The added mass of an expanding bubble

The added mass interaction of a body of variable volume translating in a fluid depends not only on the relative acceleration, but also on the rate of change of the volume. In the present study this prediction is put to an experimental test by observing the motion of a bubble rising in a pressurized tube. When the pressure is brought back to ambient by a fast-opening valve, the bubble expands and, from an analysis of its acceleration, it is possible to deduce the effect of the volume change on the added mass interaction. The results support the validity of the conventional theory for this effect. Some other observations on the transition from a straight to a spiral or zig-zag trajectory – a regime which may well be called ‘Leonardo's paradox’–are reported.