Effect of the Extended Rigid Flapping Trailing Edge Fringe on an S833 Airfoil
Round 1
Reviewer 1 Report
See the attached file.
Comments for author File: 
Comments.pdf
Author Response
Reviewer #1 comments:
Major comments:
- In the introduction, I found that author(s) have given less attention to the application, therefore I suggest to improve the introduction.
Response: Thank you for the comment.
A short description was added to mention the application of the work:
“The abovementioned studies aimed at improving the aerodynamics of an airfoil and reducing the noise. The flapping trailing edge fringe can be applied to wind turbine blades and airplanes to improve the noise reduction from the aerodynamics perspective.”
In fact, more discussions were also given in the introduction already such as “The brush-like trailing edge has been utilized as the trailing edge extension to reduce both the narrowband bluntness noise and broadband turbulent boundary-layer trailing-edge noise, as it contains the characteristics of both porosity and flexibility as the natural fringe [17–21]. A remarkable noise reduction was observed for an airfoil with a flexible trailing edge equipped to the suction side of the airfoil experimentally [21]. The attachment of flaplets on the trailing edge of an airfoil has clearly shown reductions in tonal noise, primarily resulting from the generation of small-scale vortices by the flaplets [22]. ”
- Give physical reason for the proposed numerical values and why did you specifically choose these values?
Response: The numerical values defined in the Methods section were primarily to reproduce the same Reynolds number used in our previous experimental study. Air properties were defined based on the room temperature in the lab. The Re number of 40,000 was adopted to have a similar Re number of the natural owls flight. A revision was made as
“The rectangular area of the outside domain is 20C by 10C (C: the chord length of the airfoil) with 5C from the inlet to the leading-edge of the airfoil. The flow boundary conditions applied in the simulation are adopted from the parameters in our previous experimental study [24]. The upstream velocity is set as at 6 m/s, and the density and viscosity of the air are assumed as 1.23 kg/m3 and 1.84×10-5 Pa s to match the air property at room temperature. This boundary condition corresponds to a Reynolds number (Re) of 40,000, which is the same as the Re number evaluated in the experiment. This Re number falls within the range of that generated when owls fly.”
- Its better to provide more mathematical work for better understanding of the readers.
Response: Thank you for the comment.
A paragraph was added to describe the governing equations.
“ The flow over the airfoil is governed by the full Navier-Stokes equation for two-dimensional, viscous, incompressible flow. The Reynolds averaged N-S equations can be expressed as:
(1)
(2)
where i and j are indices; u is the velocity; x is the flow direction; ρ is the density of the fluid; μ is the viscosity of the fluid.”
- If possible, provide a comparison to justify your results.
Response: Thank you for the comment.
It is challenging to make a direct comparison to validate the current simulation results due to the lack of comparable studies either in experiments or simulations. We have previously validated the computational method used for the CFD simulation as reported by Yu et al. 2020 [24]. Two figures are provided below to show the validation of the drag and lift coefficients and pressure coefficient distribution of flow over a rigid flapping airfoil (Figures 3 and 4 in the reference). But these figures can’t be reused in the current work.
Figure 3. a) Comparison of the drag coefficient with reference; b) comparison of the lift coefficient with reference.
Figure 4. a) Comparison of the pressure coefficient distribution of the laminar flow over rigid flapping airfoil model at t=0/20 T, t=7/20 T, t=10/20 T, and t=17/20 T between the current method and previous CFD results by Miao et al. 2006.
Yu, H. and Yang, Z., 2020. A numerical simulation on the airfoil s833 equipped with flapping trailing edge fringes. Journal of Applied Fluid mechanics, 13(2), pp.571-582.
Miao, J.M. and Ho, M.H., 2006. Effect of flexure on aerodynamic propulsive efficiency of flapping flexible airfoil. Journal of Fluids and Structures, 22(3), pp.401-419.”
- Conclusions could be more specific and to the point, I would suggest to look and think about it.
Response: Thank you for the comment.
