# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on MBDR
The process of bubble generation and its contribution to decreasing the frictional drag on the downstream horizontal flat wall were meticulously investigated through experiments in a circulating water channel, where the flow velocity was established at 5.6 m/s to replicate the typical cruising speed of ships. Here, \(B_{h}\) represents the horizontal width of the bubble-mixed layer and V denotes the flow velocity of the circulating water, with the water's density indicated by \(\rho\). For practical application to full-size ships, the total drag force \(F_{total}\) (Equation (2)) that is acting on a flat plate with length \(L\) is determined by the use of local friction factors with and without air bubbles, represented by \(C_{f}\) and \(C_{f0}\), respectively. \[D_{b}=D_{t}-D_{0} \tag{1}\]  
\[F_{Total}=D_{t}-\int_{0}^{L}\frac{1}{2}\rho V^{2}C_{f_{0}}\left(1-\frac{C_{f} }{C_{f_{0}}}\right)B_{h}dx \tag{2}\]  
This system, by avoiding the need for porous materials and overpressure, allows for the flow of air with the small bubbles created by the instability, demonstrating a significant advancement in drag reduction technology for maritime vessels. In Figure 5, the optimal functioning of the WAIP system is graphically depicted, showing variation with the ship's draught. In conditions where the draught is shallow (less than 5 m), the hydrofoil functions as a self-priming pump, effectively releasing small air bubbles near the hull through the hydrofoil device (Figure 5a). For ships with deeper draughts (over 5 m), where the hydrofoil's pressure reduction capacity is insufficient, a slight increase in air pressure is necessary to sustain microbubble formation (Figure 5b). In this configuration, the flow required is supplied by an air compressor, which is channelled through pipes along the hull of the ship and released above the hydrofoil in the area of low pressure. In the situation illustrated in Figure 5c, employing blowers that deliver high rates of air flow may result in a state where the hydrofoil becomes entirely separated from the adjacent water flow, resulting in the formation of only large bubbles beyond the boundary layer. This situation is not only inefficient for bubble generation but also increases the resistance caused by the hydrofoil, making the system less effective and wasting energy in bubble production. [PERSON] et al. [31] investigate the effectiveness of repetitive bubble injection (RBI) for reducing frictional drag in turbulent boundary layers under a model ship. This study explores how periodic fluctuations in bubble injection can enhance drag reduction compared to continuous bubble injection (CBI). The experiments were conducted with a 36 m long flat-bottom model ship towed at a speed of 8.0 m/s, where the drag reduction ratio achieved up to 24%, a 5% improvement over CBI. High-frequency cameras were used to monitor the flow and distribution of bubbles under the ship and the local wall shear stress was measured to evaluate the performance of the drag reduction techniques directly. Analysis showed that the downstream persistence of drag reduction can be significantly enhanced by RBI, with the effect observable up to 15 times further downstream compared to CBI. The findings suggest that the repetitive injection strategy alters the bubble dispersion in a way that extends the effective drag reduction zone along the ship's hull, potentially offering a more efficient method for reducing hydrodynamic resistance in marine vessels. The technique for generating and injecting the air cushion beneath the base of the passenger vessel's hull is shown in Figure 6.  
As shown in Figure 7, potent blowers create air bubbles that move steadily beneath the hull's bottom. The outlets for these bubbles are positioned symmetrically along the bottom on either side of the ship's centre line. Figure 5: Methods of air supply over a ship’s hull: (**a**) WAIP with natural ventilation; (**b**) WAIP with blower (where the hydrofoil is ineffective as a self-priming pump); and (**c**) WAIP with blower (air flow is excessive) (Ref.