Prediction of Seabed Scour Induced by Full-Scale Darrieus-Type Tidal Current Turbine
Round 1
Reviewer 1 Report
General Comments:
The manuscript titled “Prediction of seabed scour induced by full-scale 2 Darrieus type tidal current turbine” by Chong Sun et al. propose a numerical model implemented in ANSYS Fluent as a User Defined Function to to calculate scour development induced by Darrieus type tidal current turbine. The model is validated through experimental scour data. The validated model is used to investigate scour process of a full-scale vertical axis tidal current turbine.
This is a well written and well organized paper. The topic is interesting to the journal and, in particular, it would find readers interested in marine renewable energy, specifically in tidal and offshore wind energy.
In order to improve the manuscript, there are several aspects that need to be addressed before the manuscript can be accepted for publication.
Specific comments:
Consider include a notation section. Give a brief introduction of the different types of foundations for tidal energy turbines, i.e. monopile, gravity based, floating. Line 48: define tip clearance in Fig. 1 or Fig. 2. Line 69 to 72: References [14,15] are not relevant to the present study, their findings are towards turbine performance not about turbine induced scour. Line 72 to 74: Consider replace reference [16] with the more up to date work of Gonzalez-Gorbeña et al. (2018), where the effect of tidal array size on morphodynamics of a multi-inlet coastal lagoon is assessed. This study shows that large amounts of energy extraction may cause significant erosion in adjacent inlets. Line 97-102: Consider merge this paragraph with the previous one, some information is repeated. Line 99: What version of Fluent did you use? Section 2.1 Hydrodynamic mode: give details of the Fluent solver and solution methods used in your simulations, I.e. pressure-based or density-based, pressure velocity coupling method, spatial discretisation settings, etc. Line 110: change the style of the variable P to p (lowercase and italic) to match Eq. 1. 2. Define parts of turbines, i.e. blades and monopile. Line 137: change font style of k-w to italic. Line 137-138: What do you want to mean with “It is assumed that a “lid” at the top surface”. Line 150: show variables D and H in Fig.1 or Fig. 2. Section 2.1.3 Boundary conditions: what are the turbulence conditions at inlet/outlet boundaries? Section 2.1.4: what is the y+ value and sizing used around the blades and foundation pile in each case? Section 2.1.4: Did you use inflation layers near the bottom wall to capture the flow boundary layer? Section 2.1.4: what is the height of the region unstructured mesh grid? Line 193: correct “flied”. Section 2.2.1: Can you use a granulometric curve or just one size of sediment defined by d50? Line 215: left size of Eq. 5, both the numerator and denominator are the critical shear stress. Line 215: Lift and drag coefficients of the sediment grain? Line 217: define this angle in Fig. 5. Line 219: include in the text the variable representing the sediment transport rate. Try to define in the text all the variables that appear in the equations. Line 224: in Eq. 6, are you comparing the angle between the slope and horizontal plane with the critical Shields number? Line 232: what do you want to mean with: “when the slope of scour hole is better than bed slope angle of repose”. Line 240: is this angle alpha? Line 254: why you raise the morphologic time step? Section 2.2.1: the user defined morphologic model will be available open-source or under a license? Table 2: Dimensionless tip clearance C/H, C stands for what? Line 308: Provide a reference for Melville’s experiment. Line 310: correct “filed”. Section 3.2.1: Flow field behind the pile is stable? How do you think vortex formation behind the pile could affect sediment transport? Line 316: How much similar? Section 3.2.1: I’m not very convinced about comparing results from turbines operating in water and wind flows. Why haven’t you run your CFD model using a wind flow instead of a water flow? Section 3.2.1: Do you think the rotation of the turbine may affect the asymmetry of the wake profile shown in Fig. 8. 8. At what distance downstream was taken the profiles shown in Fig. 8? Fig. 8: show rotation direction of the turbine. Section 3.2.2: Consider to include in your analysis bed slopes of scour hole (upstream, downstream, at both sides of the pile). 9b and 9c: Consider to include longitudinal and transversal distance scales. Repeat for Figs 10 and 11. Line 408-409: Why do you think this happens? Give a brief discussions as you do in following sections, i.e. line 447-452, and Section 4.2.
Author Response
Thanks for the comments, we have revised the manuscript and made a response as following:
Responses to Reviewer#1’s comments:
Comment 1: Consider include a notation section. Give a brief introduction of the different types of foundations for tidal energy turbines, i.e. monopile, gravity based, floating.
