Air Layer Drag Reduction

Technology Summary

Air Layer Drag Reduction (ALDR) is a method of reducing skin-friction resistance in marine vessels by introducing sufficient air beneath a hull to form a continuous or near-continuous air layer between the hull surface and the surrounding water flow. Air Layer Drag Reduction is understood to be the most effective way of reducing drag by introduction of air beneath a ship.

History

The U.S. Navy were the first to experiment with pumping air beneath the hull of a ship in the 1950s. However, their goal was to reduce the noise of their vessels by creating a carpet of bubbles to isolate the ships from surrounding water, making them harder to detect with sonar. Then, in the 1980s, researchers began investigating the use of air beneath a ship to increase fuel efficiency, calling it an air lubrication system (ALS). The typical method for air lubrication is micro-bubble drag reduction (MBDR)

Modern Technology

There are currently a few providers of air lubrication systems but only one provider of an air layer drag reduction system (AirGlide AI). With the consistent tightening of green house gas regulations across the world, the business case for air layer drag reduction systems becomes more compelling and many ship owners are turning to this fuel-agnostic drag reducing technology.

STUDIES ON Air Layer Drag Reduction

"Comparing MBDR with ALDR, Elbing [5] found that the significant drag reduction of MBDR (>25%) is limited to the first few meters downstream of the injection point, whereas the drag reduction of ALDR can extend further. The drag reduction rates of both the slot injection and the porous plate injection using ALDR can maintain more than 80% at 10 m downstream of the injection point, whereas the two injection methods of MBDR both decrease significantly at 2 m, and can only maintain a drag reduction rate below 20% after 4 m." - An, H., Pan, H., & Yang, P. (2022). Research Progress of Air Lubrication Drag Reduction Technology for Ships. Fluids, 7(10), 319.

"In this paper, the objective is to identify the optimum type of air lubrication using microbubble drag reduction (MBDR) and air layer drag reduction (ALDR) techniques ... The results show that the ship model using the air layer had the highest drag reduction up to a maximum of 90%. Based on the characteristics of the SPB, which operates at low speed, the optimum air lubrication type to reduce resistance in this instance is ALDR." - Yanuar, W., Waskito, K. T., Pratama, S. Y., Candra, B. D., & Rahmat, B. A. (2018). Comparison of microbubble and air layer injection with porous media for drag reduction on a self-propelled barge ship model. Journal of Marine Science and Application, 17, 165–172

"As observed in BDR [Bubble Drag Reduction], for the lower-range of gas injection rates, drag reduction rate is less than 20%. However, above a critical gas injection rate, the ALDR with volumetric air flux being approximately 50 percent greater than BDR gives rise to drag reduction rate over 80 percent. The net energy saving rate expected for ALDR in Fig. 1 becomes higher than that for BDR. " - Park, S. H. & Lee, I. (2018). Optimization of drag reduction effect of air lubrication for a tanker model. International Journal of Naval Architecture and Ocean Engineering, 10(4), 427–438.

"Results from the BDR experiments indicate that: significant drag reduction (>25%) is limited to the first few metres downstream of injection ... once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model" - Elbing, B. R., Winkel, E. S., Lay, K. A., Ceccio, S. L., Dowling, D. R., & Perlin, M. (2008). Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction. Journal of Fluid Mechanics, 612, 201–236.