The strange three-dimensional flight behaviour of slowly spinning soccer balls is one of the most interesting and unknown phenomenon associated with the trajectories of sports balls. Many spectators have experienced numerous exciting and emotional instances while observing the curious flight behaviour of these balls. We examine the aerodynamic mechanisms of erratic ball behaviours through real flight observations, unsteady force measurements and flow pattern visualisations. The strange behaviour is elucidated by the relationship between the unsteady forces on the ball and the wake flow. The irregular changes in position for twin longitudinal vortices have already been discovered in the supercritical Reynolds number region of a sphere with a smooth surface. This finding is applicable to the strange behaviour of the flight of soccer balls with this supercritical flow. The players, spectators, and television viewers will gain greater insight into the effects of soccer ball flights.

The Japanese filefish Paramonacanthus japonicus has extreme sexual dimorphism in its overall shape, even though its mating system is monogamy with biparental care. This sexual dimorphism is mainly due to the development of secondary sexual traits in males. Males become more slender in body with elevated soft dorsal and anal fins as they mature. We examined the function of such male secondary sexual traits by field research and fluid-dynamic analysis. Underwater observations showed that movement rate and steady swimming speed of males were higher than those of females. Male and female P. japonicus showed similar feeding habits and egg-tending behavior, although males attacked potential egg predators more frequently. A windtunnel experiment using the air bearing and spring system showed that the drag coefficient of males was significantly lower than that of females, indicating a lower male hydrodynamic drag performance. Also, male elevated soft dorsal and anal fins are considered to give rise to higher thrust performance in monacanthids. Thus, these results suggest that male secondary sexual traits are hydrodynamic devices for enhancing swimming performance that seem to be actually functional under natural conditions. We discuss the evolution of such conspicuous male sexual traits in P. japonicus .

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We have measured drag coefficients of a sphere and a circular cylindrical aerodynamic model using a five axes and a six axes control magnetic suspension and balance system (MSBS) developed by us. This MSBS has the characteristics of large aperture relative to the weight, light weight, and small electric power consumption in comparison with the conventional ones. We had good agreements between the measured values of the drag coefficient and the values appearing in the common aerodynamic handbook or textbook. We also succeeded in measuring the aerodynamic influence of a supporting rod of the aerodynamic models making use of the characteristics of the MSBS. Conventionally, the MSBS can be used only in large scale laboratories because the size, weight, and electric power consumption are large. We think that successful measurements of various aerodynamic characteristics using this type of MSBS will stimulate the introduction of it into the wind tunnel experiments in small scale laboratories.

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