Response 1：We have included this notation section in lines (37-40)
Comment 2: Line 48: define tip clearance in Fig. 1 or Fig. 2.
Response 2: We have defined the tip clearance in Fig.2. Tip clearance is vertical clearance between turbine rotor and seabed.
Comment 3: Line 69 to 72: References [14,15] are not relevant to the present study, their findings are towards turbine performance not about turbine induced scour.
Response 3: References[14-15] shows the engineering application and research about Darrieus type tidal current turbine. Though these researches are not about turbine induced scour, they can act as research foundation for turbine scour. On the other hand, there is very few study about Darrieus type turbine scour.
Comment 4: Line 72 to 74: Consider replace reference [16] with the more up to date work of Gonzalez-Gorbeña et al. (2018), where the effect of tidal array size on morphodynamics of a multi-inlet coastal lagoon is assessed. This study shows that large amounts of energy extraction may cause significant erosion in adjacent inlets. Response 4: Thank you for the advice, we have replaced reference [16] with Gonzalez-Gorbeña et al. (2018) in lines(75-77).
Comment 5: Line 97-102: Consider merge this paragraph with the previous one, some information is repeated. Line 99: What version of Fluent did you use?
Response 5: We have merged this paragraph with the previous one in lines(93-97). Fluent version is Fluent 2018.
Comment 6: Section 2.1 Hydrodynamic mode: give details of the Fluent solver and solution methods used in your simulations, I.e. pressure-based or density-based, pressure velocity coupling method, spatial discretisation settings, etc.
Response 6: We have added the details of the Fluent solver in lines(110-118). We use SIMPLE algorithm, which is a kind of pressure-based solver, and we use Second-Order Upwind Scheme for spatial discretization. More details of the algorithms can be found in Fluent Help.
Comment 6: Line 110: change the style of the variable P to p (lowercase and italic) to match Eq. 1. 2.
Response 6: We have changed it.
Comment 7: Line 137: change font style of k-w to italic.
Response 7: We have changed it.
Comment 8: Line 137-138: What do you want to mean with “It is assumed that a “lid” at the top surface”.
Response 8: We use symmetry boundaries to simulate the surface of water. As seen, the model does not have a free surface facility. It’s like a “lid” at the top surface. This is a simplified method to model the water surface since the change of free surface is not significant for seabed scour problems. Many researchers used this method to simulate scour problems, as illustrated in line 150.
Comment 9: Line 150: show variables D and H in Fig.1 or Fig. 2.
Response 9: We have shown these variables in Fig.2.
Comment 10: Section 2.1.3 Boundary conditions: what are the turbulence conditions at inlet/outlet boundaries?
Response 10: The turbulence intensity at inlet/outlet boundaries is 5% for medium intensity as default value which is suggested in Fluent Help. We found that turbulence intensity at boundaries showed little impact on turbine scour development.
Comment 11: Section 2.1.4: what is the y+ value and sizing used around the blades and foundation pile in each case? Section 2.1.4: Did you use inflation layers near the bottom wall to capture the flow boundary layer? Section 2.1.4: what is the height of the region unstructured mesh grid?
Response 11: We didn’t use inflation layers near the bottom wall because the bottom wall was unstructured grid to capture its morphological changes. But we made very fine grids and ensured the y+ value was under the suitable range for k-w SST model. In Fluent Help, k-w SST model can adapt the size of mesh grid automatically with different y+. In our model, the y+ is between 20~60, k-w SST model can use wall function automatically to calculate flow field. The height of the region unstructured mesh grid is same as the height of tip clearance. In CFD calculation, this region acts as dynamic mesh and is smoothed in each time step.
Comment 12: Line 193: correct “flied”.
Response 12: We have corrected it.
Comment 13: Section 2.2.1: Can you use a granulometric curve or just one size of sediment defined by d50?
Response 13: In scour research, d50 is an important parameter to represent the size of sediment. Many researchers investigated different types of scour and proposed empirical equations to predict the scour depth based on d50 , such as pier scour, propeller scour, turbine scour and so on. The parameter d50 can be used to define the probable size of sediment. Hence we use d50 to represent sediment size in our morphologic model.
