# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 1 Introduction  
In the quest for greater efficiency and sustainability in maritime operations, the naval sector is increasingly focusing on resistance reduction strategies to enhance vessel performance and reduce environmental impact. The resistance to forward motion of a ship is fundamentally determined by three components: frictional drag, wave drag, and viscous drag. Notably, frictional drag, occurring due to viscous interactions between a ship's hull and the water, is a significant component, especially in contexts where wetted surfaces are extensive, such as in surface ships. Frictional drag contributes to 60-70% of the total drag on cargo ships and roughly 80% on tankers [1; 2; 3]. In the context of reducing movement resistance in vessels, computational fluid dynamics (CFD) provides a crucial tool for detailed analysis and optimization [4] of ship design [5], especially within the realm of air lubrication techniques. CFD enables the simulation of complex interactions between water and naval structures, thereby facilitating the development of innovative solutions that enhance operational efficiency and meet sustainability challenges in the maritime sector [6]. CFD is used to model the flow behaviour around the hull and within air cavities, assessing the effectiveness of different configurations in reducing resistance [7]. Through CFD simulations, it is possible to predict how microbubbles, air films, or air cavities affect pressure distribution and velocity profiles along the hull, thus optimizing the design to maximize drag reduction. These simulations are crucial for understanding the delicate balance between the amount of air injected, the placement of injections, and the overall impact on the hydrodynamic resistance of the ship [8]. Within the strategies for drag reduction, air lubrication techniques represent an important technological frontier. The classification of these techniques is divided into three principal categories. Microbubble drag reduction (MBDR) involves the introduction ofmicrobubbles into the ship's boundary layer, leading to a reduction in frictional drag by lowering the mean density and altering the velocity profile. Air layer drag reduction (ALDR) employs a gas film which creates a physical separation between the ship and the water, resulting in friction attenuation. Air cavity drag reduction (ACDR), in contrast, establishes a gas-filled cavity that, by isolating a portion of the wetted surface, reduces drag in that area. These techniques not only bring about improvements in terms of operational efficiency but also offer innovative responses to the pressing sustainability challenges in the maritime industry, reducing the fuel consumption of the vessels. It is essential to note that the transition from MBDR to ALDR mode and the subsequent implementation of ACDR are phenomena governed by the flow and pressure of air injected beneath the hull. The choice between these modes depends on the optimal balance between drag reduction efficacy and operational feasibility in relation to different types of ships and their specific operations. The three mechanisms of air lubrication for drag reduction are depicted in Figure 1, showcasing MBDR (Figure 1a), ALDR (Figure 1b), and ACDR (Figure 1c). The reported studies intend to deepen the understanding of air lubrication techniques, evaluating not just their efficiency in terms of drag reduction but also their overall impact on ship performance and the marine environment. In an era where sustainability has become a watchword for the industry, the potential of these technologies to contribute to a cleaner and greener future for the maritime sector is substantial. Addressing the challenge of environmental sustainability is crucial across various design fields. The integration of air lubrication technologies should be considered within the broader context of ecological impact, energy consumption, and emissions reduction.