applied sciences

Article Aerodynamic Characteristics of New for the 2020 Tokyo Olympics

Sungchan Hong 1,* , Hiroki Ozaki 2, Keita Watanabe 3 and Takeshi Asai 1

1 Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba 305-8574, Japan; [email protected] 2 Japan Institute of Sports Sciences, Tokyo 115-0056, Japan; [email protected] 3 Faculty of Culture and Sport Policy, Toin University of Yokohama, Yokohama 225-8503, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-29-853-2650

 Received: 8 April 2020; Accepted: 30 April 2020; Published: 7 May 2020 

Abstract: The pattern of a modern volleyball is greatly different from that of a conventional volleyball, with several changes being made to the shape and design of the surface on the . Furthermore, at the 2020 Tokyo Olympics, a new volleyball (V200W; Mikasa) with 18 panels will be shown as the official ball. Therefore, this study compared the basic aerodynamic characteristics of conventional volleyballs with those of new designs in a wind tunnel. We used three full-size FIVB (Fédération Internationale de Volley-Ball) official volleyballs (V5M5000; Molten, MVA200; Mikasa and V200W; Mikasa) to determine the aerodynamic forces acting on each ball. The results indicate that the critical Reynolds number (Recr) differed depending on the ball types and their orientations. The Recr for the Molten ball (conventional) was determined to be ~3.4 105 (Cd = 0.17) on panel orientation A and 5 × ~2.7 10 (Cd = 0.14) on panel orientation B. Moreover, the Recr for the conventional Mikasa ball was × determined to be ~2.6 105 (Cd = 0.14) on panel orientation A and ~3.0 105 (Cd = 0.13) on panel × × orientation B. On the other hand, the critical Reynolds number for the new volleyball (V200W) was ~2.9 105 (Cd = 0.17) in the panel orientation A and ~2.6 105 (Cd = 0.15) in panel orientation B. × × From these results, it can be hypothesized that, during a float serve, the flight trajectory will change depending on the type of volleyball and their orientation.

Keywords: aerodynamics; Tokyo Olympics; volleyball

1. Introduction The shape and design of volleyballs have substantially changed in recent years. Volleyballs are typically composed of a total of six planes, each of which is formed by three rectangle panels, totaling 18 panels. In 2008, a dimple-style eight-panel design ball (MVA200 by Mikasa), substantially different from conventional official , made its debut in the volleyball games at the Beijing Olympics. The use of this ball continued at the 2008 Beijing Olympics and 2016 Rio Olympics, with this ball still being officially used in international competitions such as the FIVB (Fédération Internationale de Volley-Ball) World League. Subsequently, a volleyball with a raised hexagon-shaped design on the surface (V5M5000 by Molten) was introduced in 2008 and has been adopted as the official ball by the NCAA (National Collegiate Athletic Association) Volleyball tournament and the USA Volleyball. The aerodynamic characteristics of these balls, having significantly different shapes and surface textures from those of traditional volleyballs, have been studied previously using wind-tunnel experiments [1–3]. Moreover, wind-tunnel experiments and CFD tests on various sports balls have been performed in previous studies [4–13]. 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,

Appl. Sci. 2020, 10, 3256; doi:10.3390/app10093256 www.mdpi.com/journal/applsci Appl. Sci. 2020,, 10,, x 3256 FOR PEER REVIEW 2 of 7 aerodynamic characteristics of volleyballs also change according to the roughness of the ball surface thereby changing the forces acting on the ball [5]. It is considered that the aerodynamic characteristics and the positional relation of the seams (e.g., width, depth, and length). of volleyballs also change according to the roughness of the ball surface and the positional relation of However, as there have not been any studies conducted on the aerodynamic and flight the seams (e.g., width, depth, and length). characteristics of the new official volleyball (V200W, 18‐panels by Mikasa) to be used in the 2020 However, as there have not been any studies conducted on the aerodynamic and flight characteristics Tokyo Olympics, an investigation into these was sought. This study examined primarily the effect of of the new official volleyball (V200W, 18-panels by Mikasa) to be used in the 2020 Tokyo Olympics, panel orientation on the aerodynamic characteristics, flight characteristics, and aerodynamic force an investigation into these was sought. This study examined primarily the effect of panel orientation on variation of the V200W ball, and compared the results with those of the balls used in the 2018 FIVB the aerodynamic characteristics, flight characteristics, and aerodynamic force variation of the V200W World League (MVA200, 8‐panels by Mikasa) and the 2018 USA Volleyball (V5M5000, 18‐panels by ball, and compared the results with those of the balls used in the 2018 FIVB World League (MVA200, Molten). 8-panels by Mikasa) and the 2018 USA Volleyball (V5M5000, 18-panels by Molten).

