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Droplet evaporation on porous materials is a complex dynamic that occurs with spontaneous liquid imbibition through pores by capillary action. Here, we explore water dynamics on a porous fabric substrate with in-situ observations of X-ray and optical imaging techniques. We show how spreading and wicking lead to water imbibition through a porous substrate, enhancing the wetted surface area and consequently promoting evaporation. These sequential dynamics offer a framework to understand the alterations in the evaporation due to porosity for the particular case of fabric materials and a clue of how face masks interact with respiratory droplets.

The trajectory of a soccer ball, kicked with a spin to curve it into the goal, is strongly influenced by aerodynamic factors such as the Magnus force. Several studies using a wind-tunnel and high-speed cameras have investigated the Magnus force acting on a spinning soccer ball. However, the exact effect of the Magnus force on the trajectory of a spinning soccer ball in free flight remains unclear. This study set out to use an optical three-dimensional motion-capture system to record the details of the flight of such a spinning soccer ball. The maximum curvature of the ball’s trajectory occurred in the middle of its flight. The sideways-directed Magnus force acting on the ball decreased as the ball’s speed decreased during the entire flight. Thus, it was concluded that the deflection of the trajectory of the ball decreases as the sideways-acting force decreases throughout the flight.

Aerodynamic coefficients were determined for Telstar 18 and Brazuca, match balls for the 2018 and 2014 World Cups, respectively. Experimental determination of aerodynamic coefficients prompted the development of computationally determined soccer ball trajectories for most launch speeds experienced in actual play. Although Telstar 18’s horizontal range will be nearly 10% shorter than Brazuca’s horizontal range for high-speed kicks, both Telstar 18 and Brazuca have similar knuckling effects due to nearly equal critical speeds and high-speed drag coefficients that differ by less than 10%. Surface comparisons suggest why aerodynamic properties for the two World Cup balls are so similar.

The purpose of this study was to measure the aerodynamic force acting upon a soccer ball spun in a wind-tunnel test. In particular, we prepared two types of spinning soccer balls (air and motor types), measured the aerodynamic force on each, and examined the validity of the measurement method. In the case of the air-type spinning ball, the ball was made to spin by applying compressed air from an air compressor via an air duster. In the case of the motor-type spinning ball, a motor was placed at the middle inside the ball, and the ball was then automatically spun. First, both the air- and motor-type spinning balls demonstrated very strong spin-parameter ( Sp ) dependence with respect to drag during spinning. In addition, the side-force coefficient showed signs of linear increase as Sp increased in the motor-type spinning ball, whereas it increased along a slight curve in the air-type spinning ball. In particular, a negative Magnus effect was observed in the motor-type spinning ball when the Reynolds number was approximately 2.0 × 10 5 and Sp was approximately 0.14 (-0.007) and when the Reynolds number was approximately 2.2 × 10 5 and Sp was approximately 0.13 (-0.005).

Various studies have been conducted on the aerodynamic characteristics of nonspinning and spinning soccer balls. However, the vortex structures in the wake of the balls are almost unknown. One of the main computational fluid dynamics methods used for the analysis of vortex structures is the lattice Boltzmann method as it facilitates high-precision analysis. Studies to elucidate the dominant vortex structure are important because curled shots and passes involving spinning balls are frequently used in actual soccer games. In this study, we identify the large-scale dominant vortex structure of a soccer ball and investigate the stability of the structure using the lattice Boltzmann method, wind tunnel tests, and free-flight experiments. One of the dominant vortex structures in the wake of both nonspinning and spinning balls is a large-scale counter-rotating vortex pair. The side force acting on a spinning ball stabilizes when the fluctuation of the separation points of the ball is suppressed by the rotation of the ball. Thus, although a spinning soccer ball is deflected by the Magnus effect, its trajectory is regular and stable, suggesting that a spinning ball can be aimed accurately at the outset of its course.

