ANOMALOUS MOTION OF A SPHERE FALLING THROUGH WATER

A pendulum motion of a sphere falling through water under the action of gravity was studied experimentally for the Reynolds number between 1.5×10 3 and 4.0×10 4 , and its explanation is given. The quantitative analysis of the path with stereoscopic photographs showed that the bending of the path from the vertical line occured through the action of a side force L, acting perpendicularly to the path and alternating its direction with a frequency whose Strouhal number was about 0.2. Visual observation with a VIDEO revealed that the force L arose from a circulating flow around the sphere, which was generated and alternated its sense of rotation by shedding asymmetric vortex loops from alternate sides of the sphere. The lift coefficient was found to be about 0.3. The drag coefficient increased from 0.4 to 0.9 when the circulating flow reversed.