2. Methods Methods

2.1.Wind Wind Tunnel Tunnel Experiment Experiment This experiment took place in the closed closed-circuit‐circuit wind wind tunnel tunnel (by (by San San Technologies Technologies Co., Co., LTD, LTD, Tochigi, Japan) at the the University University of of Tsukuba. Tsukuba. The The maximum maximum flow flow velocity velocity of ofthis this wind wind tunnel tunnel was was 55 m/s,55 m the/s, the blower blower outlet outlet size size was was 1.5 1.5m × m 1.5 m,1.5 the m, flow the flow velocity velocity distribution distribution was waswithin within ±0.5%,0.5%, and × ± theand turbulence the turbulence was was 0.1% 0.1% or less. or less. Furthermore, Furthermore, in inthe the measuring measuring system system of of this this study, study, the the dynamic pressure cancan be be measured measured automatically automatically with with 0.1 0.1 m/s m/s intervals intervals by means by means of the of Pitot-static the Pitot tube‐static placed tube placedabove the above measuring the measuring portion ofportion the volleyball. of the volleyball. In addition, In addition, because the because position the of position the ball duringof the ball the duringmeasurement the measurement procedure procedure is set almost is set at the almost center at the of the center nozzle of the cross nozzle section cross to section adjust to the adjust distance the distancebetween between the nozzle the and nozzle the ball and to the zero, ball theto zero, flow the generated flow generated from the from Pitot the tube Pitot may tube not may have not a direct have aeff directect on effect the flow on the around flow around the ball. the Furthermore, ball. Furthermore, the length the length of the stingof the used sting in used this in study this study was 0.8 was m 0.8and m its and width its waswidth 0.02 was m (Figure0.02 m 1(Figure). The diameter 1). The diameter of the volleyball of the volleyball used in this used study in wasthis uniformlystudy was uniformly0.2 m, its weight 0.2 m, was its weight 0.269 was0.002 0.269 kg, and± 0.002 its internalkg, and pressureits internal was pressure set to 0.3 was kgf set/cm to2. 0.3 The kgf/cm official2. balls The ± officialof the 2020 balls Tokyo of the Olympics 2020 Tokyo (V200W), Olympics 2018 (V200W), FIVB World 2018 League FIVB World (MVA200), League and (MVA200), 2018 USA Volleyball and 2018 USAleague Volleyball (V5M5000) league were (V5M5000) fixed and were tested fixed in the and wind tested tunnel in the as wind shown tunnel in Figure as shown1. Furthermore, in Figure 1. Furthermore,aerodynamic measurementsaerodynamic measurements for each volleyball for each were volleyball performed were three performed times and three their times average and values their wereaverage compared. values were In this compared. study, aerodynamic In this study, measurements aerodynamic measurements for each volleyball for each were volleyball performed were for windperformed speeds for (Reynolds wind speeds number) (Reynolds from 7 number) m/s (approximately from 7 m/s (approximatelyRe = 1.0 105) to Re 35 = m1.0/s × (approximately 105) to 35 m/s × Re(approximately= 5.5 105) at Re intervals = 5.5 × 10 of5) 1 at m intervals/s. of 1 m/s. × ReRe == νν D//vv (1)(1) where ReynoldsReynolds number number (Re (Re) is) ais dimensionless a dimensionless number number defined defined as the as ratio the ofratio inertial of inertial force to force viscous to 2 viscousforce, ν isforce, the windν is the speed wind (m speed/s), D (m/s),is the D ball is the diameter ball diameter (m), and (m), v is and the v kinematic is the kinematic viscosity viscosity (m /s), (mas shown2/s), as shown in Equation in Equation (1). (1).