The drag and lift of footballs have been mainly measured by wind tunnel tests. In the present study, computational fluid dynamics (CFD) and the lattice Boltzmann method were used to visualise the wakes of spinning and non-spinning footballs and analyse the dynamics of the observed vortex structures. The dominant vortex structures in the wakes of the footballs were determined to be large-scale counter-rotating vortex pairs. The fluctuation of the vortex pair for the spinning football was also estimated to be smaller and more stable than that for the non-spinning football. Although the presence of an unstable, large-scale counter-rotating vortex pair in the wake of a non-spinning ball has been previously observed in wind tunnel tests, the present study particularly found that the dominant vortex structure of a spinning ball was a stable, large-scale counter-rotating vortex pair.

Trajectory analysis is an alternative to using wind tunnels to measure a soccer\ud balls aerodynamic properties. It has advantages over wind tunnel testing such as being\ud more representative of game play. However, previous work has not presented a method that\ud produces complete, speed -dependent drag and lift coefficients. Four high-speed cameras in\ud stereo-calibrated pairs were used to measure the spatial co-ordinates for 29 separate soccer\ud trajectories. Those trajectories span a range of launch speeds from 9.3 m/s to 29.9 m/s. That\ud range encompasses low-speed laminar flow of air over a soccer ball, through the drag crises\ud where air flow is both laminar and turbulent, and up to high-speed turbulent air flow. Results\ud from trajectory analysis were combined to give speed-dependent drag and lift coefficient\ud curves for the entire range of speeds found in the 29 trajectories. Average root mean square\ud error between measured and modelled trajectory was 0.028 m horizontally and 0.034 m\ud vertically. The drag and lift crises can be observed in the plots of drag and lift coefficients\ud respectively.

Surface patterns on objects are important in aerodynamics. We show how surface patterns on volleyballs modify their aerodynamic performances. Conventional volleyballs with six panels show different aerodynamic characteristics along transverse and diagonal directions. Interestingly, isotropic surface patterning with hexagons or dimples enables us to achieve isotropic aerodynamics. This result gives insight into surface-mediated flight controls of projectiles through resisting fluid media.

This paper describes the experimental method for evaluating the flight trajectory and the aerodynamic performance of a kicked non-spinning soccer ball. The flight trajectory measurement is carried out using the digital image analysis. A centroid method and a template matching method are tested for the flight trajectory analysis using the artificial images generated by the data of a free-fall experiment. The drag coefficient obtained by the centroid method is better suited for the sports ball experiment than that by the template matching method, which is due to the robustness of the centroid method to the non-uniform illumination. Then, the flight trajectory analysis is introduced to a kicked experiment for a non-spinning soccer ball. The experimental result obtained from the stereo observation indicates that the S-shaped variation is found in the three-dimensional flight trajectory and in the side force coefficient during the flight of the non-spinning soccer ball.

The purpose of this study was to discuss the fluid mechanics acting on a non-spinning and low-spinning flight soccer ball, which called ˝Knuckling effect ball ˝ or ˝Knuckleball ˝, using the kick-robot. This robot is driven by high-pressure air in an air cylinder up to 500 kPa, being capable of generating a ball flight up to about 30 m/s. The machine with a size of 1.8 m x 2.5 m x 2.0 m in height can be moved, and be set both inside of a gymnasium and outdoor field. The ball speed can be set between 10 m/s and 30 m/s. Some condition of ball hitting can be adjusted, such as the hitting speed for the ball flight speed, the contact angle between the hitting-board and a ball for the shot angle and the off-axis distance between the normal line to the hitting board and the ball center for the ball spin rate. It is planned to conduct experiments on the knuckle effect of a non- or low-spinning soccer ball. It was determined that the peak of the irregular force in this experiment was about 2.1 N (s.d.=0.4 N). Furthermore, the knuckleball was observed to have an average lift force frequency of approximately 2.0 Hz. It can be concluded that the emergence of irregular forces is one of the characteristics of knuckle balls in soccer.