Comment 14: Line 215: left size of Eq. 5, both the numerator and denominator are the critical shear stress. Line 215: Lift and drag coefficients of the sediment grain?
Response 14: We have corrected it. The numerator is critical shear stress on the slope, and the denominator is critical shear stress in flat plan. In our model, we use /=0.85 to express the hydrodynamic coefficients of sediment which is suggested by Nagata N et al. as shown in line 222.
Comment 15: Line 217: define this angle in Fig. 5.
Response 15: We added a figure to show the slope model, and we have defined the angle in Fig.5.
Comment 16: Line 219: include in the text the variable representing the sediment transport rate. Try to define in the text all the variables that appear in the equations.
Response 16: We have defined in the text all the variables that appear in the equations in lines(229-230).
Comment 17: Line 224: in Eq. 6, are you comparing the angle between the slope and horizontal plane with the critical Shields number?
Response 17: The is real local Shields number, and is critical Shields number. So we are comparing local Shields number with critical Shields number. If , the sediment can transport. We used to represent both angle and Shields number in initial version, this is a mistake. We have corrected it. And we use to represent angle in Eq. (7).
Comment 18: Line 232: what do you want to mean with: “when the slope of scour hole is better than bed slope angle of repose”.
Response 18: We changed this sentence to be “when the local bed slope exceeds the angle of repose”. Previous researches showed that, during the development of scour holes, there are areas at the upstream face of the scour hole where the local bed slope exceeds the angle of repose, and, as a result, shear failures occur at these locations. We added the explanation in lines(242-243).
Comment 19: Line 240: is this angle alpha?
Response 19: The angle is angle between slope and horizontal plane . This is a mistake in the initial version of manuscript. We have corrected it both in Fig.(6) and line 251.
Comment 20: Line 254: why you raise the morphologic time step?
Response 20: We raise the morphologic time step because the dynamic change speed of seabed shape gets slower as time goes by. So we raise time step to accelerate the CFD calculation. This is a method to improve the computational efficiency. We added an explanation in lines(266-269).
Comment 21: Section 2.2.1: the user defined morphologic model will be available open-source or under a license?
Response 21: The user defined morphologic model should be under the license of Fluent because we use the User Defined Function in Fluent. However, the code of morphologic model can be modified to insert an open-source CFD solver. It will be an interesting work.
Comment 22: Table 2: Dimensionless tip clearance C/H, C stands for what?
Response 22: C stands for tip clearance, and H stands for turbine height. We also defined it in Fig.(2).
Comment 23: Line 308: Provide a reference for Melville’s experiment.
Response 23: We have added the reference.
Comment 24: Line 310: correct “filed”.
Response 24: We have corrected it.
Comment 25: Section 3.2.1: Flow field behind the pile is stable? How do you think vortex formation behind the pile could affect sediment transport?
Response 25: Flow field behind the pile in not stable. Because we use the Reynolds-averaged Navier-Stokes (RANS) equations. They have the same general form as the instantaneous Navier-Stokes equations, with the velocities and other solution variables now representing ensemble-averaged (or time-averaged) values. Additional terms now appear that represent the effects of turbulence. So the velocity downstream has been averaged. The vortex formation behind the pile can expand the scour hole behind pile, this phenomenon has been illustrated by many previous pile scour researches. In addition, we also observe this phenomenon in the experiment. However, our CFD model can not capture the unsteady faction due to the Reynolds-averaged Navier-Stokes (RANS) method like Roulund’s model[12]. Hence the equilibrium scour depth of simulated scour hole is a little shallower than experiments.
Comment 26: Line 316: How much similar?
Response 26: It can be seen that flow acceleration around pile and flow reduction and reversed flow behind the pile occurred either in our CFD model or Melville’s experiment. The flow return to stability after about 1D. However, our model failed to simulate the vortex after pile due to the defect of RANS model at high Reynolds number. we added the explanation in lines(326-329).
Comment 26: Section 3.2.1: I’m not very convinced about comparing results from turbines operating in water and wind flows. Why haven’t you run your CFD model using a wind flow instead of a water flow?
Response 26: Firstly, the most important reason why we compare the results of wake from CFD model of water turbine and wind turbine is that there is no researches about wake of Darrieus type tidal current turbine. The experimental study of Darrieus type wind turbine wake is the closest research to validate our hydrodynamic model.
