University of Twente researchers found injecting bubbles underneath a ship’s hull causes them to be pushed against the hull’s surface making an air bearing 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 carbon dioxide emissions.
The precise mechanism is still unknown, as the local water flow is complex and turbulent. As the scientists demonstrate, 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 them. 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 4 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.
The team found these results.
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.
Lead author Ruben Verschoof said, “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 provides valuable new insight.
The team’s paper, “Bubble Drag Reduction Requires Large Bubbles” has been published in Physical Review Letters. The research was done in the Physics of Fluids group of Professor Detlef Lohse.
Air bearing surfacing isn’t a new idea, rather its one that has been exploited at some expense in both liquid environments and in the atmosphere. Some shaft bearings in fans and other medium and higher speed motors use air bearings to good effect. But the loads are quite low.
Air bearing in liquids is more advanced, one could say a hovercraft rides on an air bearing breaking full liquid contact riding on the air bearing itself. Yet this team’s work is opening the field further, where lots more money is spent on fuel and time for ocean crossing is a significant cost on a container ship with millions of dollars worth of goods slowly crossing the ocean. Saving fuel costs and perhaps picking up speed is going to bee worthwhile.
Questions remain. Barnacles, the enemy of all ships hulls creates substantial drag and just how bubbles will work on a crusty hull is yet to be answered. Then there is the fuel cost to pump the bubbles out for injection. One wonders what part of the fuel saving will be used for pumping air.
Still, freighting, cruising and military operations use a great deal of fuel and saving it is surely a worthwhile undertaking.