In this study, the unsteady aerodynamics of a knuckle ball in soccer were investigated. The characteristics of the fluid force, i.e. the magnitude and frequency acting on the knuckle ball, were analyzed through comparison with those of the straight ball using high-speed video cameras. Furthermore, the fluid flow around the knuckleball in flight was visualized using a smoke agent (titanium tetrachloride), and an attempt was made to clarify the fundamental fluid mechanics expressed by the knuckle ball as well as to analyze the vortex dynamics. It was found that the vertical acceleration of the knuckle ball tended to vary greatly (oscillate) in comparison with the acceleration of a straight ball, despite the fact that both are influenced by gravity. Moreover, in the case of the knuckle ball, the peak value of the vortex lift reached approximately 2.0 N, which was larger than the value of approximately 0.5 N for an instep kick. This vortex lift fluctuated greatly with time and was thought to be a highly unstable phenomenon. When the large-scale vortex structure of the knuckle ball was studied, successively shed vortex loops transformed and merged, and a few massive vortices formed an extended vortex street structure. Moreover, when the vertical fluctuation along the wake axis of the large-scale vortex structure was studied, evidence of unsteady wave oscillation (undulation) was observed. This large-scale structure of the far wake was conjectured to indicate traces of vortex motion at the time of vortex shedding, and the effect of this motion was thought to have a direct influence on the motion of the ball. Furthermore, the knuckle ball was observed to have an average vortex lift force frequency of approximately 3.5 Hz. A comparison of this vortex lift frequency and a vortex oscillation frequency indicated that these frequencies tended to act in unison with a high statistical correlation (r=0.83, p

When the boundary layer of a sports ball undergoes the transition from laminar to turbulent flow, a drag crisis occurs whereby the drag coefficient (C d) rapidly decreases. However, the aerodynamic properties and boundary-layer dynamics of a soccer ball are not yet well understood. In this study we showed that the critical Reynolds number (Re crit) of soccer balls ranged from 2.2 × 105 to 3.0 × 105. Wind-tunnel testing, along with visualisation of the dynamics of the boundary layer and the trailing vortex of a ball in flight, demonstrated that both non-spinning and spinning (curved) balls had lowC d values in the super-critical region. In addition, theRe crit values of the soccer balls were lower than those of smooth spheres, ranging from ∼ 3.5 × 105 to 4.0 × 105, due to the effects of their panels. This indicated that the aerodynamic properties of a soccer ball were intermediate between those of a smooth ball and a golf ball. In a flow visualisation experiment, the separation point retreated and theC d decreased in a super-critical regime compared with those in a sub-critical regime, suggesting a phenomenon similar to that observed in other sports balls. With some non-spinning and spinning soccer balls, the wake varied over time. In general, the high-frequency component of an eddy dissipated, while the low-frequency component increased as the downstream vortex increased. The causes of the large-scale fluctuations in the vortex observed in the present study were unclear; however, it is possible that a ‘knuckle-ball effect’ of the non-rotating ball played a role in this phenomenon.

The purpose of this study is to assess the fundamental characteristics which cause a football to spin in a curve ball kick. The ball impact process was analysed initially with a high speed video camera running at 4500 frames per second to obtain the basic data for a computer simulation model. This simulation model showed suitable agreement although it slightly deteriorated during the latter half of impact. It was noted that rotation of the ball occurs, even if the kinetic coefficient of friction is nearly equal to 0 because of local deformation of the ball during impact around the foot allowing forces to be transmitted to the ball around its axis. The spin of the ball was found to increase with the offset distance between the foot and the axis of the ball and as the kinetic coefficient of friction was increased. The offset distance between the foot and the axis of the ball affects the spin more than the coefficient of friction. Varying the coefficient of friction from 0.0 to 1.0 produces an increase in spin of 13 rad s )1 at most. It was suggested that the most suitable offset distance, which makes the largest ball rotation was around 100 mm. A trade-off was found between the ball speed and spin, for different offset distances.