Secondly, water turbine and wind turbine have same operating principle only with different type of fluid. The turbine captures energy and blocks the fluid and then flow velocity recovers in wake. It can be seen the efflux velocity shows same flow trends in Fig.(9). However, the low velocity region of tidal current turbine wake is wider than wind turbine wake due to the higher density of water than air.
Thirdly, many researchers used the sliding mesh model to simulate the flow field and Hydrodynamic performance around Darrieus type tidal current turbine, such as Dai and Lam[15].
Therefore, we think we can use this comparison to prove our hydrodynamic model can work effectively. In the future, we may do the experiment of Darrieus type tidal current turbine wake and compare to our CFD model to make it more convincing.
Comment 27: Section 3.2.1: Do you think the rotation of the turbine may affect the asymmetry of the wake profile shown in Fig. 8. 8. At what distance downstream was taken the profiles shown in Fig. 8?
Response 27: Of course the rotation of turbine can affect the asymmetry of the wake profile. In Fig.(9), it can be seen that the flow velocity at Y/D=0~0.6 is a little slower than Y/D=-0.6~0. This is due to the rotation of turbine rotor. In Fig.(9), the distance is near efflux plane of turbine at about x=R downstream, as shown in the following picture.
Comment 28: Fig. 8: show rotation direction of the turbine.
Response 28: We have shown the rotation direction in Fig.(9).
Comment 29: Section 3.2.2: Consider to include in your analysis bed slopes of scour hole (upstream, downstream, at both sides of the pile).
Response 29: We have added the analysis of bed slopes of scour hole in lines(397-401).
Comment 30: 9b and 9c: Consider to include longitudinal and transversal distance scales. Repeat for Figs 10 and 11.
Response 30: We have added the longitudinal and transversal distance of Figs.(11,12) in lines(356-359, 381-383).
Comment 31: . Line 408-409: Why do you think this happens? Give a brief discussions as you do in following sections, i.e. line 447-452, and Section 4.2.
Response 31: We have added a brief discussion for the big change of scour depth from C/H=0.75 to C/H=0.5 in lines(434-438). This is because when turbine rotor is installed high enough away from seabed, the scour hole mainly induced by accelerated flow below rotor. However, when turbine is installed close to seabed (C/H<0.5), the flow compression effect due to turbine rotor and increased turbulence intensity around tip can act together to increase the scour degree. Hence the scour depth shows a big jump from C/H=0.75 to C/H=0.5.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear authors,
Please refer to the attached document for detailed comments.
Thanks.
Comments for author File: Comments.pdf
Author Response
Thanks for the comments, we have revised the manuscript and made a response as following:
Responses to Reviewer#2’s comments:
Comment 1: First of all, transport of suspended sediments neglected in the model. This might be OK, but authors must discuss why suspended load has been neglected either through analytically/numerically, or at least providing references which show suspended load is negligible for scour around piles. To me it seems like that the huge discrepancy in time when equilibrium is reached between model results and experiments (Figure 18) might be due to this fact.
Response 1: Thanks for the advice. We have added the discussion and reference in lines(465-468). The transport of suspended sediments was neglected in the proposed CFD model. The reason is as follows:
Firstly, the suspended sediments show little impact on local scour around foundation pile. Local scour around turbine was caused by downflow and horseshoe vortex around pile. The suspended sediments can be taken away by tidal current and deposited downstream. In the current study, we are proposed to predict the scour depth of full-scale turbine, and suspended sediments cannot affect the equilibrium scour depth obviously. Secondly, we use the dynamic mesh method to develop the morphologic model. This model mainly controlled the relationship between sediment transport rate and seabed profiles change. In previous researches for pile scour, the transport of suspended sediments was also neglected, such as [12,19,28,29].
In our opinion, the discrepancy in time when equilibrium is reached between model results and experiments (Figure 18) is not due to the suspended load. The main reason is the defect of RANS hydrodynamic model. Because the RANS turbulence model can not capture the vortex at high Reynolds number. It uses turbulence intensity to represent the flow’s instability. But this unstable factor is not used for calculation of seabed shear stress. So our morphologic model can’t consider these unsteady factors. However, the vortex system can promote turbine scour in reality. Therefore, the scour depth calculated by CFD model reaches equilibrium depth faster than experiment. And the scour depth is also shallower in CFD results than experiment. We have added a discussion in lines(537-542).