Figure 1. Wind tunnel setup.

InIn this this experiment, the the orientation orientation of of the the panels panels was was divided divided into into two two (Face (Face A and B), and the aerodynamic characteristicscharacteristics ofof the the balls balls in in the the respective respective panel panel orientations orientations were were measured measured (Figure (Figure2). The2). The orientation orientation where where the centerthe center of the of front the front face isface at theis at center the center was set was as set Face as A, Face and A, the and point the where point whereall the seamsall the meetseams at meet the lower at the leftlower of theleft frontof the face front was face set was as Face set as B. Face The B. aerodynamic The aerodynamic force on force the on the respective balls was measured between wind velocities (U) of 7 m/s and 35 m/s at 1 m/s intervals. Air forces acting on a mounted ball were measured during a 10 s time interval by a sting‐ Appl. Sci. 2020, 10, 3256 3 of 7 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 7 typerespective six‐component balls was measuredforce detector between (model wind number velocities LMC (U‐)61256 of 7 m by/s andNissho 35 m Electric/s at 1 m Works/s intervals. Co,Ltd). Air Dataforces recording acting on was a mounted done on ball a werepersonal measured computer during with a 10 an s time A/D interval (Analog by to a Digital) sting-type converter six-component board thatforce has detector a 1000 (model Hz sampling number LMC-61256 rate. The byaerodynamic Nissho Electric forces Works measured Co, Ltd). in Data this recording experiment was were done convertedon a personal into computer a drag coefficient with an A(Cd/D), (Analog lift coefficient to Digital) (Cl), converter and side boardforce coefficient that has a 1000(Cs), Hzas shown sampling in Equationrate. The (2), aerodynamic (3), and (4), forces respectively. measured in this experiment were converted into a drag coefficient (Cd), lift coefficient (Cl), and side force coefficient (Cs), as shown in Equations (2)–(4), respectively. 2𝐷 𝐶 (2) 𝜌𝑈2D𝐴 Cd = (2) ρU2A 2𝐿 𝐶 (3) 𝜌𝑈2L𝐴 Cl = (3) ρU2A 2𝑆 𝐶 2S Cs = (4)(4) ρ𝜌𝑈U2A𝐴 wherewhere ρ ρ isis the the air density, expressed as ρ = 1.2 kg/m kg/m3;; UU isis the the flow flow rate; rate; and A is the projected area of 2 2 the volleyball, expressed as A = π 0.1 2= 0.0314 m 2. of the volleyball, expressed as A = π × × 0.1 = 0.0314 m .

