Air lubrication: large bubbles do the trick

UT research shows 'dramatic difference' between large and small bubbles.

Injecting bubbles at a ship’s hull is an effective way of reducing drag, and fuel consumption of the ship. That is, if those bubbles have the right size. Researchers of the University of Twente show that the reduction is negligible when tiny bubbles are used. Large, deformable bubbles do the trick, the scientists conclude in Physical Review Letters of September 2.

Blowing bubbles underneath a ship’s hull, causes them to be pushed against the surface. In the surface layer between the ship and water, these air bubbles cause less friction: it’s also known as air lubrication. In practice, friction can be reduced 20 percent, with a huge impact on fuel consumption and CO2 emission. The precise mechanism is still unknown, as the local water flow is complex and turbulent. As the UT scientists prove now: the size of the bubbles make a big difference: tiny bubble don’t have a net effect at all. This may seem counterintuitive, but large bubbles that can be deformed easily, give the strongest effect.

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 friction/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).

The paper ‘Bubble drag reduction requires large bubbles’, by Ruben Verschoof, Roeland van der Veen, Chao Sun and Detlef Lohse, appears in Physical Review Letters, September 2.

Wiebe van der Veen
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