Comment 2: Also, authors are convinced that their model works quite well based on qualitative comparisons with experiments. However, Figures 7 to 10 suggest otherwise to me. In Figure 7, where they compare flow field predicted by their tool with Melville’s experiment results, it is clear that their model fails to generate the wake region behind the pile properly. Additionally, I don’t know why the flow vectors in Figure 7a are distributed unevenly. Since their model uses a structured mesh, the flow vectors should appear uniform in the computational domain.
Response 2: In Figure 7, our model captures the main trends of. It can be seen that flow acceleration around pile and flow reduction and reversed flow behind the pile occurred either in our CFD model or Melville’s experiment. The flow return to stability after about 1D. However, our model failed to simulate the vortex after pile due to the defect of RANS model at high Reynolds number. This phenomenon has also been pointed out by Xiong et al.[29].
We use k-w SST turbulence model to calculate the flow field. It is a kind of Reynolds-averaged Navier-Stokes (RANS) equations. They have the same general form as the instantaneous Navier-Stokes equations, with the velocities and other solution variables now representing ensemble-averaged (or time-averaged) values. Additional terms now appear that represent the effects of turbulence. So the velocity downstream has been averaged and wake region behind the pile shows a little different.
On the other hand, the wake behind the pile can affect the horizontal development of scour hole downstream, but it shows little impact on scour depth[6]. Because the equilibrium scour depth mainly depends on downflow and horseshoe vortex. Hence our model underestimates the scour depth and scour hole width. This is the defect of dynamic mesh method, we have added a discussion in lines(454-468).
Additionally, the reason why the flow vectors in Figure 7a are distributed unevenly because we uses the unstructured mesh in flow areas below turbine, as said in lines(165-167). The overall mesh combined technologies of structured grids(upper domain contains turbine rotor) and unstructured grids(lower domain between rotor and seabed). The upper structured grids can speed up the numerical calculation. The lower unstructured grids can better adapt the updating bed boundary under scour process.
Comment 3: The mismatch in flow speeds shown in Figure 8 is very obvious, which implies that Bianchini’s experiment isn’t a good test case to begin with.
Response 3: Thanks for the advice. The mismatch phenomenon occurs because Bianchini’s experiment is about wind turbine, but our model is tidal current turbine. The experimental study of Darrieus type wind turbine wake is the closest research to validate our hydrodynamic model.
Secondly, water turbine and wind turbine have same operating principle only with different type of fluid. The turbine captures energy and blocks the fluid and then flow velocity recovers in wake. It can be seen the efflux velocity shows same flow trends in Fig.(9). However, the low velocity region of tidal current turbine wake is wider than wind turbine wake due to the higher density of water than air.
Thirdly, many researchers used the sliding mesh model to simulate the flow field and Hydrodynamic performance around Darrieus type tidal current turbine, such as Dai and Lam[15].
Therefore, we think we can use this comparison to prove our hydrodynamic model can work effectively. In the future, we may do the experiment of Darrieus type tidal current turbine wake and compare to our CFD model to make it more convincing.
Comment 4: Lastly, the comparison of morphological changes shown in Figures 9 and 10 also don’t show similar patterns with experiments. In the model, maximum scour occurs in a limited region on the side of the pile whereas it wraps the pile around according to experiments. There is also a clear shift in the location of the maximum scour compared to experiments. In short, I cannot agree with authors that their model has a good agreement with the experiments.
Response 4: This is the first time to use dynamic mesh model to investigate seabed scour induced by tidal current turbine. In previous study, some researchers used this model to predict scour depth around piles. This model can simulate the main factor of scour development: high shear stress around pile and downflow in front of monopile. This model also has efficient computing performance.
However, this model cannot capture the unsteady factors like the fluctuating components of horseshoe vortex and lee-wake vortex flows. Hence the model underestimate the maximum scour depth and horizontal extent of scour hole. This result has been pointed out by Roulund et al.[12]. Based on our turbine scour experiment and analysis of previous researchers’ data, we found the Underestimating proportion is about 15%-20%.
In short, even though the proposed model cannot simulate the scour hole shape perfectly, it can provide a new CFD model for turbine scour. This model can provide a relatively shallower scour depth and give researchers or engineers a reference value for turbine scour.
Comment 5: They just present three figures (Figs 12,13 and 15) for comparison of large scale tests with experiments and these figures also fall short to convince me that their model can be useful provide some insight about the overall process.