The conclusion was revised as follows:
“The airfoil S833 equipped with an extended flapping fringe is used to investigate the effects of two factors, i.e., the trailing fringe length and the flapping frequency, on the vortex shedding characteristics and the aerodynamic performance of the airfoil. The extended fringe with a flapping frequency significantly below the natural shedding frequency (~140 Hz), such as 80 Hz, can alter the coherence structure. Likewise, an irregular vortex structure in the wake can be generated, thus cause decrements in the swirling strength and faster decay of the vortices. The flapping motion can decrease the pressure coefficient over the upper surface of the airfoil, leading to increases in the lift coefficient about 40% relative to the bare airfoil model. Out of the cases in the present study, the model with Le=0.01 m (10% of the airfoil chord) at a flapping frequency of fe =110 Hz outperforms other cases and shows an overall substantially better aerodynamic performance of the airfoil as well as the favorable vortex characteristics downstream of the airfoil. In spite of the limitations of the study, we can observe the aerodynamic benefits of using an extended flapping fringe at the trailing edge. The pressure alteration around the airfoil would potentially reduce the noise generation especially for wind turbines, which will be studied in our future work as well.”
- The novel terms in the physical model pertaining to the current study should be highlighted and discussed. Also, explanations are required for imposition of boundary conditions.
Response: Thank you for the comment.
The novelty of the study was highlighted and added into the introduction as follows:
“In this study, to leverage the influence of the extended flapping trailing edge fringe on the aerodynamic performance of the airfoil, CFD simulations are performed for the S833 airfoil with an extended flapping trailing edge fringe. This numerical simulation model is different from our previous experimental study. First, the experimental model was equipped with real feathers at the trailing edge of the airfoil as a passive flow control without any controlled flapping motion. Due to the complexity of the real feather structure, the porosity and biological material properties, such numerical studies haven’t been conducted yet. In this work, a simplified bare fringe without any porosity in a 2D fashion, together with specific flapping motions worked as an active control, is adopted in the numerical modeling.”
The Explanaiton for the boundary condition is given as
“The rectangular area of the outside domain is 20C by 10C (C: the chord length of the airfoil) with 5C from the inlet to the leading-edge of the airfoil. The flow boundary conditions applied in the simulation are adopted from the parameters in our previous experimental study [24]. The upstream velocity is set at 6 m/s, and the density and viscosity of the air are assumed as 1.23 kg/m3 and 1.84×10-5 Pa s, respectively, to match the air property at room temperature. This boundary condition corresponds to a Reynolds number (Re) of 40,000 which is the same as the Re number evaluated in the experiment. This Re number falls within the range of the Re number of owls’ flight.”
- In the conclusion, please show how the work advances the field from the present state of knowledge. Please provide a clear justification for your work in this section, and indicate uses and extensions if appropriate. Moreover, you can suggest future experiments/simulations and point out those that are underway.
Response: Thank you for the comment. The conclusion was revised as follows:
“The airfoil S833 equipped with an extended flapping fringe is used to investigate the effects of two factors, i.e., the trailing fringe length and the flapping frequency, on the vortex shedding characteristics and the aerodynamic performance of the airfoil. The extended fringe with a flapping frequency significantly below the natural shedding frequency (~140 Hz), such as 80 Hz, can alter the coherence structure. Likewise, an irregular vortex structure in the wake can be generated, thus cause decrements in the swirling strength and faster decay of the vortices. The flapping motion can decrease the pressure coefficient over the upper surface of the airfoil, leading to increases in the lift coefficient about 40% relative to the bare airfoil model. Out of the cases in the present study, the model with Le=0.01 m (10% of the airfoil chord) at a flapping frequency of fe =110 Hz outperforms other cases and shows an overall substantially better aerodynamic performance of the airfoil as well as the favorable vortex characteristics downstream of the airfoil. In spite of the limitations of the study, we can observe the aerodynamic benefits of using an extended flapping fringe at the trailing edge. The pressure alteration around the airfoil would potentially reduce the noise generation especially for wind turbines, which will be studied in our future work as well.”
- The new model was derived in the manuscript. However, some limitations were ignored or assumption with the model. Therefore, could the final resolution support the results or application? Or how to assess the deviation?
Response: Thank you for the comment.
A limitation section was added in the discussion section, and future work was also discussed.
