Review of

Dong et al.

"On Turbulence Models and LiDAR Measurements for Wind Turbine Control"

submitted to Wind Energy Science, wes-2021-51

The paper numerically investigates the spectral coherence between a

simulated lidar measurement of the wind inflow to a wind turbine on

the one hand, and the rotor-effective wind speed (REWS) at the turbine

on the other hand. The incoming wind field is simulated by two

different, commonly used turbulence models, namely the Kaimal and the

Mann model. Results are evaluated for three different rotor diameters,

namely 52 m, 126 m, and 178 m. It is found that the above-mentioned

coherence significantly differs between the turbulence models as well

as between the rotor diameters. Namely, the coherence decreases with

the rotor size, and this decrease is stronger for the Mann model.

The methodology and investigations are based on results by Held and

Mann (2019) where a significant difference in coherence was found

between the same turbulence models. In that paper, only a 52 m rotor

diameter was investigated, and additional validation against

experimental field data was performed. As a consequence, it was found

that the predictions of the Mann model came closer to reality than

those of the Kaimal model.

The paper under review investigates a relevant and interesting topic,

namely systematic differences between turbulence models, and their

relevance for wind turbine applications. The numerical investigations

are performed systematically and they deliver clear results. The main

conclusion of the paper is that the systematic differences in

coherence depending on the turbulence model should be considered as

uncertainties for relevant applications.

One fundamental weakness of the paper is that turbulence models,

including the two under investigation, differ significantly from real

turbulence. Conclusions from the results of the paper for real-world

applications are therefore very difficult and should be discussed with

care. Other than in Held and Mann (2019), no experimental validation

was performed here.

General remarks

The text of the paper is not strictly systematic. A more strict

systematic would improve the text.

Especially, conclusions are already drawn in section 4.3 (last

paragraph), but they should be moved to section 5. In the conclusions

themselves, these findings are missing. Moreover, two further

important findigs of the paper are completely missing:

* The rotor size influences the difference in coherence between the

  turbulence models.

* An experimental validation of the findings, especially for large

  rotors, is essential for further application.

A few inappropriate terms make the understanding of the unneccessarily

difficult. Those are

* Lidar measurement quality (P1L23)

  This term is used frequently throughout the paper. It does make no

  sense at all in a simulation study, because the quality of the lidar

  measurement is not accessible and also not investigated here. This

  causes unneccessary confusion. What is meant (to my understanding)

  is the quality of the prediction of the REWS by the two turbulence

  models, given the information of a simulated lidar measurement

  upstream of the rotor.

  The term should be replaced by something more appropriate, like

  "REWS prediction quality" (this is probably not the best term

  either), and it should be clarified and explained when first used.

  In contrast to this, the term "lidar measurement coherence", which

  is also used frequently, does actually make sense, even though no

  real measurement is performed. 

* Value creation of LAC (P1L8)

  Without any relation the field measurements, it is not appropriate

  to speak of "value creation" in this context. There is no way to

  evaluate the real benefits of LAC based on the presented results.  

* Lidar measurement error (P5L138)

  It is not clear what this term means here. Error of what compared to

  what exactly?

Moreover, please make sure that all technical terms are clearly

defined or explained at their first occurence.

Detailed remarks

1. P7L159: "the choice of turbulence model strongly influences the

coherence of LiDAR measurements"

See above, "lidar measurement quality". Reformulate to what is

actually meant.

2. P9L178: "the true velocity measured by a lidar"

What is meant by "true velocity"? Is is the LOS component a a certain

point? Keep in mind that velocity is a vector by definition. Make clear. 

3. Section 4.1: Was the lidar measurement modeled after section 3.2?

Was the REWS evaluated after section 3.3? What are the time constants

in the measurement?

4. Section 4.2: The discussion of eddy sizes (line 219 ff) is

confusing. First, the mentioned integral length scale is questionable

and will most probably depend strongly on the time window used for the

analysis. Moreover, what is meant by "the eddy size" is probably "the

size of the largest eddies", which in turn is a questionable quantity.

A spherical eddy is hard to imagine. Morover, if it would exist,

pitching the blades to "feather" would not help in decreasing the

loads.

These aspects are, however, unneccessary for the relevant part of the

discussion. Namely that, given Taylor's hypothesis of frozen

turbulence, the smallest relevant time and length scale for collective

pitch control is of the order of the rotor diameter. The authors

should restrict the discussion to this aspect.

In this paper, the authors compare two different turbulence models (the Kaimal model and the Mann model) and two different LiDAR scanning protocols to assess their potential for LiDAR assisted control (LAC). The focus of this work is on larger rotor sizes. The comparisons are based on numerical implementations of the turbulence models and simulations of the LiDAR scanning protocols. The authors find a good agreement of the magnitude-squared coherence between simulated LiDAR measurements and the rotor effective wind speed with the theoretically expected ones.

Overall, the work is interesting, clearly structured, and well written. The main finding is that the asserted coherence depends strongly on the specific type of turbulence model. In fact, it is shown that the assertion of the benefit for LAC depends more on the turbulence model than on the specific LiDAR scanning pattern.

This brings me to my main question. Whether LAC is beneficial or not ultimately depends on the coherence of the atmospheric turbulence. This means that the major question should be which turbulence model is better suited to capture atmospheric turbulence. In that sense, sentences like "the value creation of LAC, evaluated using the Kaimal turbulence model, will be diminished if the Mann turbulence model is used instead." seem inappropriate to me since no direct conclusions for field measurements can be drawn. I think the authors should clarify their scope and in particular the limitations of their study.

Another question concerns the choice of parameters. Many of the parameters in this study are kept fixed, which makes me wonder how much the conclusions depend on the specific parameter choices. Can the authors comment on that?

Minor comments:

* Eq. (5): The argument of R_ij should be boldfaced.

* I don't understand the statement below eq. (7): "When the two indices i=j, then Delta_y=Delta_x=0..." Can't I vary i, j, Delta_x, and Delta_y independently? I think this statement needs clarification.

* Line 114: I think "the" in "especially for the Mann model" should be deleted.

* Eq. (9): The notation is a bit difficult to understand: Shouldn't all vectors be boldfaced? Perhaps it is worth checking the manuscript once more regarding the consistency of notation.

Once these aspects are addressed, I can recommend publication in WES.