Figure 2. Volleyballs used in the experiment and their panel orientation settings. In this figure, Figure 2. Volleyballs used in the experiment and their panel orientation settings. In this figure, a and (a,b) represent the Face A and B orientation of the new Mikasa volleyball (V200W), respectively; b represent the Face A and B orientation of the new Mikasa volleyball (V200W), respectively; c and d (c,d) represent the Face A and B orientations of the old Mikasa volleyball (MVA200), respectively; and represent the Face A and B orientations of the old Mikasa volleyball (MVA200), respectively; and e (e,f) represent the Face A and B orientation of the old Molten volleyball (V5M5000), respectively. and f represent the Face A and B orientation of the old Molten volleyball (V5M5000), respectively. 3. Results 3. Results 3.1. Drag Coefficient Variation by Ball 3.1. Drag Coefficient Variation by Ball Figure3 shows the drag characteristic curves of the respective balls based on their panel orientation. The panelFigure orientation 3 shows ofthe the drag respective characteristic balls was curves divided of into the two respective faces (Face balls A and based B), and on the their respective panel orientation.drag coefficients The panel were measuredorientation for of each the respective face. Most balls notably, was the divided drag generatedinto two faces for the (Face three A typesand B), of andballs the varied respective substantially drag coefficients according were to the measured ball type. for The each results face. showed Most notably, that drag the on drag the traditionalgenerated forMikasa the three (MVA200) types and of balls Molten varied (V5M5000) substantially balls varied according significantly to the ball depending type. The on theresults orientation showed of that the dragball panels on the (Figuretraditional3c–f). Mikasa The new (MVA200) volleyball and for Molten the 2020 (V5M5000) Tokyo Olympics balls varied (V200W), significantly on the otherdepending hand, onshowed the orientation relatively of small the dragball panels variation (Figure with changes3 c, d, e, inand panel f). The orientation new volleyball (Figure 3fora,b). the 2020 Tokyo Olympics (V200W), on the other hand, showed relatively small drag variation with changes in panel orientation (Figure 3 a and b). Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 7 Appl. Sci. 2020, 10, 3256 4 of 7

Figure 3. Drag coefficient variation of the balls. In this figure, (a,b) represent the Face A and B Figure 3. Drag coefficient variation of the balls. In this figure, a and b represent the Face A and B orientation of the new Mikasa volleyball, respectively; (c,d) represent the Face A and B orientations orientation of the new Mikasa volleyball, respectively; c and d represent the Face A and B orientations of the old Mikasa volleyball, respectively; and (e,f) represent the Face A and B orientation of the old of the old Mikasa volleyball, respectively; and e and f represent the Face A and B orientation of the Molten volleyball, respectively. old Molten volleyball, respectively. The critical Reynolds number of the respective balls was also found to vary according to the The critical Reynolds number of the respective balls was also found to vary according to the orientation of the panels. The moment at which the flow state of the ball spin changes from laminar orientation of the panels. 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 to turbulent is called a transition, and the Reynolds number of this transition is called5 the critical Reynolds number (Recr). The Recr of the new Mikasa volleyball (V200W) was ~2.9 10 (Cd 0.17) Reynolds number (Recr). The Recr of the new Mikasa volleyball (V200W) was ~2.9 ×10× 5 (Cd ≈ 0.17)≈ for for Face A and ~2.7 105 (Cd 0.15) for Face B. The Cd value of Face A of the new volleyball was Face A and ~2.7 × 10× 5 (Cd ≈ 0.15)≈ for Face B. The Cd value of Face A of the new volleyball was particularly high relative to the other balls. The Recr of the old Mikasa volleyball (MVA200) was particularly high relative to the other balls. The Recr of the old Mikasa volleyball (MVA200) was ~2.7 ~2.7 105 (Cd 0.14) for Face A, and ~3.1 105 (Cd 0.13) for Face B, indicating that the Cd value of × 10×5 (Cd ≈ 0.14)≈ for Face A, and ~3.1 × 105× (Cd ≈ 0.13)≈ for Face B, indicating that the Cd value of Face Face B of the old Mikasa ball was low relative to the other balls. The Recr of the old Molten volleyball B of the old Mikasa ball was low relative to the other balls. The Recr of the old Molten volleyball (V5M5000W) was ~2.2 105 (Cd 0.16) for Face A and ~2.8 105 (Cd 0.14) for Face B. (V5M5000W) was ~2.2 ×× 105 (Cd ≈ ≈0.16) for Face A and ~2.8 ×× 105 (Cd ≈ ≈0.14) for Face B. 3.2. Changes in the Lift and Side Forces Over Time 3.2. Changes in the Lift and Side Forces over Time The next graphs show the irregular force fluctuation in the horizontal and vertical directions of eachThe volleyball next graphs when show a wind the velocity irregular of 15force m/ sfluctuation is applied forin the 10 shorizontal (Figure4). and Therefore, vertical thesedirections are the of scattereach volleyball diagrams when of the a changeswind velocity in the of lift 15 and m/s side is applied forces applied for 10 s to(Figure different 4). Therefore, types of volleyball these are and the panelscatter orientations diagrams of for the 10 changes s. From in Figurethe lift4 and, it is side possible forces to applied verify to that different the irregular types of forces volleyball (lift and and sidepanel forces) orientations acting onfor the 10 s. ball From varied Figure depending 4, it is possible on the type to verify of volleyball. that the irregular These results forces also (lift showed and thatside if forces) the panel acting orientation on the ball changed varied fordepending a volleyball on the being type tested, of volleyball. the forces These also results changed also drastically showed (Tablethat if1 the). With panel the orientation old Molten changed ball, thefor a forces volleyball changed being considerably tested, the forces when also the changed panel orientation drastically changed.(Table 1). However, With the old because Molten the ball, new the volleyball forces changed (V200W) considerably for the Tokyo when Olympics the panel is less orientation dependent on panelchanged. orientation However, changes, because similar the new results volleyball were obtained.(V200W) for Therefore, the Tokyo the Olympics flight trajectory is less dependent of the new volleyballon panel orientation for the Tokyo changes, Olympics similar is expected results were to be obtained. more stable Therefore, than that the of flight the old trajectory volleyball. of the new volleyball for the Tokyo Olympics is expected to be more stable than that of the old volleyball. Appl. Sci. 2020, 10, 3256 5 of 7 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 7

