Clarification on ABL perturbation parameters (cutoff_height, perturb_ref_height, Uperiods, Vperiods, deltaU, deltaV)

[passboy666]

Hello,

I am running neutral ABL precursor simulations with AMR-Wind and I would like to better understand the role of several perturbation-related parameters:

ABL.cutoff_height = 50.0
ABL.perturb_ref_height = 960.0
ABL.Uperiods = 4.0
ABL.Vperiods = 4.0
ABL.deltaU = 1.0
ABL.deltaV = 1.0

  From the documentation, I understand that ABL.perturb_velocity = true introduces initial velocity perturbations to help trigger turbulence. However, I am unclear about the detailed meaning of these specific parameters:

1.cutoff_height: Does this mean that perturbations are only applied above 50 m, or below 50 m, or that the profile is scaled differently near the surface?
2.perturb_ref_height: Is this the vertical reference height used to normalize the perturbation shape? If I set it close to my hub height (150 m), will it strengthen perturbations near the rotor?
3.Uperiods / Vperiods: Do these control the number of sinusoidal modes (periods) in the streamwise and spanwise directions of the perturbation field?
4.deltaU / deltaV: Are these the amplitude (in m/s) of the perturbations in the streamwise and spanwise directions?

In my case, the turbine hub height is 150 m and the domain top is at 960 m. I want to understand how adjusting these parameters influences turbulence levels near the surface vs. at hub height and aloft.

Could you please clarify the physical meaning of these parameters and provide guidance on how to tune them for wind turbine simulations?

Thank you!

[passboy666]

I am running neutral ABL precursor cases in AMR-Wind. From my simulation results (see figure below), I observe that:

Near the surface (e.g. ~30 m), turbulence intensity and TKE are too high.

At hub height (150 m) and blade tip height (270 m), turbulence is too low.

This results in an unrealistic vertical turbulence distribution: the lower part of the boundary layer is very energetic, but turbulence decays too quickly with height.

I would like to ask:

Which parameters most strongly control this balance? For example:

ABL.surface_roughness_z0

Smagorinsky_coeffs.Cs

ABL.cutoff_height

ABL.perturb_ref_height, deltaU, deltaV

ABL.wall_shear_stress_type (e.g. "local" vs "Moeng")

If the goal is to reduce turbulence near the surface but enhance it at hub height and aloft, what adjustments would you recommend?

Does the temperature profile (e.g. neutral with inversion cap at 750 m) also play a role in how turbulence is distributed across the boundary layer?

For reference, my current settings include:

Hub height: 150 m
Tip height: 270 m
ABL.surface_roughness_z0 = 0.2 m
Smagorinsky_coeffs.Cs = 0.135
ABL.perturb_velocity = true
ABL.deltaU, deltaV = 1.0
ABL.perturb_ref_height = 960 m
ABL.cutoff_height = 50 m

Any guidance on how to tune these parameters to achieve a more realistic turbulence distribution would be very helpful.

Thank you!

[hgopalan]

The perturbations are a means to trigger the unsteady flow structures in LES. After the initial transience, the mean wind speed and turbulence profiles is independent of the values.

Capping inversion plays a role in the hub-height wind speed. You can always set a very high capping inversion to begin.

Turbulent intensity at a desired height is related to the roughness length of the lower surface. You need to adjust the value to get the roughness height, for a fixed Geostrophic/Driving wind.

[passboy666]

How to adjust parameters so that turbulence above the rotor increases and matches the standard turbulence profile?
Which parameters are most effective for increasing turbulence above the rotor so that the profile matches the standard ABL?

Should I tune the cutoff height or perturbation parameters (ABL.cutoff_height, deltaU, deltaV)?

Or is it better to adjust the surface roughness z0, turbulence length scales, or forcing height?

Has anyone encountered a similar mismatch between simulation and standard ABL profiles?

[hgopalan]

None of those parameters affect the generation of turbulence.

I do not know what you mean by standard profile?

We are running LES with a specified Geostrophic wind/ABL forcing and surface roughness and an optional temperature inversion. You need to play around with those parameters to obtain the desired turbulence intensity at your hub height.