parameter (gTI) provides a simple, IEC-compatible way to impose such a gradient in TurbSim wind fields.

What gTI means

The gTI parameter is defined as the ratio of turbulence intensity at the bottom of the rotor disk to turbulence intensity at the top:

gTI = TI(z_bottom) / TI(z_top)

A value of 1.0 (the default) gives uniform turbulence — identical to standard IEC NTM behaviour. A value greater than 1.0 means turbulence is higher at the bottom of the rotor than at the top, reflecting typical boundary layer conditions.

Implementation in TurbSim

TurbSim supports user-defined wind speed and turbulence profiles through its USR (user-specified) profile type. FlowUrja Studio uses this mechanism to impose a gTI gradient while keeping the hub-height turbulence intensity consistent with the IEC NTM target for the selected turbulence class.

Sigma profile derivation

For a given hub-height reference turbulence intensity $\sigma_{hub}$ (derived from IEC 61400-1 NTM at the selected wind speed and class), FlowUrja Studio computes the along-wind standard deviation $\sigma_u(z)$ at each grid height $z$ as:

σ_u(z) = σ_top × (1 + (gTI − 1) × α(z))

where $\alpha(z)$ is a normalised height parameter ranging from 0 at the top of the rotor to 1 at the bottom. The value of $\sigma_{top}$ is back-computed so that the mean of $\sigma_u$ over the rotor disk equals the IEC NTM target $\sigma_{hub}$:

σ_top = σ_hub / mean(1 + (gTI − 1) × α(z))

This ensures that:

• The hub-averaged turbulence intensity matches the IEC class target
• The bottom-to-top turbulence ratio equals exactly gTI
• The profile transitions smoothly across the rotor height

USR profile file

For cases where gTI > 1.0, FlowUrja Studio automatically switches TurbSim from a standard power-law mean wind profile to a USR wind profile (WindProfileType = USR). A companion .profiles file is written containing five columns per height:

z (m)  U (m/s)  WindDir (deg)  σ_u (m/s)  L_u (m)

The mean wind speed column follows the power-law shear profile. The $\sigma_u$ column carries the gTI gradient. Integral length scale $L_u$ is held constant with height at the IEC-specified value.

Each batch case writes its own .profiles file, preventing race conditions when multiple TurbSim processes run in parallel.

When gTI = 1.0

When gTI is set to 1.0, no USR profile is written and TurbSim uses its standard IEC mode — the behaviour is identical to a conventional TurbSim run. This means gTI can be included as a sweep axis with 1.0 as a baseline without any change to the reference cases.

Setting gTI in FlowUrja Studio

Single wind field

In the Wind Field (TurbSim) panel, the TI asymmetry (gTI) slider in the Rotor Parameters section sets the gTI value for that case. The default is 1.0.

Batch sweep

In the Wind Field Batch panel, gTI is one of the sweep axes. Enter a comma-separated list of values in the TI asymmetry (gTI) field:

1.0, 1.15, 1.2

Cases with gTI = 1.0 are generated using standard IEC mode. Cases with gTI > 1.0 use the USR profile. Both types are seamlessly mixed in the same batch run and the case names reflect the gTI value:

Custom_IECKAI_TI004_NTM_V12ms_149m_15x15_PLX010_s01        ← gTI = 1.0 (omitted)
Custom_IECKAI_TI004_NTM_V12ms_149m_15x15_PLX010_gTI115_s01 ← gTI = 1.15
Custom_IECKAI_TI004_NTM_V12ms_149m_15x15_PLX010_gTI120_s01 ← gTI = 1.20

Physical interpretation

A gTI value of 1.15 means the bottom of the rotor experiences 15% higher turbulence intensity than the top. For an IEC Class A turbine (hub TI ≈ 18% at 15 m/s), this translates to roughly:

Height	σ_u	TI
Top of rotor	lower	~16.5%
Hub height	reference	~18%
Bottom of rotor	higher	~19%

The magnitude of this gradient depends on the rotor diameter, hub height, and site conditions. Values in the range 1.1–1.3 are physically representative of many onshore and nearshore sites.

Effect on loads

Imposing a turbulence gradient increases the asymmetric loading on the rotor — the upwind blade encounters a different turbulence environment at the bottom of the sweep than at the top. This contributes to:

• Increased tower side-side bending moments
• Increased blade flapwise fatigue loads at the lower positions
• Increased rotor tilt and yaw moments

The magnitude of these effects relative to uniform-turbulence IEC loads can be quantified by comparing gTI > 1.0 cases directly against gTI = 1.0 reference cases run at the same wind speed and seed.