It is somewhat puzzling that a heavy object sinks straight in water, while a buoyant air-bubble or a balloon zig-zags or spirals while rising through water. This observation has been famously coined as “Leonardo’s paradox”. A popular explanation has been proposed: that the mass (or weight) of the rising or falling body governs its path-oscillations, and when the body becomes buoyant, it starts to oscillate vigorously.
Recently, researchers at the Physics of Fluids Group at University of Twente and the Center for Combustion Energy at Tsinghua University have revealed that it is not the mass of the body, but instead, mainly its moment of inertia which causes a rising particle or air bubble to zig-zag. They uncover the various kinds of path- and wake-instabilities that result for freely rising/falling particles in still fluid. Their results are supplemented by predictions for the existence of two regimes of motion and wake-transitions, all induced by moment of inertia change.
These results will have strong bearing on our understanding of everyday observations, for example: on the zig-zag paths of air bubbles rising in an aquarium tank. It is likely that the underlying rotation of the rising bubble, which causes it to zig-zag or spiral. The authors estimate that these insights will also trigger new directions towards the control of the path-oscillations of light particles in flows, with implications to many fluid-structure-interaction and energy harvesting problems.
The researchers conclude their findings in Physical Review Letters (119, 054501, 2017).
Wake transitions for rising cylinders. From (A) to (D), the 2S wake mode transitions to 2P wake mode.