摘要：The problem of a tactically well performed straight service in volleyball is under investigation. The aerodynamical forces acting on the ball and the vortex wake behind the sphere are the main subjects of the research work. These parameters determine the tactics in volleyball service, because they can “spoil” the expected trajectory of the ball and consequently disturb the defences. Two different wind tunnels in diameter 1.8 m and 3 m, respectively, with distinguished turbulence intensities were used for measuring the interval of critical Reynolds’ numbers for two volleyball balls with different roughness of the surface. The longitudinal and the transversal forces were also measured for five airflow velocities ranging from 8 m/s to 25 m/s. It was found that the transversal forces acting on the ball are two orders of magnitude smaller than the frontal drag force. It was also noticed that the position of the seams, in regard to the direction of the airflow velocity vector, is crucial for the determination of the transversal forces components. All the facts stated above give the reason for the deviation of the ball away from its expected ballistic trajectory. The periodic nature of the phenomena enabled the performance of the discrete Fourier trans- formation on the transversal forces. It turned out that the error made with the Fourier transformation when taking only the first four summands is negligible compared to the measuring error.
关键词：volleyball; straight service; ball flight; vortex wake behind the sphere; Fourier transformation; transversal forces
