Air lubrication
Air lubrication systems are very promising to reduce the fuel consumption of vessels. By injecting air between the ship’s hull and the moving water the frictional resistance can be reduced, resulting in a power saving. Although the idea is simple, air lubrication is challenging in practice and still a lot of research and development is needed. MARIN is closely cooperating with Delft University of Technology and University of Twente to get a full understanding of the physics and application of air lubrication systems.
air lubrication systems
There are several types of air lubrication systems. Each system has a different working principle and is therefore facing different challenges.
The working principle of a micro bubble lubrication system is injecting air bubbles in the boundary layer of the vessel. Based on several experiments (deformable) air bubble supply does seem to give significant friction drag reduction, but only over limited downstream length, order of 2 m for full scale ships (SANDERS et al. 2006).
External air cavity systems create air cavities typically of half a free surface wave length behind a cavitator (little wedge upstream of the air injector) and need relatively little air supply (Zverkhovskyi et al. 2014). An example of such a system is the Damen Air Cavity System (DACS) tested at MARIN in 2019.
Air layer systems aim at a much longer air film, since the length does not depend on such a free surface wave. These systems do not necessarily need a cavitator, however need significant larger air supply (Elbing et al. 2008).
Air chamber systems have large chambers on the bottom of the vessel, which are filled with air to reduce the wetted surface. This system has been investigated in the EU projects PELS and SMOOTH on both model and full scale as well as in flat plate lab conditions (Rotte et al. 2016).
Based on its experience, MARIN is currently focussing research on the stability of air layers. External air cavities and air chambers have proven their effectiveness and are still high on the (applied) research agenda of MARIN. These last three types of air lubrication have the potential to reduce the total resistance of a vessel in the order of 3-15%.
The working principle of a micro bubble lubrication system is injecting air bubbles in the boundary layer of the vessel. Based on several experiments (deformable) air bubble supply does seem to give significant friction drag reduction, but only over limited downstream length, order of 2 m for full scale ships (SANDERS et al. 2006).
External air cavity systems create air cavities typically of half a free surface wave length behind a cavitator (little wedge upstream of the air injector) and need relatively little air supply (Zverkhovskyi et al. 2014). An example of such a system is the Damen Air Cavity System (DACS) tested at MARIN in 2019.
Air layer systems aim at a much longer air film, since the length does not depend on such a free surface wave. These systems do not necessarily need a cavitator, however need significant larger air supply (Elbing et al. 2008).
Air chamber systems have large chambers on the bottom of the vessel, which are filled with air to reduce the wetted surface. This system has been investigated in the EU projects PELS and SMOOTH on both model and full scale as well as in flat plate lab conditions (Rotte et al. 2016).
Based on its experience, MARIN is currently focussing research on the stability of air layers. External air cavities and air chambers have proven their effectiveness and are still high on the (applied) research agenda of MARIN. These last three types of air lubrication have the potential to reduce the total resistance of a vessel in the order of 3-15%.
Contact
Karola van der Meij
Head of Onboard Specialists
air lubrication TOOLS AND FACILITIES
MARIN can also be your research partner in the development of an air lubrication system, or perform an independent evaluation of the present system used on your ship.For this we have several tools and facilities available.
Full-scale measurements
MARIN has the equipment and experience to accurately measure the required speed-power relation to determine the effect of air lubrication systems. In addition we can determine the presence of either an air layer, a water layer or bubbly flow beneath a ship, by means of measurement probes developed in-house.
Model tests
MARIN has already tested several systems on model scale and thus built up experience in testing and extrapolating model test results. During the tests the airflow rate can be optimised, the performance is accurately measured and air layers can be observed by means of (high-speed) underwater cameras. However, scaling the model test results can be challenging depending on the type of air lubrication. Model tests can be performed in MARIN’s Shallow Water Basin or Deep Water Basin.
CFD computations
CFD computations for air lubrication systems is still work in progress and not yet available. However, steps are being made and we hope to share our research soon. Is it clear that CFD computations can be helpful in determining the additional resistance of an air lubrication system when turned off, or in providing information for the correct positioning of the air inlets. For this we use our in-house developed RANS code ReFRESCO.
References
- Elbing, Brian R, Eric S Winkel, Keary A Lay, Steven L Ceccio, David R Dowling, and Marc Perlin. 2008. “Bubble-Induced Skin-Friction Drag Reduction and the Abrupt Transition to Air-Layer Drag Reduction.” Journal of Fluid Mechanics 612: 201–36. https://doi.org/10.1017/S0022112008003029.
- Rotte, Gem, Oleksandr Zverkhovskyi, Maarten Kerkvliet, and Tom van Terwisga. 2016. “On the Physical Mechanisms for the Numerical Modelling of Flows Around Air Lubricated Ships.”
- SANDERS, WENDY C., ERIC S. WINKEL, DAVID R. DOWLING, MARC PERLIN, and STEVEN L. CECCIO. 2006. “Bubble Friction Drag Reduction in a High-Reynolds-Number Flat-Plate Turbulent Boundary Layer.” Journal of Fluid Mechanics 552 (1): 353. https://doi.org/10.1017/S0022112006008688.
- Zverkhovskyi, O, T Van Terwisga, M Gunsing, J Westerweel, and R Delfos. 2014. “Experimental Study on Drag Reduction by Air Cavities on a Ship Model.” In 30th Symposium on Naval Hydrodynamics, 9. Hobart, Tasmania, Australia.
Damen Air Cavity System (DACS)
PELS: Project Energy saving air-Lubricated Ships