GENERAL COMMENTS

This is an interesting and well-written paper, relevant to the airborne wind energy community. The authors develop a framework to simulate multiple ground-gen AWES with LES, while running a Model Predictive Controller. With this tool, they simulate a system from the literature and analyze the performances of two systems, where the second system is operated to avoid or to encounter the wake of the first system. The results show that the second system can be operated such that it almost entirely avoids the wake of the first, producing higher power.

I have minor reviews and a few suggestions for further analyses, which could be incorporated here or in future works.

SUGGESTED ADDITIONAL ANALYSES

It would be nice to have a deeper explanation of how the aerodynamic force contributes to the reduction of wind speed for a Ground-gen AWES. Indeed, the wing is moving downstream, because of the reel-out, and generates an aerodynamic force. From the force of the lifting line and its velocity you should be able to reconstruct the work done on the fluid from the AWES. Note that this work should be directly comparable with the power available at the ground station, taking in account the work done by the tether drag, the change in kinetic energy of the AWES and the exchange of potential gravitational energy. These two last quantities are conservative, so that after a full periodic cycle there should be a balance between the aerodynamic work and the electric energy. It would be nice to have some insights on the work done on the fluid from the AWES, which can hardly be reconstructed by the plots given in the paper because of the reel-out speed and of the kite attitude. This could provide a more detailed explanation on the skewed reduction in wind speed.

DETAILED REVIEW

Line n. 1) Given that the journal is Wind Energy Science, I think it is not necessary to start the abstract  with 7 lines of introduction. I would reduce it to max 2 lines.  

30) Please cite the original paper from Loyd

55) please use the same tense in all the manuscript. This should be proposes

121) A comment on Sect. 2.3 is missing

Eq6) It is not clear from the text why the non-dimensional coordinate s should be in front of the parenthesis. I believe it is because this expression comes from a moment equation: The moment at the ground station generated by the distributed drag (you right-hand-side times l) should be equal to the moment generated by the concentrated drag with the kite (you left-hand-side times l). It would be nice to have a brief explanation of this type, so that it is clear why s appears in the expression

211) remove “of which”

219) the citation should go inside the parenthesis ()

229) even if the meaning of r and e is understandable (elevator and rudder), you could write it out, so there is no doubt

235) Are the commas supposed to be multiplication signs? If yes, add the effective lift slope coefficient symbol in front for clarity

245) is the chord-wise length of the aileron equal to 60% of the wing chord? that seems a lot compared to the wing drawing in the appendix.

265) This should be a vector with square brackets. The \hat{omega} should be highlighted with \boldsymbol, in accord to the WES guidelines. Moreover, I would also add the dependence of the control inputs u on the time t.

299) maybe refer to x_0 as x(t=0)? and eq 13d consequently.

330) repetition of “the”

334) Can you give a reference for the mollification width? It is still Trigaux et al (2024a)?

Fig 13 to 22) there is some text missing in the figures, for example in Fig13 a “B” in the legend and a “v” in the fig b y-label are missing. Please makes sure the figures are well-rendered in the manuscript.

Fig 14) here it seems that the wake is being partially deflected towards the ground a few D after the 4^th loop of the trajectory. That makes sense because the aerodynamic force has a vertical component. It would be nice to add a comment on this and maybe to reproduce Fig 14b for different distances downstream.

An additional point on the discussion at line 445 might be that the kite fly the lower part of the loop always at the same altitude (probably because of some constraints in the MPC), while the altitude of the top part changes during the reel-out. This means that the loads are distributed over a larger swept area and thus lower induction can be found. However, I’m not sure if this effect is negligible.  Other considerations might come from the suggested analyses.

465) I don’t really understand the meaning of the term “2^e”. Could you write it out? If it just stands for “second”, then you could use “ 2’ ” or “2^\circ” or “II”.  

481) please write it out that the free stream velocity is 11m/s: “.. is estimated to be approximatively equal to the free-stream velocity of 11 m/s”

495) if you refer here to Fig. 23, then you should explain it better. I think here you refer to Fig. 23 (a) and (b), where the in-phase cases are shown. Then please refer just to these two images, I tried to understand the figs (c) and (d), and the realized the explanation was at line 516.

543) I don’t really understand this sentence. An influence of -6% in power production is somehow relevant and it is caused by the reduction in wind speed. Since the controller is in-the-loop you might not see it in flight speed. To clarify this aspect, you could add the numerical values of the period T_p of the various solutions somewhere in Sect. 5.2.2 (all plots show the normalized time, so we don’t know the actual value of T_p). In alternative, you could also show a plot of the time evolution of the flight speed. For example, in Fig. 20b you show that the case phase-shifted has lower values of power, but it seems that it has the same flight speed as the loops have the same length.

General Comments

The manuscript presents a high-fidelity numerical framework for simulating ground-gen AWES integrated with a MPC strategy. The authors validate the framework's accuracy regarding grid resolution and domain size. Subsequently, the study investigates two primary scenarios: a single kite and a tandem configuration of two kites. Key findings indicate that a downstream kite experiences only moderate power loss, even when operating directly in the wake of an upstream kite. The paper is well-structured, the methodology is clearly described, and the results are presented logically. The investigation into tandem flight offers valuable insights for future AWES layouts. However, there are specific aspects regarding inflow conditions and the interpretation of wake physics that require further clarification prior to publication.

Specific Comments

1. Inflow conditions

The authors state that a uniform inflow wind condition (superimposed with Mann turbulence) is used, neglecting wind shear on the basis that the kite flies "relatively far from the ground." While this simplification effectively isolates the influence of turbulence, it removes a critical component of real atmospheric boundary layer physics. Even at altitudes between 200 m and 400 m (typical for the loops shown in Fig. 7), wind shear and veer can be significant, particularly in thermally stable conditions. Wind shear creates an aerodynamic load asymmetry between the top and bottom of the loop, which the controller must compensate for.

The authors should discuss the potential impact of neglecting shear on both the controller's effort and the wake structure. Specifically, would the inclusion of shear significantly alter the vertical drift or recovery rate of the wake? If running a new simulation is computationally prohibitive, a discussion of this limitation and its implications is necessary in the text.

2. Wake transient effects

The analysis relies on six-cycle averaging to quantify velocity deficits. As noted by the authors, the wake is discontinuous and released in discrete batches. Time-averaging a pulsed, moving wake tends to smooth out the intensity of the vortex structures. A follower kite does not experience an "average" wake; rather, it interacts with specific, localized vortex structures.

In phase-shifted configuration, could the authors provide a plot showing the instantaneous forces (or effective velocity) experienced by the second kite during the specific moments it traverses the wake? This would better quantify the load transients that the controller must handle, which are more critical for structural fatigue than the reported 6% average power loss.

Technical Corrections

1. Abstract and Introduction: The broad overview of the general wind energy context in the Abstract and Introduction is somewhat lengthy. I recommend condensing this background material to focus more quickly on the specific challenges of AWES wakes and control.

2. Line 16: "foor-loop" should be corrected to "four-loop".

3. Line 465: "latter referred as" should be corrected to "hereinafter referred to as" (or simply "hereafter").