Innovative Device to Control Self-Induced Instabilities Associated with the Swirling Flow from the Discharge Cone of Hydraulic Turbines

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article deals with the development of a pulsed jet actuator for flow control in a laboratory environment. The article has a good topic and is important in aerodynamics and hydrodynamics. I agree to publish the article after the following major revisions.

1- The abstract should be rewritten according to the IMRD or IMRaD format. Sentences such as lines 20 to 22 in which you have cited the results of others should be presented in a different way.

2- The introduction needs major revisions and should be revised for several reasons and requirements. The references are very limited (18 paper) and references from the last 5 years have not been considered and cited. The references should be increased to at least 30 to 40 references, and recent articles in the Actuator’s journal should also be considered. The introduced actuator has only suggested use for the Francis turbine, and this has reduced the comprehensiveness of this tool, and it is better to present it comprehensively for flow control issues in aerodynamics and hydrodynamics to attract more readers. Similar actuators with a similar pulse approach are not considered. A paragraph should be devoted to recent flow control actuators and the role of the developed actuators in various applications such as mixing enhancement for combustion applications and flow control on aerodynamic and hydrodynamic surfaces to be emphasized. Abdolahipour [1] first presented the modulated pulse jet for flow control, which is capable of simultaneous excitation with two high and low frequencies and used it in recent research on high-lift devices. Also, the effects of low and high frequency actuation on aerodynamic performance of a supercritical airfoil by pulse jet [2] have been studied recently. Refer to these cases and their applications on high-lift. Also, the effect of flow control by blowing on a full-scale aircraft vertical tail by creating vortex through discrete slots and increasing the aerodynamic efficiency of the vertical tail has been successfully demonstrated [3].

[1] DOI: https://doi.org/10.1088/1402-4896/ac2bdf

[2] DOI: https://doi.org/10.3389/fmech.2023.1290074

[3] DOI: https://doi.org/10.1007/s42405-024-00826-1

3- The distinction of your method with previous techniques should be more clearly defined. The innovation should be clarified accordingly.

4- Explain the importance of the 2% reduction in the required flow rate compared to axial injection with more precise explanations. It is necessary to clarify that this improvement is of significant importance considering the added complexity of the proposed actuator.

5- In Figure 6, why at the L3 level, the pulse jet has the opposite effect compared to its other forms. Has this led to negative effects?

6- Given that the experiments were performed in a model laboratory, understanding the generalization of this method to a real turbine requires further discussion and explanation. Discuss the dimensionless numbers governing the problem and the effects of scaling. Reference should be made to the recent review article [4] which reviews two important parameters on fluidic actuators for flow control and separation control.

[4] DOI: https://doi.org/10.3389/fmech.2024.1380675

7- The authors can provide a proposed parameter or index based on the ratio of efficiency improvement to energy consumption so that it can be referenced and compared in the future.

8- In general, very limited results have been presented. The pressure term in the Navier-Stokes equations is inviscid in nature, and in order to see the effects of viscosity, measuring the instantaneous velocity can help to better understand the flow. The instantaneous or turbulent velocity characteristics or high-speed imaging of the jet spread or the use of PIV can provide better and more detailed information about the jet dynamics produced by the actuator, and also give more comprehensiveness to the applications of this actuator. The characteristics of the jet in a quiescent environment such as [1] and matching the input frequency with the output pulse jet frequency.

9- The operational cycle of the jet output velocity or flow rate should be presented in terms of time so that the duty cycle and output response to the input, i.e. its rotation speed and frequency, can be examined.

10- Some minor errors

-In line 39, the reference 60-70% does not make sense.

-Line 147, -+0.15% Fig.3a

-The image quality is too low. Use a vector format like Wmf.

-Provide more details of the actuator shape in Figure 2.

-Avoid presenting other people's results in the conclusions section.

Author Response

Thank you very much for your valuable recommendations for improving our research paper. Please find in the attached file our answers and also see our revised version of the paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I have read the manuscript entitled Innovative device to control self-induced instabilities associated with the swirling flow from the discharge cone of hydraulic turbines”, with interest, given the theme the authors choose to tackle. The manuscript presents a potentially interesting and valuable contribution to the field of hydraulic turbine stability, due to the innovative device (VO) for pulsating water jet injection to mitigate vortex rope formation. The results may appear of interest to the community, in reducing pressure fluctuations associated with vortex rope, but further refinement and analysis are needed to determine if the manuscript is warrant publishing.