Response 5: Considering the fact that no previous work investigate the full-scale Darrieus type tidal current turbine scour, we set up two steps to validate our model. Firstly, we set up two scour models(T1 and T2). Model T1 was pile scour model without turbine rotor. This model was aim to validate the numerical scour model with the comparison of Melville’s experiment of scour at piers. Model T2 was turbine scour model including rotating rotor with same pile diameter as model T1. This model was aim to investigate the impact of rotating rotor on seabed scour compared to single pile scour. The results can be found in section(3).
Secondly, we set up four scour models with different tip clearance from C/H=0.25 to C/H=1.0 (T3-T6). This group was also aim to validate the numerical model compared to experimental model tests for tidal current turbine.
After validation, we use this model to investigate the full-scale turbine scour. Fig.13 shows the equilibrium scour depth at different tip clearance. It can be seen the proposed CFD model underestimates about 15%-20% of scour depth compared to experiment. we added discussion in lines(434-438). Fig.14 shows the horizontal extent of scour hole and Fig.16 shows the comparison among different experiments and CFD models. We have reviewed all the closest researches to prove the applicability of the proposed model. In addition, Figs(17-19) also show the results of full-scale tidal current turbine scour.
In summary, we think section 4 has described the scour depth, scour profiles and temporal scour development in detail. The proposed model has also been validated in section(3). Maybe the proposed model cannot simulate the scour process very accurately, but the model can be useful provide some insight about the overall process.
Comment 6: Overall, like I said, the results are explained qualitatively, rather than quantitatively. I haven't seen any mention of the error in the paper at all. Therefore, authors must provide quantitative comparisons and also need to explain the observed differences between modeled and observed results. Shortcomings of the model must be discussed, and the probable causes must again be addressed. Right now, they only blame “unsteady effects” that cannot be captured by the model they use. So, maybe they should consider using a different hydrodynamic solver.
Response 6: Thank you for the advice, we have added a paragraph to discuss the shortcomings of the proposed model, and provided quantitative comparisons and explanations in lines(464-468). We also discussed the “unsteady effects”. However, the use of RANS hydrodynamic solver is the coordination results between computational efficiency and accuracy. If we consider the unsteady effects and use like LES model, the computing cost can be very expensive. Maybe we will try to use different hydrodynamic solver in the future.
Comment 7: L51 – L53 – Authors cite Zhang [8] and Sun [9], say these two references have empirical models. Have you attempted to compare your results with these empirical models?
Response 7: Yes, we have compared our results with their empirical models in Figs.(16, 19). The results shows that scour depth of Darrieus type tidal current turbine is about 30% deeper than horizontal axis turbine.
Comment 8: L112 – Why do you use SIMPLE to calculate the flow conditions?
Response 8: We have added the details of the Fluent solver in lines(124-128). We use SIMPLE algorithm, which is a kind of pressure-based solver, and we use Second-Order Upwind Scheme for spatial discretization. The strength of the SIMPLE method is, that together with implicit time treatment of the flow variables you can efficiently obtain a steady state solution or use rather large time steps for unsteady flow computations. In our model, we need to update the mesh grid in each time step and adjust the time step size according to the seabed scour development. The SIMPLE method shows great convergence and accuracy for turbine scour problems.
Comment 9: L113-114 – I don’t see the continuity equation.
Response 9: We have added the continuity equation in lines().
Comment 10: L119 – What are these aforementioned positive features of k-w and k-e models?
Response 10: In the proposed model, we used SST k-w turbulence model to simulate the flow field. The Shear Stress Transport (SST) k–w turbulence model combines the positive features of both the k–w and the k–e turbulence models. The k–o turbulence model can accurately predict the near-wall region. In contrast, the k–e turbulence model can accurately predict the far field. A blending function, F was applied to combine both models. At the near wall region, the k–w model is multiplied by F equal to1, and the k–e model is multiplied by (1–F). The factor of the blending function for the k–w model reduces to zero at the boundary layer and the k–e model will replace the k–w model to predict the free stream region. Furthermore, the model was modified to account for the
transport of the shear stress inside the boundary layer. The purpose is to improve the accuracy of prediction of the flows with strong adverse pressure gradient and separation.
Comment 11: L132-133 – Inlet and outlet boundary conditions need a bit more information. What does pressure-outlet mean for velocity components for instance?