FigureFigure 4. 4. Scatter diagrams diagrams of ofchanges changes in the in lift the and lift side and forces side forcesacting on acting the balls on the in the balls wind in tunnel the wind tunneltest. In test. this In figure, this figure, a and (ba ,brepresent) represent the the Face Face A and A and B orientation B orientation of the of the new new Mikasa Mikasa volleyball, volleyball, respectively;respectively; ( c ,andd) represent d represent the the Face Face A A and and B B orientations orientations of of the the old old Mikasa Mikasa volleyball, volleyball, respectively; respectively; andand (e e,f and) represent f represent the Facethe Face A and A and B orientation B orientation of the of oldthe old Molten Molten volleyball, volleyball, respectively respectively (10 (10 s, winds, velocitywind velocity of 15 m of/s). 15 m/s).

TableTable 1. 1.Mean Mean and and standard standard deviation deviation (SD) (SD) of the of liftthe and lift side and forces side forces acting onacting the respectiveon the respective volleyballs (10volleyballs s, wind velocity (10 s, wind of 15 velocity m/s). of 15 m/s).

aa b b c cd de ef f TypeType Sideside Liftlift sideSide liftLift sideSide liftLift sideSide liftLift sideSide lift LiftsideSide lift Lift Forceforce Forceforce forceForce forceForce forceForce forceForce forceForce forceForce forceForce forceForce forceForce forceForce MeanMean -0.060.06 -0.66 0.27 -0.460.46 -0.380.38 -0.770.77 0.21 0.21 -0.200.20 -0.480.48 -0.610.61 0.35 0.350.33 0.33 − − − − − − − − SDSD 0.810.81 0.920.92 0.88 0.73 0.73 0.73 0.73 0.70 0.70 0.84 0.84 0.86 0.86 0.19 0.19 0.26 0.26 0.80 0.800.66 0.66