 

My first point is around the clearly demonstrating the novelty of this work as well as over past works. Can the authors be clear early on the document (I.e. by the end of the literature review) about the relevant gaps in the current pertinent literature and how this research aims to tackle these? Is it correct to deduce that the novelty of this work is, especially in the specific implementation of the VO device to generate a pulsating water jet for dynamic fragmentation of the vortex sheet and secondary in the potential for reduced water jet flow rate? These need be clearly stated in the manuscript and contrasted to other works (including their own work eg 2019), such as other recent studies on vortex rope control techniques, particularly those focusing on active control methods (e.g. Aras, Ozkan, et al. “Numerical Analysis of Pulsating Water Jet Injection for Vortex Rope Mitigation in Hydraulic Turbines.” IEEE Xplore, 2019, https://ieeexplore.ieee.org/document/8937652).

 

Along this comment, I would like to ask the authors to enrich their literature search at the introduction (with the objective mentioned above - to establish the research gaps), but also including recent publications thematically relevant (noting that many are old references and up to only 18, that may give the reader the false impression that this is not a topic worthy of studying or with any late developments). For example, research on the use of machine learning for active control of vortex rope could be included to contextualize your work within current trends. Recent studies explore the use of sensors and real-time control algorithms to optimize jet injection parameters.

 

Could the authors explain how the design parameters of the VO device (e.g., number of slots, frequency range) were determined based on existing knowledge of vortex rope characteristics. Citing literature that discusses the typical frequencies and Strouhal numbers associated with vortex rope formation in similar turbine geometries would strengthen the justification for your design.

 

The manuscript mentions advantages/disadvantages of other existing techniques, but a more in-depth comparison, especially regarding efficiency, cost, and complexity, could be valuable. So what are the potential advantages of your method compared to existing ones? Can they be quantified?

Additionally the setup needs be better detailed/discussed, and the following points can be considered:

*   What criteria were used to select the frequency of the pulsating water jet?

*   How was the flow rate of the pulsating jet optimized?

*   What is the expected lifespan and maintenance schedule for the VO device?

*   How does the energy consumption of the VO device compare to the energy savings achieved by mitigating vortex rope instabilities?

*   Have you considered using CFD simulations to validate your experimental results and gain a deeper understanding of the flow physics? Or similar results from the literature?

 

Regarding the FFT results, the authors could discuss the changes in the frequency spectrum beyond the dominant frequency further. For example, are there any other harmonics that are affected by the VO device?

 

Can the uncertainty in the pressure measurements and flow rate measurements be quantified and can the authors discuss how these uncertainties might affect the final conclusions?

 

I would be keen to see the authors explain the rationale behind choosing the specific flow rates for the pulsating jet (3.6 l/s and 3.8 l/s). Were these values chosen arbitrarily, or was there a specific criterion/insight used to determine them? This will be useful for other readers who may wish to develop their own design.

 

Elaborating further on the physical mechanism by which the pulsating water jet mitigates the vortex rope would be useful. How does the pulsating jet interact with the vortex sheet to disrupt its formation?

 

Kindly address the increase in amplitude at level L3 more thoroughly. Is this a concern? Could it be related to the specific geometry of the test rig?

 

A more detailed analysis of the energy input required for the pulsating jet and its impact on the overall turbine efficiency is needed. How does the energy consumption of the VO device compare to the energy savings achieved by mitigating vortex rope instabilities?

 

It may be interesting to the reader to discuss the potential long-term effects of using the VO device on the turbine components. Could the pulsating jet induce fatigue or erosion? Address the durability and maintenance requirements of the VO device.

 

Ensure all figures are clear, legible and well-labeled. Please consider adding error bars to the graphs. Currently a lot of the text is not clear or legible.

 

The conclusions should clearly quantify all improvements achieved with the VO device. For example, "The VO device reduced pressure pulsations by X% and the dominant frequency by Y%”. Kindly thoroughly discuss efficiency considerations.

 

Any limitations of the study need be carefully acknowledged. For example, the experiments were conducted on a test rig and may not fully represent the behavior of a full-scale turbine.

Author Response

Thank you very much for your valuable recommendations for improving our research paper. Please find in the attached file our answers and also see our revised version of the paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors responded well to the comments and made appropriate corrections in the revised version. Therefore, I agree with the acceptance and publication of the article.

There are a few extra Hyphen in the highlighted text that need to be corrected in final version. Also, the order of citation of reference numbers in the text.

Author Response

Dear Reviewer 1,

Thank you very much for your suggestions, we modified the paper as indicated. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I appreciate the replies the author shared. I am well aware of the expertise the authors develop in this field and my comments only aim is that the authors get an opportunity to refine some points that may benefit the lay readership of Water MDPI. 
So with this, I encourage the authors to include any of the aspects they find useful (to non expert readers) from their reply, to their main text. 

Author Response

Dear Reviewer 2,

Thank you very much for your suggestions, we modified the paper as indicated. 

Author Response File: Author Response.pdf