A16 硬式野球ボールの空力特性 : 縫目の及ぼす影響と負のマグナス力(ボール)

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We investigated aerodynamic properties of a hard-baseball and a sphere by field experiments, in which we measure the trajectory of the projectiles using a high-speed video-camera. We determine the drag- and lift-coefficients by analyzing the video images. The drag coefficient (C_D) of the fastball pitch (back-spinning hard-baseball) with the spin parameter (SP)≈0.12, 0.23, decrease gradually with the Reynolds number (Re), for Re<1.4×10^5 and for Re<1.2×10^5, and attains its minimum at Re≈1.5×10^5 and Re≈1.2×10^5, respectively. Similarly, C_D of the back-spinning hard-baseball of SP≈0.35 attains its minimum at about Re≈0.8×10^5, increases gradually in the range 0.8×10^5<Re<1.3×10^5, and becomes almost constant for larger Re. In contrast, C_D of the back-spinning sphere with SP≈0.23 is almost constant for any Re considered. A rotating object placed in a uniform flow is subjected to the cross-flow force (Magnus effect), which lifts the back-spinning projectile upwards. We show that the direction of the Magnus force is inverted (negative Magnus effect) in a certain Re-SP parameter range, both for a hard-baseball and for a sphere.
2007-11-13