Response 11: We have added more information for inlet and outlet boundary conditions in lines(140-144).
Comment 12: L167 – Is the angular velocity constant throughout the simulations? What is the relation between angular velocity of rotor to flow velocity? Is there any correlation?
Response 12: The angular velocity is constant throughout the simulations in one test. We set the turbine rotor rotates at a certain angular speed to simulate real operated turbine. Researchers usually used this method to study Darreius type turbine, such as Dai and Lam[15]. However, the angular velocity is not same in different CFD test models. the hydrodynamic conditions like inlet velocity and rotational speed were set under same Froude number ( ) as experiments. This is illustrated in lines 289.
Comment 13: L196 – 201 – Discussion in this paragraph is more relevant to result.
Response 13: We agree with this comment, and we moved this paragraph to section 4 in lines (534-548). This section discuss the overall scour process of full-scale turbine.
Comment 14: Section 2.2.1 – How do you calculate instantaneous shear stress to compare with critical shear stress?
Response 14: For no-slip wall conditions, ANSYS Fluent uses the properties of the flow adjacent to the wall/fluid boundary to predict the shear stress on the fluid at the wall. So the shear stress at seabed can be accessed in Fluent to compare with critical shear stress.
Comment 16: L222 – Why did you select 1.5? Have you test the model’s sensitivity to C?
Response 16: The empirical coefficient C=1.5 was suggested by B. Brørs[28] in his research for numerical model of flow and scour at pipelins. We have also tested different coefficient C and found the scour development speed was reasonable when C was set as 1.5.
Comment 17: L228 – What is Porosity (n) value used?
Response 17: The porosity n affect the relationship between sediment transport rate and seabed height change rate. The height of seabed sinks quicker with bigger porosity. The relationship is expressed by Eq.(10).
Comment 18: It seems that there is no mechanism to control sediment mass conservation. If so, must be shown here, and must be explained in the text. What is the model performance in terms of mass conservation?
Response 18: Yes, the sediment mass conservation is not controlled in the proposed model. We simulated the scour hole around pile and neglected the sand deposition. This is controlled by Eq.(). If , the sediment can transport and seabed sinks, otherwise the seabed keeps the original height. We have added the explanation in the text in lines(238-240).
Comment 19: Section 2.2.3 – What do you mean by morphologic time step? Do you update the seabed elevation at every time step?
Response 19: Yes, we used the transient simulation. We update the seabed elevation at every time step. Hence this model can also simulate the temporal scour development. The scour speed is controlled by sediment transport rate in Eq.(10). We need to raise the morphologic time step because the dynamic change speed of seabed shape gets slower as time goes by. This is a method to improve the computational efficiency.
Comment 21: L280 – Reynolds number seems too high based on the parameters given in Table 2. Can you explain how did you find 287500?
Response 21: We are sorry to find that we wrote wrong Reynolds number. The calculated wake Reynolds number the wake is about 25640. , where ,L=0.1126m(diameter of turbine rotor), V=0.23m/s, and . Re>10000, so the Reynolds number in turbine scour tests is also high enough to neglect the viscosity based on Rajaratnam’s suggestion[32]. We have corrected this in line(293).
Comment 22: L288 – Do Melville experiments have the same setup? Not all the readers might have access to cited paper, thus more info is needed, maybe a sketch?
Response 22: We have added more information of Melville’s experiments in lines(302-305). Melville experiments have the same setup of inlet velocity, sediment diameter d50 and pile diameter.
Comment 23: L46 – Serious how? A bit vague. Do you mean scouring is more profound under the turbines than around the piles for similar flow conditions?
Response 23: Yes, this is what we want to express. We have revised this sentence in line().
Comment 24: L79 – Replace limit with limited.
Response 24: We have replaced it.
Comment 25: L89 – not only… but also… usage is wrong. It is a tricky phrase even for native speakers. I’d suggest authors to replace this with two simpler sentences.
Response 25: We have replaced it. Thanks for the advice.
Comment 26 L108 – Replace “solving” with “solves”
Response 26: We have replaced it.
Comment 27: L110 – In equation (1) small letter p is used for pressure, whereas capital P is used in text. Pick either for consistency.
Response 27: We have corrected it.
Comment 28: L111 - μ is “dynamic” viscosity and should be introduced as such.
Response 28: We have replaced “viscosity” with “dynamic viscosity”.
