We thank the reviewer for the time spent reviewing the article and the feedback given. Below are the answer to the questions asked.

1. The abstract has been re-written to be longer and make it clear that the paper is about 2D CFD simulations and analysis:

A commercial wind turbine blade equipped with root spoilers is analysed using 2D URANS Computational Fluid Dynamics (CFD) to assess the unsteady impact of passive devices. In this work, we present the 2D CFD unsteady results from a non-rotating single thick section located at the root end of the blade with and without spoiler. Computations were performed at the chord-based Reynolds number $Re_c = 3 \times 10^6$. The analysed spoiler is of commercial size with a height of approximately 33\% of the local chord. Comparatively to the existing literature, it is at least one order of magnitude larger than the size of the well known Gurney flaps. The analysis is first performed in the steady state at a single angle of attack using global aerodynamic forces, the local pressure distributions and flow field analysis. It shows a very important flow rearrangement in the presence of a spoiler, responsible of the mean lift force enhancement. Then, simulations are extended over a large range of angle of attack (from $-20^\circ$ to $20^\circ$), to identify the spoiler behaviour across the polar. Analyses are then continued accounting for the flow unsteadiness. The spoiler induces an important wake behaviour linked to the apparition of global load fluctuations. Using the wall pressure distributions and the associated spatio-temporal organisation of the flow field those fluctuations are well characterised. The detailed analysis performed at one angle of attack, is then extended to assesses the load fluctuations at other angles of attack, showing the evolution of the unsteady loads in relation to the wake

1. Unfortunately such experimental data does not exist to the best of our knowledge, therefore we could not validate our simulations against wind tunnel measurements. However, in Potentier et al. [1] a validation of the code has been done against the existing DANAERO database for aerofoils up to 33% thick, giving confidence in our approach and results.

[1]: T. Potentier, E. Guilmineau, and C. Braud, ‘CFD solver and meshing techniques verification using the DANAERO database’, p. 8, 2020.

1. The Strouhal number is calculated using the definition of Yarusevych et al. [2]. As detailed on page 16 line 178 of our manuscript, the characteristic length (L) is the distance between two peaks of RMS of the horizontal velocity. The paragraph has been modified as follow :

In the far wake region, a single peak frequency emerges, with its harmonics, that can be extracted at $\frac{x}{c} = 3.0$ using PSD analysis (see Figure \ref{PSD_Ux_R6}). The energy content for the "no spoiler" case is several order of magnitude lower than the "spoiler" case as expected.

Figure 11. Horizontal velocity PSD, in the wake, at $\frac{x}{c} = 3.0$. The blue square (\textcolor{blue}{\large$ \blacksquare $}) shows the "no spoiler" case while the orange dot (\textcolor{orange}{\Large$ \bullet $}) shows the "spoiler" case.

At last, following the definition of \cite{yarusevychVortexSheddingAirfoil2009} a Strouhal number of $S_{t-spoiler}^* = 0.15$ is found. In this definition, the velocity used is the mean free stream velocity and the characteristic length ($L$) is the distance between two mean horizontal velocity Root Mean Square extrema at $\frac{x}{c} = 3.0$. The RMS peaks represents the aerofoil upper side vortex centre and aerofoil lower side vortex centre, therefore the vertical distance can be viewed as a representation of the wake width. As seen in Figure \ref{PMS_Ux_R6}, the “no spoiler” case does not present two distinct peaks, only a single bell-type curve representing the velocity deficit in the wake. On the other hand, the “spoiler” case shows a larger velocity deficit accompanied with a pair of RMS peaks showing the presence of vortices centre.

\begin{equation}

 St^*_{spoiler} = \frac{f \times L}{U} = \frac{31.53 \times 0.2197}{45} = 0.15

 \end{equation}

Where $f$ is the main vortex shedding frequency, $L$ is the characteristic length and $U$ the incoming velocity.\\\

This result falls in line with their study where $S_t^* \thickapprox 0.17$ was found, albeit in our case at a much higher Reynolds number and for a much thicker aerofoil and equipped with spoiler.

