CFD in Urban Wind Resource Assessments: A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for submitting your work. It is clear effort was put into it, but I am not sure it is the effort required to publish a paper under your name. 

As a review paper, you are expected to cover a lot of previous publications, however, your contribution is required. A final discussion and judgement based on the collected knowledge should be provided and made clear to the reader. I do not believe I got this from the manuscript, which is my main concern. The manuscript advocates for a certain use in a certain field, but does not show the true limitations and challenges opposing the work.

I have the following comments and suggestions:

Strengths:
The work covers a wide range of CFD-related topics, including turbulence modelling, meshing, validation, grid metrics, and LES/RANS comparisons.

Extensive literature review, with many relevant citations from reputable journals.

The mention of hybrid LES–RANS models (e.g., DES, ELES) is relevant and timely.

Weaknesses:

Lack of synthesis and critical analysis

No comparing of the listed studies.

Conclusions on which CFD approaches perform best in specific urban conditions should be provided.

Lacks explanation of databases searched, if there were any inclusion/exclusion criteria.

No statistics on the reviewed papers.

The manuscript lacks quantitative meta analysis and has very limited real-world insights.

Suggestions:
More comprehensive comparisons between models is required.

Provide metrics-based synthesis of model performance.

Include research gaps and future directions

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The study presents a comprehensive analysis of how CFD techniques are applied to assess wind resources in urban settings, specifically for small wind turbine SWT applications. The authors critically evaluate a range of CFD modeling approaches, emphasizing their suitability and effectiveness within complex urban landscapes for harnessing wind energy. The manuscript is well-structured and could be of interest for readers in the field. Some comments and suggestions can be seen below:

1- First of all, you need to keep a space between the citation and the text, for instance: k[19], models[20]. Check the manuscript. 

2- Referring to equation 4, you should mention how the parameters alpha and beta are determined or estimated from real wind speed data. These parameters are crucial for accurately fitting the Weibull distribution to observed wind speeds, and without a discussion on their estimation methods, the equations might appear somewhat abstract to readers unfamiliar with the process.

3- Referring to equation 6, you clarify that this equation represents the idealized power calculation, and the actual power output from a wind turbine is typically calculated by multiplying the ideal power by the power coefficient.

4- Figure 2 would benefit from a clearer explanation of the differences between the two models in terms of their actual performance metrics (e.g., computational time, accuracy in predicting turbulent flow characteristics).

5- Figure 3, how sensitive is the performance of a rooftop-mounted SWT to variations in the ABL profile caused by different ground roughness conditions? Can these inhomogeneities in the ABL lead to over- or under-estimations of wind speed at rooftop height, which would directly affect energy yield calculations for SWTs? 

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

1. This paper takes the application of CFD in urban wind energy resource assessment as the theme, which closely fits the era background of global low-carbon urban development and distributed energy demand, and has significant practical significance. The complexity of urban built environment and the adaptation of SWT are the key challenges in the field of urban energy. This paper systematically combs the application framework of CFD in this scenario, and builds the technical chain step by step from urban wind environment characteristics to turbulence model selection. In particular, the study clearly points out the limitations of traditional measurement methods in spatial resolution and boundary condition manipulation, highlighting the irreplaceability of CFD. However, it is recommended to further compare the core differences of energy systems at different scales in the introduction to strengthen the uniqueness of the research problem;
 At the same time, it can supplement the differentiated demand analysis of typical city cases and enhance the practical guiding value of the research.

2. This paper makes a comprehensive review of the mainstream models and boundary condition treatment methods of CFD in urban wind field simulation. The literature covers important researches in the past ten years, which reflects a solid theoretical foundation. By comparing the turbulence model selection of different studies in the table, the current academic community's tradeoff between computational efficiency and accuracy is clearly presented. However, there are two points that can be optimized in the literature review: First, the discussion of the emerging hybrid model is a little brief, and it is suggested to supplement its specific application cases and effect comparison in complex urban terrain; Secondly, the literature review of data-driven methods can be more in-depth, especially the latest progress in large-scale urban modeling, so as to reflect the cutting-edge research.
 In addition, some references are repeated, so it is suggested to optimize the distribution of references to avoid redundancy.

3. The CFD-assisted wind energy evaluation framework constructed in this paper has a clear logic, forming a complete technical route from turbulence model selection, calculation grid division to boundary condition verification. The analysis of key technologies such as WFs has both theoretical depth and practical guiding significance, especially the parameter standard in engineering application is clarified through formula derivation. However, the methodology part can further strengthen the following contents: (1) Error comparison of different turbulence models in UCL simulation, and suggestions for applicable scenarios are given; (2) Combined with concrete examples to illustrate the impact of mesh refinement on the calculation cost, enhance the practical reference for engineering personnel;
 ③ Add special treatment methods for boundary conditions under extreme conditions to avoid only staying in the theoretical level of discussion.

4. The characteristics of UBL and RSL are analyzed in depth, and the modulation effect of building geometry on wind field is revealed through logarithmic law formula and measured data. However, it can be further improved in the following aspects: (1) The specific definition and simulation method of extreme conditions in wind resource assessment, and it is suggested to illustrate the input parameters of extreme scenarios combined with climate model data; (2) The effects of different building forms on turbulence intensity are different, and quantitative comparison data can be introduced; (3) The application cases of Venturi effect in actual architectural design enhance the combination of theory and engineering practice.
 In addition, the discussion on the applicability of Weibull distribution in complex urban terrain is a little simple, which can supplement the error analysis of the fitting between measured data and the model.

