Terminal velocities of spherical particles in a vertically oscillating liquid

The effect of vertical oscillations on the average terminal velocities of plastic spheres in water has been measured. The sphere diameters ranged from 18 to 12 in. and oscillation frequencies were from 0·88 c/s to 2·37 c/s with amplitudes up to 4 in. at the lower frequency and 1·1 in. at the higher. It was found that liquid oscillations brought about a decrease of up to 28 per cent in the terminal velocity. This retardation was greater than that expected from the drag mechanism under steady conditions and it was concluded that the additional retardation was due to the shedding of a large wake during each oscillation. The alternating wake could be observed when the spheres were coated with dye. The retardation was correlated dimensionlessly with the frequency as a Strouhal number, and the amplitude as the amplitude/diameter ratio.