Shinichiro Ito

Fundamental aerodynamics of a new volleyball

This study compared the basic aerodynamic characteristics of conventional volleyballs with those of new designs in a wind tunnel. Furthermore, to examine the aerodynamic instability near the critical Reynolds regime (14 m/s), balls were propelled with an impact-type ball ejection device and the variation in the coordinates of the landing point was measured. It was found that the critical Reynolds number for a conventional volleyball (Molten MTV5SLIT) was approximately 2.7 × 105, whereas that for a design with honeycomb surface protrusions (Molten V5M5000) was slightly lower at approximately 2.2 × 105. The honeycomb-type balls showed a drag coefficient of approximately 0.17 when supercritical, which is rather large compared with that of conventional balls (0.1). It is believed that the honeycomb surface protrusions increase the roughness of the ball surface, and this presumed increase in roughness implies a trend towards reduced aerodynamic instability close to critical Reynolds numbers.

Aerodynamic characteristics and PIV analyses concerning tennis balls

The structure of the current tennis ball is covered with a uniform felt fabric on the outer surface made of hard rubber. As the ball continues to hit, the surface felt wears. Also, air leaks out through the structure inside. These factors cause a difference in the aerodynamic characteristics of new and used balls. In this research, the aerodynamic characteristics of the old and new tennis balls rotating were investigated and the PIV results of the flow around the ball due to the wear of the felt producing these characteristics were showed. For the four types of tennis balls tested, used balls showed higher in drag coefficient than new balls, and new balls tended to be higher than used ball in lift coefficient. It is believed that deformation due to deterioration of ball rubber brings about a change in drag coefficient, which causes a change in lift coefficient due to wear of the surface felt. In the new ball, felt fluff stands, which indicates that the wake flow is large and inclined diagonally backward compared to the used ball, and the air around the ball is found to be caught by the felt fluff.

Aerodynamic characteristics and heat radiation performance of sportswear fabrics

Sports such as swimming, speed skating, and marathon are sports competing for time. In recent years, reduction of the fluid drag of sportswear is required for these competitions in order to improve the record. In addition, sweating and discomfort due to body temperature rise during competition are thought to affect competitor performance, and heat radiation performance is also an important factor for sportswear. The authors have measured fluid force drag by wrapping cloth around a cylinder and have confirmed their differences due to the roughness of the fabric surface, differences in sewing. The authors could be verified the drag can be reduced by the position of the wear stitch. This time, we measured the heat radiation performance of 14 types of cloths whose aero dynamic properties are known using cylinders which are regarded as human fuselages, and found elements of cloth with heat radiation performance. It was found to be important for raising the heat radiation performance of sportswear that the fabric is thin and flat surface processing.

222 Analysis on swimming behavior of dolphin with artificial tail flukes

Dolphins have high propulsion and maneuvering performances. Dolphin’s tail flukes play an important role to generate propulsive force. We paid attention to a dolphin that lost most part of its tail flukes by disease kept in an aquarium, Okinawa, Japan. The dolphin could swim by attaching artificial tail flukes as well as a normal dolphin. The objective of this study is to discuss the effectiveness of the artificial tail flukes for the swimming of the dolphin by estimating the propulsive forces obtained theoretically using data on the behavior of the tail flukes and swimming number.

Ratio of Propelling Force Generated by a Palm and by a Forearm in Freestyle Swimming

The propelling force in swimming is mainly generated by hands and arms rather than by legs. However, the ratio of the thrust force by a hand to that by the whole forearm with hand has not been experimented. Replicas of a hand and a forearm with hand were made from an excellent swimmers. Those characteristic data of the lift-drag forces and the propelling force ratio were taken by wind tunnel tests based on the angle of attack and the sweepback angle. The following results were found. As for the S-shaped pull stroke, the thrust force by the whole forearm with hand was 3 to 4 times larger than that by the hand itself in the catch and the finish phases while about 1.8 times larger in the pull phase. In the drag pull type stroke, the I-shaped pull, the thrust force by the whole forearm with hand was 1.8 times larger than that by the hand itself throughout all the three phases. Furthermore, the fluid characteristic of the whole forearm with the hand showed similar to that of the hand itself.