Analysis of force and impulse of a hydrophobic rod rotating into water

This paper investigates the change of the relative magnitudes of force and impulse components generated by the rotational motion of a hydrophobic carbon rod moving into water from the water surface, by varying the rod length. Whereas added mass force and drag force were dominant regardless of the rod length, buoyancy force and surface tension force were relatively small. For relatively short rod lengths (2, 4, 6 cm), drag forces were larger than added mass forces in the beginning of the rotational motion; the magnitudes of the two forces, however, reversed during the rotation. It was found that the reversals of magnitudes of the two forces occurred when the velocity at the end of the rod was 0.5 m/s. On the other hand, for a long rod length of 8 cm, added mass force was higher than drag force throughout the rod motion. The change of impulse by varying the rod length showed a similar tendency to that of force. Added mass impulse and drag impulse were also considerably larger than buoyancy impulse and surface tension impulse. Furthermore, drag impulse was larger than added mass impulse for short rod lengths, and the magnitudes of the two impulses reversed for long rod lengths. Compared to the shorter rod, the larger rod has a relatively low momentary force, but its force has much larger duration time, resulting in an increase of impulse.