Figure 12. Mean Horizontal velocity Root Mean Square value for the radial position R6 at $\alpha = 6\circ$ ($\frac{r}{R}=13\%$) and at $\frac{x}{c} = 3$. The blue square (\textcolor{blue}{\large$ \blacksquare $}) shows the RMS for the no spoiler case and the orange dot (\textcolor{orange}{\Large$ \bullet $}) shows the RMS for the spoiler case.

[2]:  S. Yarusevych, P. E. Sullivan, and J. G. Kawall, ‘On vortex shedding from an airfoil in low-Reynolds-number flows’, J. Fluid Mech., vol. 632, pp. 245–271, Aug. 2009, doi: 10.1017/S0022112009007058.

1. After a time step convergence study based on the chosen mesh (not initially presented in the manuscript for conciseness) we chose to use a time step of  of the time necessary for an air particle to travel the chord distance. For the aerofoil and velocity used (45m/s) in the present manuscript, it is equivalent to . The difference between the chosen time step and the finest one is small enough so that the calculation time benefit outweighs the small uncertainty introduced. The following will be added to the revised manuscript:

A time step convergence study using the “Fine grid” has also been performed and summarised in Table 3.

The chosen time step for the rest of the study is Δt=4.44 ×10^(-5) s because of the good balance between result accuracy and time prior convergence.

Table 3. Time step independence study for the scanned blade with spoiler at α = 0° and Re_c = 3 ×10⁶

You will find in attachment a difference file between the the original and the revised manuscript. I have also attached a plot showing the good agreement between ISIS-CFD (FINE/Marine) against the DANAERO database (from comment 2) and a plot, now included in the revised manuscript, showing the RMS in the aerofoil wake.

We thank the reviewer for the quality of the feedback. Below are the answers to the several points noted.

1. It is noted that ISIS-CFD code has not been properly cited in the current state of the paper. Therefore we have updated the section 3.1:

ISIS-CFD, developed by Centrale Nantes and CNRS and available as a part of the FINE\texttrademark/Marine computing suite, is used in the present study to solve the incompressible Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. It is based on the finite volume method to build the spatial discretization of the transport equations. The  unstructured discretization is face-based, which means that cells with an arbitrary number of arbitrarily shaped faces are accepted (unstructured mesh). A second order backward difference scheme is used to discretize time. The solver can simulate both steady and unsteady flows. In the case of turbulent flows, transport equations for the variables in the turbulence model are added to the discretization. \\\

All flow variables are stored at the geometric cent of arbitrary shaped cells. Volume and surface integrals are evaluated with second-order accurate approximations. The method is face-based, which means that the net fluxes in the cells are computed face by face. Thus, the cells with an arbitrary number of arbitrarily shaped faces are accepted. Numerical fluxes are reconstructed on the mesh faces by linear extrapolation of the integrand from the neighbouring cell centres. A centred scheme is used for the diffusion terms, whereas for the convective fluxes, a blended scheme with 80\% central and 20\% upwind is used.\\\

The velocity field is obtained from the momentum conservation equations and the pressure field is extracted from the mass equation constraint, or continuity equation, transformed into a pressure equation. The pressure equation is obtained by the Rhie and Chow interpolation \cite{rhie_chow_83}. The momentum and pressure equations are solved in an segregated manner as in the SIMPLE coupling procedure \cite{issa_85}. A detailed description of the discretisation is given by \cite{queutey_visonneau_07}.\\\

The turbulence model used is SST k-$\omega$ (see \cite{menterZonalTwoEquation1993}). The flow characteristics are representing the air at sea level at a temperature of \unit{15}{°C}, i.e.: $\nu$ = 1.81$\times10^{-5}$\unit{}{kg m$^{-1}$ s$^{-1}$} (dynamic viscosity) and $\rho$ = \unit{1.225}{kg m$^{-3}$} (air density). A uniform inflow of \unit{45}{m/s} is set, which induces a chord Reynolds number of: $Re_c = 3\times 10^6$ for the presented section of $\frac{r}{R}=13\%$.

