Air lubrication: Only big bubbles do the trick

For investigating the effects, the University of Twente has a unique 'Taylor Couette' setup, capable of generating fully developed turbulent flow. This machine consist of two large cylinders with fluid in between. When the inner cylinder is turning fast, injected bubbles will be pressed against the surface, just like they do at the ship's hull. At the surface of the cylinder, they start influencing drag. This setup enables the scientists to search for the relevant parameters in efficient air lubrication.

With four percent of air in the water, a reduction of 40 percent is feasible in the experimental setup, using large, millimeter size bubbles. By adding a tiny amount of 'surfactant', the scientists were able to vary the surface tension between bubbles and water, and they could vary bubble dimensions. The other properties, like flow speed and density, were kept the same. What was the result? On average, the bubbles get much smaller, because the surfactant prevents bubbles getting together, coalescing, forming larger bubbles. Within the turbulent flow, the bubble have a uniform distribution and moreover, they will not be pushed against the surface. With, again, four percent of air that is in microbubbles now, there is four percent reduction: there is no net air lubrication at the ship's hull. Ruben Verschoof: "From previous experiments, we knew that deformable bubbles work well, but in no way we expected a dramatic difference like this.

By doing the experiments in real life turbulent flows, and not in the simplified situation of slow and laminary flow, the outcome of this research is directly applicable in the naval sector. For reducing drag in pipelines, the experiments also provide valuable new insight.

The research has been done in the Physics of Fluids group of Professor Detlef Lohse. This group is part of UT's MESA+ Institute for Nanotechnology. Research is funded by Dutch Technology Foundation STW and Dutch Foundation for Fundament Research on Matter (FOM).