# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on ALDR  
The experimental method of air layering was studied by authors [PERSON] et al. [3]. Experiments were conducted using a flat plate measuring 7.5 m \(\times\) 1.0 m \(\times\) 8 cm, horizontally mounted in the test section of a water tunnel (Figure 8). Air was injected through a flush slit near the front end of the plate, and two floating-plate-type friction sensors measured changes in local frictional drag caused by air layers. Additionally, six independent air injection units were installed on the ship model, flush with the hull's bottom surface to avoid unwanted drag increase. Compressed air was stored in receivers on the ship model's deck, and the air flow to the injection units was regulated by PC-controlled mass flow meters (Figure 9). \begin{table}
\begin{tabular}{c c c c} \hline
**Year** & **Vessel Name** & **Type** & **Length (m)** \\ \hline
2015 & Quantum of tire Sea & Cruise & 348 \\
2016 & AIDA prima & Cruise & 300 \\
2017 & AIDA perla & Cruise & 300 \\
2017 & Norwegian Joy & Cruise & 333 \\
2018 & Diamond Princess & Cruise & 290 \\ \hline \end{tabular}
\end{table} 
Table 1: The list of cruise vessels with air lubrication systems delivered 2015–2019 [32]. Figure 7: Compressed air supply system (ALS) of the underwater part of the hull on a large cruise liner. (Ref. [32] reproduced under a CC BY 4.0 DEED license). The experiments demonstrated that a transitional air layer generated by air injection on the hull's bottom can effectively reduce ships' frictional drag. For a 66K DWT Supramax wide-beam bulk carrier, net power savings, accounting for air injection energy consumption, were estimated between 5% and 6%. This energy saving translates to an 8-10% reduction in effective power and a 7-9.5% reduction in delivered power due to less than 1% loss in quasi-propulsive efficiency. The results suggest significant potential for energy savings with air lubrication if there is no significant performance deterioration in real sea conditions. In another study conducted by authors [PERSON] et al. (2019), experiments were performed to investigate friction reduction using an air layer beneath a flat plate in a towing tank. They observed the air layer formation and measured changes in drag. Drag and self-propulsion tests were also conducted on a ship model based on a 95,000 DWT container ship, using the method of friction reduction through a lower cavity air layer. Air layers were observed, and estimated energy savings were calculated. The results indicated that the provided power could be reduced by up to 15% in a full-scale ship, not accounting for the energy consumption for air injection. In the study conducted by [PERSON] et al. (2019), the application of an air cavity on the bottom of a ship model for friction reduction was explored. The model (Figure 10), inspired by a large container ship, featured a flat bottom structure ideal for the implementation of ALDR technology. Two air injection devices were used to form an air layer within the cavity, and the cavity's depth was varied to study its effects on the ship's drag and propulsion. Air was injected through holes in the base plate of each device, and the depth of the cavity was adjusted to study its impact on the ship's drag. Figure 8: Flat plate in water tunnel for generation of air layer (Ref. [3] reproduced under a CC BY-NC-ND 4.0 DEED license). Figure 9: Compressed air supply chain for air injection (Ref. [3] reproduced under a CC BY-NC-ND 4.0 DEED license). To further understand the impact of air flow rate and inflow velocity on air layer formation, the non-dimensional air flow rate coefficient (Equation (3)), \(C_{q}\), is defined using the air flow rate (\(Q\)), inflow velocity (\(V\)), transverse width of the air injection entrance (B), and the thickness of the boundary layer at the air injection entrance without air injection (\(\delta\)). \[C_{q}=\frac{Q}{VB\delta} \tag{3}\]  
The coefficient proposed allows to relate the flow rate of the air injected with the velocity condition of the boat.