# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 5 Discussion  
In this paper, techniques for reducing drag in maritime applications through air lubrication are explored and classified into three main categories: MBDR, ALDR, and ACDR. A brief description of mathematical models has been included in the paper in order to give the possibility to explore numerical solutions in case it is impossible to have experimental data or conduct tests in a naval tank. While the distinction between MBDR and ALDR depends on achieving a critical drag reduction rate, the primary difference between ALDR and ACDR lies in the presence of a sealed air chamber in ACDR that alters the water-gas interface. The existence of this chamber further reduces drag to motion but also introduces additional drags. In MBDR, the main theories involve the use of jets to alter the fluid structure in the boundary layer, the effect of microbubbles on turbulent flow, and the formation of a thin film between the water and the wall that reduces shear force. Of great consideration is the implementation of the microbubble injection system on ships; it could be technically complex and usually it requires additional equipment. Regarding ALDR and ACDR, it is considered that gas injection locally modifies the fluid's viscosity and density, thus affecting the turbulent Reynolds number. Factors such as bubble size, incoming flow velocity, jet distribution, and gas type significantly impact the effectiveness of drag reduction. In MBDR, effectiveness increases with the reduction in bubble size and the increase in their concentration near the wall. The effectiveness of microbubble injection can vary with the size and type of the ship. It may not provide significant benefits for all types of vessels, and the system's design must be tailored to the specific characteristics of each ship. For ALDR, the increase in air injection flow improves the drag reduction rate up to a certain saturation point, after which its influence becomes negligible. The size of the air chamber greatly affects the amount of air needed for saturation and the stability of the chamber itself. Additionally, ALDR and ACDR are influenced by sea conditions, and the ship's design can play a crucial role in the effectiveness of drag reduction. The appropriate design of the ship's shape and the grooved structure at the bottom improve the attachment of the air layer or cavity, enhancing the efficiency of drag reduction. In Figure 22, the papers considered in this research are categorized by year and by topic, using the SCOPUS database as a reference. Before 2016, the major interest was in MBDR, then the number of papers related to ALDR and ACDR increased rapidly going towards 2023. This could possibly be due to the fact that MBDR is a more established technology and is now implemented mainly on cargo ships, so the research interest is shifting towards less industrially ready technologies like ACDR and ALDR. All the drag reduction techniques have disadvantages, and they share various challenges in terms of feasibility. For the microbubble system, for example, the implementation of a microbubble injection system could be technically complex, and it requires additional equipment. This system is highly influenced by the marine ecosystem, particularly in its very harsh environment. The hull of the ship is attacked by microorganisms that produce fouling and the efficiency of the microbubble system can decrease in this situation. To maintain the efficiency of the microbubble, the ship needs to be taken to the dry dock and cleaned more regularly, leading to longer downtime during the vessel's useful commercial life. Also, for systems such as ALDR and ACDR, implementing air layer drag reduction systems can be challenging due to the need for precise control and maintenance of the air layer. Designing systems that are effective across a range of ship speeds and conditions can be complex. In particular, concerning air cavity drag reduction, designers must include a cavity along the bottom of the hull during the design phase.