“There are several limitations of this study. First, a RANS turbulence model was used to conduct the simulation. This would cause less accurate simulation results on the structure of shedding vortices. Due to the lack of comparable experimental data, direct validation of the simulation results could not be made, but, qualitatively, the predicted vortex reduction is similar to what we observed in our previous experiment [24]. In addition, to simplify the computational effort, a 2D model geometry was used, and thus the span-wise porosity effect of feather fringes as used in the experiment could not be studied. To improve the understanding, 3D simulations on porous fringes attached at the trailing edge will be investigated in the future work. To better mimic feather fringe of an owl’s wing, a soft material with the biological property should be adopted as well. The flapping motion will then be determined by the fluid-structure interaction as a passive flow control. “
“In spite of the limitations of the study, we can observe the aerodynamic benefits of using an extended flapping fringe at the trailing edge. The pressure alteration around the airfoil would potentially reduce the noise generation especially for wind turbines, which will be studied in our future work as well.”
Minor comments:
- Figures and tables could be rechecked once for resolution and correctness.
- This would be good to use units in the nomenclature where applicable.
Response: Thank you for the comments. The Figures and tables have been checked.
Author Response File: Author Response.pdf
Reviewer 2 Report
Bioinspiration is very interesting to me. I liked your article and presentation of the results
Author Response
Reviewer #2 comments:
Bioinspiration is very interesting to me. I liked your article and presentation of the results.
Response: The authors appreciate the reviewer’s comment. We will try our best to present a insightful paper.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments are attached in a separate file.
Comments for author File: Comments.pdf
Author Response
Reviewer #3 comments:
- In reviewer’s opinion, it is not peculiar to see the change in vortex shedding frequency
with the change in flapping frequency. Authors should be careful to interpret the
observed lower number of vortices and large vortices at lower flapping frequencies as
vortex dissipation in lower flapping frequencies. Because there is no conclusive evidence
for vortex dissipation unless the author quantify the vortex dissipation according to the
vortex transport equation !
Response: Thank you for the insightful comment.
The authors agree that the effect of the flapping frequency of the fringe on the vortex dissipation was not interpreted clearly, and there may not be a direct effect between them. It is speculated that the accelerated decay of shedding vortices could be an “offset effect” of the vortices generated by the flapping fringe and natural shed. This was also observed in our previous experimental study. In the revised manuscript, we have reinterpreted this prediction of accelerated vortex decay.
- In the introduction the authors mention that this is an extension of the experimental work they did previously about a similar problem. However, the reviewer could not find a
distinct difference between the objectives of the previous study and the current study.
Reviewer suggests author clearly state the difference between the previous study and this
study. In the reviewer’s strong opinion, this difference must be stronger than the different
methodologies used in two studies, experimental for the previous and numerical for this.
Reviewer also think that the experimental study must have provided data that closely
resembles the flow field behind a birds’ feathers as it is explained in the background
information.
Response: Thank you for the insightful comment.
There are a number of differences between the current simulation and previous experimental study. First, the experiment equipped a rigid real feather at the trailing edge of the airfoil as a passive flow control without the flapping motion. However, in the simulation, it is challenging to create the geometry of a real feather and to simulate the property and porosity of a feather, and thus a bare fringe was adopted with a specific flapping motion as an active control. In addition, since a 2D model was designed in the simulation, the interactions between fringes can’t be studied.
In the introduction, a short description was added to emphasize the differences between two studies, as follows:
“This numerical simulation model is different from our previous experimental study. First, the experimental model was equipped with real feathers at the trailing edge of the airfoil as a passive flow control without any controlled flapping motion. Due to the complexity of the real feather structure, the porosity and biological material properties, such numerical studies haven’t been conducted yet. In this work, a simplified bare fringe without any porosity in a 2D fashion, together with specific flapping motions worked as an active control, is adopted in the numerical modeling..”
- Reviewer also think that it is good to mention the rational for choosing S833 airfoil for
this study in the introduction.
Response: Thank you for the insightful comment.
