# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on ALDR
It is very important in order to take into account the effects of the speeds of the boat on the drag reduction of the system. In the study, the variation in the ship model's relative drag reduction rate was observed in relation to wave length, considering different cavity depths and \(C_{q}\) values (0.112, 0.168, and 0.224) at a Froude number of 0.155. It was noted that the change in relative drag reduction rate with wave length followed the same trend across different \(C_{q}\) values. Generally, the relative drag reduction rate was higher in the 25 mm cavity and in long waves, reaching over 30% in long waves at a \(C_{q}\) of 0.224, which is lower than that in calm water. The results showed an increase in drag due to the presence of the cavity, with a 13.9% increase at a cavity depth of 25 mm at the design speed. The investigation also highlighted the adaptability of ALDR technology to the mode of air injection, demonstrating no direct correlation between friction reduction and the mode of injection in this experiment. The study's findings indicate that when the 25 mm cavity is constantly provided with air at an air flow coefficient \(C_{q}\) of 0.224 in calm water, there is an approximate 20% reduction in absolute drag at the ship's design speed. Additionally, this effect of drag reduction persists even in the presence of long waves. [PERSON] et al. [35] conducted an experimental study on air drag reduction using a 12.9 m long flat-plate model, altered by adding a 13 mm protrusion at the air injection site. The tests, performed at a speed of 6.3 m/s, evaluated the effect of vortex generators on the boundary layer, both with and without their use. A novel scaling for critical air flux was proposed, based on the analysis of a single bubble in shear flow. This scaling, suggesting that ALDR's transition is influenced by the ratio of buoyancy to turbulent shear forces, successfully unified data across different surface conditions and air injection methodologies. [PERSON] et al. [36] studied the effectiveness of porous media in the injection of microbubbles and air layers for drag reduction on a self-propelled barge model. They used a 30 cm wide porous medium as a microbubble injector to generate bubbles and layers beneath the hull surface, with an average hole diameter of 100 um, custom-built to meet experimental requirements. The study concluded that the microbubbles and air layers generated beneath the hull surface could significantly decrease water resistance, demonstrating the potential for efficiency improvements in maritime transport. [PERSON] et al. [37] conducted an experimental study on the reduction of skin friction drag using superhydrophobic-coated flat plates in high Reynolds number boundary layer  
Figure 10: Fitting the ship model with two air injection apparatuses (**a**) and arrangement of air injection devices (**b**) (Ref. [34] reproduced under a CC BY license). flows. The research involved coating aluminium flat plates with hydrophobic nanoparticles to create two distinct types of superhydrophobic surfaces--one with a lower contact angle and higher contact angle hysteresis, and another with a higher contact angle and lower contact angle hysteresis. These coatings were tested in a high-speed towing tank system to measure the skin friction drag across various flow regimes, comparing the performance to that of an uncoated bare aluminium plate. Their findings demonstrated significant drag reduction in certain conditions, attributed to the air layer's shear-reducing effects on the superhydrophobic surface  
Additionally, the study delved into basic drag reduction (BDR) by employing a traversing probe system to assess void fraction and gas-phase velocity profiles. Despite the varying configurations of the vortex generators, these measurements indicated a consistent interfacial velocity profile, akin to that in single-phase flows. However, discrepancies in void fraction profiles were observed, likely attributable to measurement uncertainties near the wall.