In downhill alpine skiing, skiers often exceed speeds of 120 km h−1, with air resistance substantially affecting the overall race times. To date, studies on air resistance in alpine skiing have used wind tunnels and actual skiers to examine the relationship between the gliding posture and magnitude of drag and for the design of skiing equipment. However, these studies have not revealed the flow velocity distribution and vortex structure around the skier. In the present study, computational fluid dynamics are employed with the lattice Boltzmann method to derive the relationship between total drag and the flow velocity around a downhill skier in the full-tuck position. Furthermore, the flow around the downhill skier is visualised, and its vortex structure is examined. The results show that the total drag force in the downhill skier model is 27.0 N at a flow velocity of 15 m s−1, increasing to 185.8 N at 40 m s−1. From analysis of the drag distribution and the flow profile, the head, upper arms, lower legs, and thighs (including buttocks) are identified as the major sources of drag on a downhill skier. Based on these results, the design of suits and equipment for reducing the drag from each location should be the focus of research and development in ski equipment. This paper describes a pilot study that introduces undergraduate students of physics or engineering into this research field. The results of this study are easy to understand for undergraduate students.

The aerodynamic properties and boundary-layer dynamics of a non-spinning or slowly-spinning soccer ball are not well understood. The purpose of this study is to discuss the magnitude and the frequency of the side force of non-spinning or slowly-spinning flight soccer ball, which called “knuckling effect ball”, using a high speed VTR image of a real place kick. The direct liner transformation method was used to obtain three dimensional coordinates of ball position. The magnitude and the frequency of the side force were measured by a digitizing software system in PC. The magnitude of the side force in real flight was measured to range from about 1 N to 8 N. Additionally, the frequency of the side force in real flight was estimated to range from 1.0 Hz to 3 Hz.

Wind tunnel test with a full size non-spinning soccer ball was carried out to measure the aerodynamic forces (drag, lift and side force). The drag coefficient of a non-spinning soccer ball in the subcritical regime was nearly 0.43 and that of in the supercritical regime was nearly 0.15. It is estimated that the critical Reynolds number of a soccer ball is about 2.2×105. In the visualization experiment using titanium tetrachloride, we compared the flow around the ball during a non-spinning, low-speed kick (5 m/s) and a high-speed kick (29 m/s). During the low-speed kick, the boundary layer separation point was about 90 degrees from the front stagnation point, while during the high-speed kick the separation point had receded to about 120 degrees from the front stagnation point.

The purpose of this study was to analyze the swing characteristics of the kicking leg in order to elucidate the technical mechanisms of the drive curve shot (topspin curve shot). The orientation of the ankle joint at the point of ball impact closely resembles the orientation during a general curve shot or inside shot. Moreover, for the ankle joint movement at the point of ball impact, there is less mediolateral movement (y direction) compared to a curve shot, and more movement in the vertical direction (z direction). Based on the fact that the angle of the vertical rotational axis at that point is larger than that in other shots, this is likely to be one of the factors in generating topspin on the ball. Therefore, we believe that some of the most important and fundamental characteristics of drive curve shots are a smaller angle of attack and a larger vertical movement of the ankle joint at the point of impact, as compared to normal curve shots.

The purpose of this study is to analyze the fluid mechanics and frequency of forces acting on a knuckleball, specifically a soccer ball that was kicked and was in flight, was visualized using a smoke agent (titanium tetrachloride), and its unsteady aerodynamics were investigated. In the case of the knuckleball, the peak value of the vortex lift reached approximately 2.0 N, and the average vortex lift force frequency of approximately 3.5 Hz. A comparison of this vortex lift frequency and vortex oscillation frequency indicated that these frequencies tended to act in unison with a high statistical correlation ( r = 0.83 , p 0.01 ) .