Air lubrication for ships: friction can be reduced 20 percent, with a huge impact on fuel consumption



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.



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ARGO Network Senses Ocean Changes

More than 3,600 robots probe the seas down to 2,000 meters

Scientists worldwide have been capturing and analyzing detailed data about the atmosphere for decades. Information about the oceans has been much more spotty, however. Ships have taken readings along many isolated transects, but each effort has occurred at a moment in time, and significant portions of the seas have gone unexamined.

That is changing. Since 2007 a network of thousands of floating robots covering the seven seas, named Argo, has been generating real-time data for use in ocean and climate research. Each robot probe, a tube a little more than meter long, dives to about 1,000 meters, then drifts locally for nine days. On the 10th day it sinks to 2,000 meters and then rises straight up, sensing temperature and salinity along its ascent. Each probe also measures the subsurface and surface currents that push it around. When a probe surfaces, a satellite picks up the data, and the probe sinks again for another 10-day cycle.

Each robot uses a hydraulic piston and bladder to dive and surface and lasts about five years. From 700 to 800 new units have been deployed in each of the past several years to replace defunct robots or to beef up the network in remote regions to improve global coverage.

The Argo program, named after the mythical Greek ship, is run by 50 research institutions in more than 30 countries.

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via Scientific American – Mark Fischetti

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‘Hot Body’ Could Help Ships Reduce Drag

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New research into drag reduction has the potential to help industries such shipping to reduce energy use and carbon emissions.

Professor Derek Chan from the University of Melbourne’s Department of Mathematics and Statistics said the research demonstrates a new way to minimise drag of fast moving projectiles in water.

A collaboration between the University of Melbourne and the King Abdulla University of Science and Technology in Saudi Arabia, the research was based on the 255 year-old Leidenfrost effect.

The Leidenfrost effect describes the phenomenon where a liquid produces an insulating vapour layer when it comes in contact with a solid surface that is hotter than its boiling point.

The new research used high-speed video footage to assess the drag produced from polished balls dropped into liquid. The results found that the drag on the ball is reduced to almost the minimum possible through the creating of an insulating vapour as it falls through the liquid.

Professor Chan said that the new drag reduction method has the potential to reduce energy costs for a broad range of applications, such as ocean transport and high-pressure pumping of liquid through pipelines.

“An obvious area of application is shipping,” he said.

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Fish Robot As An Alternative Marine Propulsion System Of The Future

A giant grouper.
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The team of Darmstadt researchers analyzed videos of fish’s motions and then developed a prototype fish robot that duplicated them, and are now testing it using the locomotional patterns of various species of fish in order to refine it and improve its efficiency.

The researchers hope that use of their fish robot for ship propulsion will help prevent shoreline erosion and the underminings of submarine installations caused by ships’ screws. The fish robot’s “soft” drive action should also prevent the churning up of seabeds and riverbeds and its effects on marine plants and aquatic-animal populations.

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