Aerodynamic Characteristics of New Volleyball for the 2020 Tokyo Olympics
Round 1
Reviewer 1 Report
Introduction:
- Is there any CFD study on ball aerodynamics? It should be noted here
- Is there any wind tunnel test on balls for other sports?
- The effect of critical Reynolds in ball trajectories and the characteristics of typical volley paths would be also welcome here
2.1 how are these balls inflated? And fixed?
Line 70: are all identical in size? May small changes in size affect largely to Cd?
How do you measure air density?
What is the blocking coefficient of the tunnel? Is the tunnel wall boundary layer an issue?
Range and precision of LMC-61256 by Nissho Electric Works should be included and checked against the expected aero forces.
How is the attachment effect evaluated/removed?
Which is the readout frequency for the instrument? Are you averaging? for how long? Is it readout stable? Standard deviation of measurements?
Line 89: how is this Reynolds estimated? Any regression formula? Based on slopes? Etc… please clarify
Line 94: what is the error un Cd figure?
Line 100: how is attitude changed? Is it stationary? Is there a rotation in the model or just a static attitude position?
In general, the paper lacks from detailed info on the experiment and some analysis on the causes and effects of the achieved measurements. A better explanation of experiment and data processing is required together with some verification (numerical or maths…)
Regards
Author Response
Response to Reviewer 1 Comments
We have revised the paper following your guidance and comments as described below. Please verify and let us know if further revisions/clarifications are required.
Introduction:
Is there any CFD study on ball aerodynamics? It should be noted here
Is there any wind tunnel test on balls for other sports? The effect of critical Reynolds in ball trajectories and the characteristics of typical volley paths would be also welcome here
A: Citations of previous studies were added to the paper as follows:
“Moreover, wind tunnel experiments and CFD tests on various sports balls have been performed in previous studies [4-13].”
- Naito, K., Hong, S., Koido, M., Nakayama, M., Sakamoto, K. & Asai, T. Effect of seam characteristics on critical Reynolds number in footballs, Mechanical Engineering Journal, 5(1), 17-00369 (2018) https://doi.org/10.1299/mej.17-00369
- Hong, S. & Asai, T. Effect of panel shape of soccer ball on its flight characteristics. Scientific Reports, 4, 5068 (2014) DOI:10.1038/srep05068
- Hong, S., Asai, T. & Seo, K. Visualization of air flow around soccer ball using a particle image velocimetry. Scientific Reports, 5, 15108 (2015) DOI: 10.1038/srep15108
- Goff, J.E., Hong, S. & Asai, T. Effect of a soccer ball’s seam geometry on its aerodynamics and trajectory, Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 234(1), 19-29 (2020) https://doi.org/10.1177/1754337119876485
- Goff, J.E., Hong, S. & Asai, T. Aerodynamic and surface comparisons between Telstar 18 and Brazuca, Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 232(4), 342-348 (2018) https://doi.org/10.1177/1754337118773214
- Goff, J.E., Asai, T. & Hong, S. A comparison of Jabulani and Brazuca non-spin aerodynamics, Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 228, 188-194 (2014) DOI: 10.1177/1754337114526173
- Passmore, M., Rogers, D., Tuplin, S., Harland, A., Lucas, T. & Holmes, C. The aerodynamic performance of a range of FIFA-approved footballs. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 226, 61-70 (2012) https://doi.org/10.1177/1754337111415768
- Aoki, K., Muto, K. & Okanaga, H. Aerodynamic characteristics and flow pattern of a golf ball with rotation. Procedia Engineering, 2, 2431-2436 (2010) https://doi.org/10.1016/j.proeng.2010.04.011
- Djamovski, V., Rosette, P., Chowdhury, H., Alam, F. & Steiner, T. A comparative study of rugby ball aerodynamics. Procedia Engineering, 34, 74-79 (2012) https://doi.org/10.1016/j.proeng.2012.04.014
- Price, D., Jones, R., & Harland, A., Computational modelling of manually stitched soccer balls. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology,220, 259-268. (2006) https://doi.org/10.1243/14644207jmda83
2.1 how are these balls inflated? And fixed?
Line 70: are all identical in size? May small changes in size affect largely to Cd?
How do you measure air density?