4.4. Conclusions Conclusions ThisThis study study examined examined the the aerodynamic aerodynamic characteristicscharacteristics of the new volleyball (V200W) (V200W) to to be be used used in thein 2020the 2020 Tokyo Tokyo Olympics, Olympics, and comparedand compared them them with thewith characteristics the characteristics of two of types two oftypes balls of currently balls incurrently use (MVA200 in use and (MVA200 V5M5000) and V5M5000) using a wind using tunnel a wind experiment. tunnel experiment. TheThe results results revealedrevealed thatthat the drag on on the the volleyballs volleyballs varied varied according according to totheir their design. design. For For the the traditionaltraditional balls balls (MVA200 (MVA200 andand V5M5000),V5M5000), it it was was found found that that the the Reynolds Reynolds number number increased increased and and the the dragdrag coe coefficientfficient varied varied substantially substantially depending depending on on thethe ballball orientation.orientation. For the the Molten Molten ball, ball, the the shift shift in itsinCd itsvalue Cd value to the to supercriticalthe supercritical region region was was revealed revealed to be to dramaticallybe dramatically slower slower for for Face Face B (2.8 B (2.8 10× 105)5 than) × forthan Face for A Face (2.2 A10 (2.25). On× 10 the5). basisOn the of thebasis results, of the drag results, on the drag conventional on the conventional Molten ball Molten substantially ball × variessubstantially depending varies on depending its orientation, on its and orientation, thus, in comparisonand thus, in withcomparison the new with ball, the the new Molten ball, the ball is thoughtMolten to ball be is more thought affected to be by more panel affected orientation. by panel The orientation. speed of aThe volleyball speed of in a avolleyball serve ranges in a betweenserve approximatelyranges between 50 approximately and 80 km/h, 50 or and 2.0 8010 km/h,5 and or 3.0 2.0 ×10 105 5if and converted 3.0 × 10 to5 ifRe converted. In a volleyball to Re. In spike a × × 5 (attack),volleyball the spike ball flies (attack), at up the to ball 100 flies km/ hat (approximately up to 100 km/h (approximately 4.2 105), but the4.2 × changes 10 ), but in theCd changesowing toin the × volleyballCd owing type to the and volleyball its orientation type and were its demonstrated orientation were to be demonstrated relatively low to at be this relatively speed. Moreover, low at this with speed. Moreover, with a velocity range of approximately 15 m/s (Re of approximately 2.0 × 105), which a velocity range of approximately 15 m/s (Re of approximately 2.0 105), which is used in float serves, is used in float serves, the Cd results change even more prominently× with the ball type and panel the Cd results change even more prominently with the ball type and panel orientation. Presumably, orientation. Presumably, this is because the air resistance acting on the ball in float serves varies this is because the air resistance acting on the ball in float serves varies according to the volleyball according to the volleyball type and panel orientation, and as a result, the flight trajectory is modified. type and panel orientation, and as a result, the flight trajectory is modified. For this reason, it is For this reason, it is considered effective to consider the ball type and orientation when executing considered effective to consider the ball type and orientation when executing different types of serves different types of serves in a volleyball, including float serves. in a volleyball, including float serves. Appl. Sci. 2020, 10, 3256 6 of 7

Therefore, it can be determined that the flight distance of the Molten ball can be changed by changing its orientation. The newly designed volleyball for the 2020 Tokyo Olympics (V200W, 18-panels by Mikasa) is predicted to have a relatively stable flight distance, as the dependence of its drag variation on panel orientation is relatively small, and ball orientation has a lesser effect on its Cd. 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. These results suggest that the raised dimple-like pattern on the surface of the ball is effective at keeping the boundary line around the ball constant. The ball surface consists of complex factors such as the shape, number, width, and depth of the panels. Therefore, a more detailed examination of the air flow on the ball surface using visualization methods such as PIV (Particle Image Velocimetry) should be conducted. Furthermore, these results are likely to be used as scientific evidence to show that, in a volleyball, service aces can be affected by the variation of trajectory as a result of ball orientation, thus complementing prior studies that have reported on how the success of float serves greatly affects the positive outcome of a match [14–16]. In summary, knowledge about the flight characteristics of various official volleyballs can lead to better floating serves, which are thought to substantially contribute to service aces and the positive outcome of a match.

Author Contributions: Conceptualization: S.H., H.O., K.W. and T.A.; methodology: S.H., H.O., K.W. and T.A.; software: S.H.; data curation: S.H. and T.A.; writing—original draft preparation; S.H. and T.A.; writing—review and editing: S.H., H.O., K.W. and T.A. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by JSPS KAKENHI (Grant No. 15K16442) of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government. Conflicts of Interest: The authors declare no conflict of interest.

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