2. ISIS-CFD has been validated many times over regarding 3D cases for hydrodynamics and automotive cases (see [4, 5, 6, 7, 8]). However, the 2D cases validation were not explicitly cited or showed in the present paper. As part of [10], a comparison between the existing DANAERO literature both experimental and numerical and ISIS-CFD was performed. The following figures illustrates the good agreement between the 2D The following lines will be added to the introduction of the section 3.2:

“A comparison between the existing DANAERO literature both experimental and numerical and ISIS-CFD was performed (see \cite{potentier_cfd_2020}). The outcome showed the good agreement between the 2D wind tunnel experiment and the 2D URANS ISIS CFD simulations, thus validating the use of ISIS-CFD for 2D external applications. The domain size study has also been performed and the recommended square domain of 80 chords in length was used.”

[1] C.M. Rhie, and W.L. Chow, ‘A numerical study of the turbulent flow past an isolated aerofoil with trailing edge separation’, AIAA Journal, vol. 17(11)  pp. 1525–1532, 1983, doi: 10.2514/6.1982-998.

[2] R. Issa, ‘Solution of the implicitly discretized fluid flow equations by operator-splitting’, Journal of Computational Physics, vol. 62, pp 40–65, 1985, doi: 10.1016/0021-9991(86)90099-9.

[3] P. Queutey and M. Visonneau, ‘An interface capturing method for free-surface hydrodynamic flows’, Computers and Fluids, vol. 36, pp. 1481–1510, 2007,  doi: 10.1016/j.compfluid.2006.11.007.

[4] J. Wackers et al., ‘Can adaptive grid refinement produce grid-independent solutions for incompressible flows?’, Journal of Computational Physics, vol. 344, pp. 364–380, Sep. 2017, doi: 10.1016/j.jcp.2017.04.077.

[5] J. Wackers, G. Deng, E. Guilmineau, A. Leroyer, P. Queutey, and M. Visonneau, ‘Combined refinement criteria for anisotropic grid refinement in free-surface flow simulation’, Computers & Fluids, vol. 92, pp. 209–222, Mar. 2014, doi: 10.1016/j.compfluid.2013.12.019.

[6] E. Guilmineau, G. B. Deng, A. Leroyer, P. Queutey, M. Visonneau, and J. Wackers, ‘Numerical Simulations for the Wake Prediction of a Marine Propeller in Straight-Ahead Flow and Oblique Flow’, Journal of Fluids Engineering, vol. 140, no. 2, p. 021111, Feb. 2018, doi: 10.1115/1.4037984.

[7] G. B. Deng, E. Guilmineau, P. Queutey, and M. Visonneau, ‘Ship Flow Simulations with the ISIS CFD Code’, CFD Workshop Tokyo 2005 (CFDWS2005), p. pp 474--482, Mar. 2005.

[8] E. Guilmineau, G. B. Deng, A. Leroyer, P. Queutey, M. Visonneau, and J. Wackers, ‘Assessment of hybrid RANS-LES formulations for flow simulation around the Ahmed body’, Computers & Fluids, vol. 176, pp. 302–319, Nov. 2018, doi: 10.1016/j.compfluid.2017.01.005.

[9] M. Visonneau, E. Guilmineau and S. Toxopeus, ‘Study on the onset and development of the three-dimensional separation around a diamond wing with incompressible flow solvers’, Aerospace Science and Technology, vol. 57, pp. 52-75, 2016, doi: 10.1016/j.ast.2016.02.012.

[10] T. Potentier, E. Guilmineau, and C. Braud, ‘CFD solver and meshing techniques verification using the DANAERO database’, p. 8, 2020.

3. This is indeed an interesting question that can be answered only by comparing 2D URANS simulations and LES simulations. However, LES simulations of this present configuration do not exist in the literature and is therefore a full work by itself. We believe that the present URANS simulations have a sufficient representation of the wake dynamics (until x/c = 5.0) to perform a first analysis of the spoiler impact. Ongoing RANS-DES simulations of the present configuration will be used in the future as a comparison with the present URANS simulations, which is out of the scope of the present paper.

You will find in attachment a difference file between the the original and the revised manuscript. I have also attached a plot showing the good agreement between ISIS-CFD (FINE/Marine) against the DANAERO database (from comment 2) and a plot of the PSD at several position in the wake (comment 3).