What is the blocking coefficient of the tunnel? Is the tunnel wall boundary layer an issue?
A: The following information was added to answer this:
“Furthermore, in the measuring system of this study, the dynamic pressure can be measured automatically with 0.1 m/s intervals by means of the Pitot-static tube placed above the measuring portion of the volleyball. In addition, because the position of the ball during the measurement procedure is set almost at the center of the nozzle cross section to adjust the distance between the nozzle and the ball to zero, the flow generated from the Pitot tube may not have a direct effect on the flow around the ball. Furthermore, the length of the sting used in this study was 0.8 m and its width were 0.02 m. The diameter of the volleyball used in this study was uniformly 0.2 m, its weight was 0.269±0.002 kg, and its internal pressure was set to 0.3 kgf/cm2.”
Range and precision of LMC-61256 by Nissho Electric Works should be included and checked against the expected aero forces.
How is the attachment effect evaluated/removed?
Which is the readout frequency for the instrument? Are you averaging? for how long? Is it readout stable? Standard deviation of measurements?
A: It was corrected as follows:
“Air forces acting on a mounted ball were measured during a 10s time interval by a sting-type six-component force detector (model number LMC-61256 by Nissho Electric Works Co,Ltd). Data recording was done on a personal computer with an A/D converter board that has a 1000-Hz sampling rate.”
Line 89: how is this Reynolds estimated? Any regression formula? Based on slopes? Etc… please clarify
Line 94: what is the error un Cd figure?
A: The following information was added to answer this:
“Furthermore, aerodynamic measurements for each volleyball were performed three times and their average values compared. In this study, aerodynamic measurements for each volleyball were performed for wind speeds (Reynolds number) from 7 m/s (approximately Re = 1.0 × 105) to 35 m/s (approximately Re = 4.5 × 105) at intervals of 1 m/s.
Re = ν D /v (1)
Where Reynolds number (Re) is a dimensionless number defined as the ratio of inertial force to viscous force, ν is wind speed (m/s), D is ball diameter (m), and v is the kinematic viscosity (m2/s), as shown in equation (1).”
Line 100: how is attitude changed? Is it stationary? Is there a rotation in the model or just a static attitude position?
A: It was corrected as follows:
“The next graphs show the irregular force fluctuation in the horizontal and vertical directions of each volleyball when a wind velocity of 15 m/s is applied for 10 s (Fig. 4). Therefore, these are the scatter diagrams of the changes in the lift and side forces applied to different types of volleyball and panel orientations for 10 s. From Fig. 4, it is possible to verify that the irregular forces (lift and side forces) acting on the ball varied depending on the type of volleyball. These results also showed that if the panel orientation changed for a volleyball being tested, the forces also changed drastically (Table 1). With the old Molten ball, the forces changed considerably when the panel orientation changed. However, because the new volleyball for the Tokyo Olympics is less dependent on panel orientation changes, similar results were obtained. Therefore, the flight trajectory of the new volleyball for the Tokyo Olympics is expected to be more stable than that of the old volleyball.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The reviewer appreciates the work and has the following comments:
- a major English language review is needed. Simple mistakes, like mixing up the word "panel", preposition and others written twice. Major mistakes in other sentences
- there are various abbreviations that authors miss to explain their meaning, for example: FIVB, NCAA, and others
- please, write about the expected accuracy of the wind tunnel measurements. I think also the set-up could be better described (for example the support for the ball)
- How do the Re values run in the tunnel compared to usual Re in volleyball playing? (surving, smashing, etc…)
- please define Recritical
- I think that the results and conclusions are a bit poor. Most of it, just says what the results of the wind tunnel were. Fig. 3 could be further explain as why the Cd for a same ball change depending on position A,B. I mean, why do the curves cross each other several times? I think the authors should explain more what the consequences are for each ball and configuration (A or B), and actually make a recommendation. There is an attempt to do so at that end, but I miss a better discussion of the results.
- Between lines 92-99, there seems to be extra line spacing
- I don’t quite understand Fig. 4. What do the authors want to show? Please, explain it further.
Author Response
Response to Reviewer 2 Comments
We have revised the paper following your guidance and comments as described below. Please verify and let us know if further revisions/clarifications are required.