5. The paper emphasizes the importance of verifying CFD results and measured data, and introduces FAC2, FB and other indicators to reflect the normalization of the method. However, there are two shortcomings in the verification part: (1) The application scenarios of "indirect verification" (model verification) and "direct verification" (target region verification) are not clearly explained, and it is suggested to illustrate the complementarity of the two with cases; (2) The limitation analysis of wind tunnel experiment data is not deep enough, so the accuracy difference between CFD and wind tunnel experiment in urban micro-scale wind field simulation can be compared.
 In addition, the verification indicators focus on speed and turbulence intensity, and it is recommended to supplement the direct verification of the power output of wind turbines to be more close to the needs of engineering applications.

6. The paper combines the wind energy formula with SWT to build the core model, which reflects the attention from wind field simulation to actual power generation. However, it can be deepened in the following aspects: (1) The efficiency difference of different types of SWT in complex urban flow field is suggested to introduce the experimental data comparison of specific models; (2) Quantitative analysis of the impact of wake effects on the layout of multiple units, the existing content only mentions the lack of models, and does not explore the technical path; (3) Considering the influence of non-flow field factors such as building shadow and temperature stratification on SWT performance, the comprehensiveness of the evaluation model is expanded.
 In addition, the method of obtaining the measured data of the power curve can be briefly explained to enhance the operability of the model.

7. The challenges presented in the paper are clearly research-oriented and forward-looking in the potential analysis of data-driven approaches. However, it is suggested to refine in the following aspects: (1) To solve the problem of high computing cost of LES, engineering optimization means such as parallel computing and GPU acceleration can be discussed, rather than just staying in the theoretical appeal; (2) Extreme conditions simulation can be combined with specific urban disaster cases to illustrate the shortcomings and improvement directions of existing models; (3) The coupling mechanism of WRF-UCM and CFD in multi-scale modeling needs to be further clarified to avoid fuzzy technical path. 
In addition, the application to the policy and planning level can complement the preliminary concept and enhance the reference value of the research for the decision-making level.

8. The structure of the full paper follows the classic framework of background - method - challenge - prospect, and each chapter is logically coherent, especially in the CFD auxiliary evaluation part, technical disassembly is realized through sub-modules. However, there are two connecting problems: (1) The correlation between urban wind environment characteristics and CFD model selection is slightly loose, so it is suggested to add the analysis of how building geometric parameters affect model parameter setting in chapter transition; Some parts of the challenges and prospects are repeated, and can be re-integrated to make the challenges focus on the problem and the prospects focus on the solution.
 In addition, the reference order of chart numbers in the text is occasionally confused, so it is recommended to check uniformly.

9. This paper contains a large number of fluid mechanics formulas, most of which are defined when they first appear, ensuring the readability of theoretical derivation. However, there is still room for improvement: (1) The physical meaning of some formulas is not fully explained, so it is recommended to supplement the parameter sensitivity analysis; (2) The formula derivation of complex concepts can be supplemented by flow charts or diagrams to reduce the difficulty of understanding;
 ③ There are inconsistencies in the symbol system, so it is recommended to unify the definition of terms to avoid confusion.

11. The paper focuses on CFD technology itself, but the interdisciplinary integration of urban planning, energy system and architectural design is not discussed enough. It is suggested to add: (1) Quantitative analysis of the influence of urban planning parameters on wind field to provide a basis for the formulation of planning standards; (2) The influence of building material roughness on boundary conditions, connecting building physics and fluid mechanics; (3) The compatibility case of SWT installation and urban landscape design reflects the application flexibility of the study.
 In addition, the migration value of CFD simulation in adjacent fields can be discussed to expand the radiation range of research.

12. The paper mentions in data availability that data sets can be obtained on demand, but does not explain the data type and acquisition method, which reduces the repeatability of the research. It is suggested to add: (1) Publicly shared benchmark examples and CFD simulation parameters, which are convenient for peers to reproduce; The source and processing method of measured data to enhance the transparency of the verification process;
 ③ A brief description of the code or tool chain to provide a technical starting point for subsequent research.

13. The conclusion part systematically reviews the core content, emphasizes the key role of CFD in urban wind energy assessment, and points out the future direction. However, it can be further strengthened: (1) The specific contribution of research results to the dual carbon goal, and strengthen the policy orientation; (2) The differentiation value from similar reviews, and the innovation points in the methodology framework of this paper are clarified; ③ The brief discussion of the technical landing obstacles makes the conclusion more realistic. In addition, the conclusion paragraph is a little lengthy, and it is suggested to summarize the core findings and prospects at different points to improve the information density.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Happy for paper to be published

Author Response

Thank you very much for your time and the positive feedback of our manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

I regret to inform you that although the author has made efforts to address some of the issues raised during the initial review, the revised manuscript still fails to meet the publication standards of the journal. The modifications made are superficial and have not fully addressed the identified fundamental issues. The authors failed to provide convincing evidence to support their claims, nor did they demonstrate a clear understanding of the key gaps in this field. In view of these persistent shortcomings, I do not recommend further consideration of publishing this manuscript in its current form.

Author Response

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Author Response File: Author Response.docx