正多角形板まわりの流れに作用する凹部の影響

抄録

Frogs and ducks have concaved web on their hands and feet. In this study, we investigate the influence of concaved thin plate nonconcaved thin plate for a drag coefficient. A hexagonal thin plate was used as the reference for the model shape. The concaved models were made with the removing of isosceles triangle from on the sides of reference. The motion of model was free-fall in a water tank. The tank was filled with the water. The model motion was recorded by using the high-speed camera. The image analysis was carried out to investigate the velocity and acceleration of the model. The drag coefficient was determined from the force balance during the motion. At this time, we compared the difference of the drag coefficient among the degree of the concave. The drag coefficient has minutely changed from 0 to 30 degrees of concave. The drag coefficient increased from 30 to 45 degrees. The drag coefficient was converged at the terminal velocity. It also had a periodic motion around the terminal velocity. The periodic motion relates the formation and detachment of vortex behind the model. This vortex relates to form the small vortices as the Kelvin-Helmholtz instability.