The reviewer appreciates the work and has the following comments:
a major English language review is needed. Simple mistakes, like mixing up the word "panel", preposition and others written twice. Major mistakes in other sentences there are various abbreviations that authors miss to explain their meaning, for example: FIVB, NCAA, and others
A: We have made the necessary corrections.
please, write about the expected accuracy of the wind tunnel measurements. I think also the set-up could be better described (for example the support for the ball)
A: We have made the necessary corrections.
“Furthermore, in the measuring system of this study, the dynamic pressure can be measured automatically with 0.1 m/s intervals by means of the Pitot-static tube placed above the measuring portion of the volleyball. In addition, because the position of the ball during the measurement procedure is set almost at the center of the nozzle cross section to adjust the distance between the nozzle and the ball to zero, the flow generated from the Pitot tube may not have a direct effect on the flow around the ball. Furthermore, the length of the sting used in this study was 0.8 m and its width were 0.02 m (Fig. 1). The diameter of the volleyball used in this study was uniformly 0.2 m, its weight was 0.269 ± 0.002 kg, and its internal pressure was set to 0.3 kgf/cm2.”
How do the Re values run in the tunnel compared to usual Re in volleyball playing? (surving, smashing, etc…)
A: The following information was added to answer this question:
“The speed of a volleyball in a serve ranges between approximately 50 and 80 km/h, or 2.0 × 105 and 3.0 × 105 if converted to Re. In a volleyball spike (attack), the ball flies at up to 100 km/h (approximately 4.2×105), but the changes in Cd due to the volleyball type and its orientation have been demonstrated to be relatively low at this speed. For this reason, it is considered effective to consider the ball type and orientation when executing different types of serves in a volleyball, including float serves. Moreover, because the new volleyball is expected to have a more stable trajectory than that of the old ball, a longer rally can also be expected.”
please define Recritical
A: The following information was added to answer this:
“The moment at which the flow state of the ball spin changes from laminar to turbulent is called a transition, and the Reynolds number of this transition is called the critical Reynolds number.”
I think that the results and conclusions are a bit poor. Most of it, just says what the results of the wind tunnel were. Fig. 3 could be further explain as why the Cd for a same ball change depending on position A,B. I mean, why do the curves cross each other several times? I think the authors should explain more what the consequences are for each ball and configuration (A or B), and actually make a recommendation. There is an attempt to do so at that end, but I miss a better discussion of the results.
A: The following information was added to answer this question:
“A recent study on the aerodynamic characteristics of soccer balls has indicated that the positional relation of the seams of the ball varies depending on the position of the panels, thereby changing the forces acting on the ball (air resistance).(4) It is considered that the aerodynamic characteristics of volleyballs also change according to the roughness of the ball surface and the positional relation of the seams (e.g., width, depth, and length).”
“With a velocity range of approximately 15 m/s (Re of approximately 2.0×105), which is used in float serves, the Cd results change even more prominently with the ball type and panel orientation. Presumably, this is because the air resistance acting on the ball in float serves varies according to the volleyball type and panel orientation, and as a result, the flight trajectory is modified.”
Between lines 92-99, there seems to be extra line spacing
A: We have made the necessary corrections.
I don’t quite understand Fig. 4. What do the authors want to show? Please, explain it further.
A: We have made the necessary corrections.
“The next graphs show the irregular force fluctuation in the horizontal and vertical directions of each volleyball when a wind velocity of 15 m/s is applied for 10 s (Fig. 4). Therefore, these are the scatter diagrams of the changes in the lift and side forces applied to different types of volleyball and panel orientations for 10 s. From Fig. 4, it is possible to verify that the irregular forces (lift and side forces) acting on the ball varied depending on the type of volleyball. These results also showed that if the panel orientation changed for a volleyball being tested, the forces also changed drastically (Table 1). With the old Molten ball, the forces changed considerably when the panel orientation changed. However, because the new volleyball for the Tokyo Olympics is less dependent on panel orientation changes, similar results were obtained. Therefore, the flight trajectory of the new volleyball for the Tokyo Olympics is expected to be more stable than that of the old volleyball.”
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments have been addressed. Still numerical analysis missing but experimental aerodynamic conclusions are ok.
Regards
Reviewer 2 Report
Dear authors,
Thank you for responding to my comments. I believe the paper has improved in its contents.