The rational for selecting S833 airfoil for the study was added in the introduction as follows:
“The NREL’s S833 airfoil has been extensively used to design wind turbine blades [26]. However, with the development of the wind turbine energy, noise emission has become a significant issue. The ultimate goal of this study is to explore a novel airfoil design to reduce the noise generation associated with the shedding vortices. Thus, the S833 airfoil is selected to investigate the aerodynamic interactions between the vortex shedding in the wake and the rigid flapping fringe installed at the trailing edge.”
- Authors have not clearly stated whether they use or Unsteady Reynolds-Averaged
Navier-Stokes (URANS) or Large-eddy simulation (LES) for solving the flow around the
airfoil. I presumed they didn’t use either Reynolds-Averaged Navier-Stokes (RANS) or
Direct numerical simulations (DNS). Because instantaneous flow fields are not possible
RANS, and turbulence modeling they mentioned in the manuscript is not required for
DNS. For the readers’ benefit and the clarity of the manuscript, I suggest authors clearly
specify the method they used in the section corresponds to computational model.
Response: The turbulence model LKE K-KL-ω is one of RANS-based approaches to predict he transitional flow proposed by Walters et al. 2004.
Additional information was added in the revised manuscript:
“The turbulence model LKE K-KL-ω was adopted to evaluate the turbulence region over the airfoil. This model is one of Reynolds-Averaged Navier-Stokes (RANS) based approaches to predict the transitional flow [25,27,28]. In the LKE model, the energy of the disturbances in the pre-transitional region of a boundary layer is expressed as "Laminar Kinetic Energy" while the turbulence energy is as k, and the transport equation of KL is solved with two equations of fully turbulent model.”
- Reviewer suggest including all the variables in the equation 1. and are named as − component of velocity and − component of velocity. Reviewer suggest clearly specify the whether these velocity components are instantaneous or time averaged or fluctuation component of velocity.
Response: the velocity components are instantaneous, and the time averaged velocity was only used to present the averaged results. This has been specified in the revised manuscript.
- On line 172, authors state “when flapping frequency 4 is lower than ”. Reviewer
suggest include how much should the frequency be. Because 110 Hz is also lower than
the vortex shedding frequency of the airfoil without the flap.
Response: Thank you for the comment.
The statement has been modified.
“From the comparison of the four flapping frequencies, it is suggested that when the flapping frequency fe is 60Hz (40%) lower than f (natural vortex shedding frequency of the bare airfoil model), the regular vortex shedding is interfered and results in an irregular vortex distribution with non-uniform scales and shortened gaps between each pair.”
- What does authors mean by the “enhance velocity distribution” on line 189-190?
Response: This statement has been modified.
“Higher flapping frequencies tend to increase the velocity over the top surface of the airfoil, as a consequence, it results in a decrease in the pressure leading to an increase in the lift coefficients.”
- What do authors think the reason for higher lift coefficient at higher flapping frequencies. This explanation in terms of flow physics might increase the value of the manuscript.
Response: The main reason for the higher lift coefficient at higher flapping frequencies could be attributed to the fact that the flapping fringe serves as a propulsion mechanism, like a fish tail. This was explained in the manuscript as
“For this case, the flapping fringe serves more like a propeller tail, such as the tail of a fish, thus reduces the drag and improves the lift.”
- On lines 229-231, authors mention “lower 4 can break down the large-scale vortex into small-scale vortices”. In reviewer’s knowledge, vortex breakdown refers to a different phenomenon. The reviewer suggest using the term vortex break up instead of vortex breakdown.
Response: Thank you for the suggestion. The sentence is modified as
“The flapping motion with a relatively lower fe (<140 Hz) can break the large-scale vortex into small-scale vortices at the trailing edge”
- Reviewer suggest authors use either gap or distance instead of using both the words on line 232.
Response: Thank you for the comment.
The description has been modified throughout the paper.
- In reviewer’s opinion, the instantaneous values of the lift coefficients at the different
flapping frequencies and fringe lengths adds more complexity than clarity to the
investigation. Reviewer suggest including only figure 17 rather than figures 15 and 16.
The conclusion about lift and drag coefficients are readily seen in figure 17.
Response: Thank you for the comment.
Figures 15 and 16 were removed as suggested.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Accepted