Comment 29: L118 – Replace “to specific” with “specifically”.
Response 29: We have replaced it.
Comment 30: Letter “w is used” in k-w instead of letter “omega”.
Response 30: We have corrected it.
Comment 31: L225 and L228 – Riverbed?
Response 31: It should be “seabed”, we have corrected it.
Comment 32: L229 – Replace “real-timely” with “in real time”.
Response 32: We have corrected it.
Comment 33: L231-232 – Slope is better? Replace “better” with “steeper”.
Response 33: We have revised this sentence.
Comment 34: L255-256 – Can be stated clearer, it is hard to comprehend as is.
Response 34: We have revised this sentence in lines(266-269).
Comment 35: L265 – Papers, make it singular. Only one paper has cited.
Response 35: We have revised it.
Comment 36: L275 – Rotational what? I guess they meant “rotational velocity”?
Response 36: Yes, this is rotational velocity, we have corrected it.
Comment 37: L287 and L293 – Replace “was aim to” with “was aimed to”.
Response 37: we have replaced it.
Comment 38: L296 – L305 – English got really rough in this paragraph. Please try to rewrite concisely.
Response 38: We rewrite this paragraph in lines(310-316).
Comment 39: L316 – Replace “similarly” with “similar”.
Response 39: We have replaced it.
Comment 40: L380 and L426 and L426– Replace “due to the ignore” with “since unsteady effects are neglected”.
Response 40: We have replaced it.
Comment 41: L386 – Replace “height from seabed” with “above from seabed”.
Response 41: We have replaced it.
Comment 42: L408 – Correct remove “A”.
Response 42: We have removed it.
Comment 43: L415 – Replace “processed” with “illustrated”.
Response 43: We have replaced it.
Comment 44: L419 – Rephrase it as “sand dune formed behind the turbine”.
Response 44: We have rephrase it.
Comment 45: L425 – Replace “reasonably” with “reasonable”.
Response 45: We have replaced it.
Comment 46: L433 – Replace “predict” with “prediction”.
Response 46: We have replaced it.
Comment 47: L477 – Replace “lesser” with “less”.
Response 47: We have replaced it.
Comment 48: L486 – Replace “Time” with “Temporal”.
Response 48: We have replaced it.
Comment 49: Table 2 and Table 4 – Units of velocity is wrong.
Response 49: We have revised it as m/s.
Comment 50: Figure 2. Can benefit from re-plotting. Due to chosen view angle, currently it is hard to grasp the set-up. It looks as if the center of the turbine is not matching the origin of the coordinate system although Figure 4 suggests otherwise.
Response 50: We have replotted the coordinate system in Fig.4 to make it more clearly.
Comment 51: Figure 6. Photograph of setup on the left doesn’t match the sketch on the left. For example, location and direction of propeller is different. Sketch needs to be updated for sanity purposes.
Response 51: We have replotted this figure and made the sketch match experimental set up. The new figure is shown in Fig.(7).
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Authors seem to address most of my editorial comments, however the major, fundamental items are left unanswered and my concerns therefore still persist.
The model evaluation is still very qualitative. I still don’t see any mention of the error in the paper. Authors should have provided quantitative comparisons and needed to explain the observed differences between modeled and observed results. They can employ a method as simple as Brier Skill Score (BSS), but I would like to see some proper quantitative comparisons. Comparison with Melville’s experiment; author’s claim that their model cannot capture the lee-wake due to defects of RANS at high Re, which I don’t agree with. They apparently only consider the steady-state flow conditions and neglect the oscillating flow components. However, this never is discussed properly in the text. They should clearly state that only steady-state flow conditions are considered and remove Figure 8 from the MS or present a figure from one of their simulations where oscillating components are also included. As is Figure 8 only proves that their hydrodynamic set-up is not appropriate. Flow acceleration along the sides of the pile can be achieved by any model with advection, and the essential part of the flow past a cylinder is the generation of wake region behind the pile. Thus, capturing the flow acceleration on the sides doesn’t enough to validate the hydrodynamic solver. Lastly, in addition to their observations they need to discuss the physics behind these observations/findings. As is, this MS doesn’t go beyond an engineering application. They should be able to make more scientific deductions out of all these numerical/physical experiments which would ultimately be a contribution in this field.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 3
Reviewer 2 Report
Dear authors,
Thank you for your edits, I do not have any more comments.
