# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on ACDR  
Research on the Air-Cavity Ship (ACS) is being studied as one of the most promising concepts for economical marine transport. The ACS employs an air injection system to isolate the ship's lower wetted surface from the water, thus reducing frictional drag. Air cavities are created by injecting air at the keel level, extending along large portions of the hull bottom. The experiment described in [PERSON]. [PERSON] et al. 's [PERSON] _et al._ (2018) work used a simplified flat-plate model to study the effect of air cavities on reducing hydrodynamic drag. Conducted in the Emerson Cavitation Tunnel at Newcastle University, the primary objective was to understand the correlation between the length and volume of air cavities and the potential reduction in resistance. The study generated both fundamental and practical knowledge about the air cavity phenomenon. The results demonstrated that air cavities could significantly reduce hydrodynamic drag, effectively showing the relationship between cavity length, volume, and drag reduction. These findings contribute to a better understanding of air cavities and their potential practical applications. The naval tank experiment described by [PERSON] et al. (2019) aimed to assess the effect of an air cavity on a ship's hull to reduce hydrodynamic resistance. The study used a 2.2 m container ship model, modified to include an air cavity, and conducted a series of tests in a towing tank (Figure 11). Figure 11: Air cavity accommodated on the bottom of the hull (**a**) and schematic of air delivery system (**b**) (Ref. [PERSON] _et al._ (2018) reproduced under a CC BY license). The outcomes of the study indicated a fluctuating decrease in resistance, influenced by both the air flow rate and the ship's velocity. Alongside assessing the reduction in drag, the research also focused on the influence of the air cavity on the ship's stability and seakeeping capabilities. To evaluate the effects of integrating an air cavity into the hull, three types of tests were carried out on a container ship model: static stability, resistance, and motion response. The results pointed towards a negligible effect of the air cavity on the ship's static stability. The introduction of the air cavity increased the hull's drag by 10% compared to the standard model. Using various air flow levels resulted in nonlinear outcomes in drag reduction, achieving a 4% saving in drag by merely reducing the wetted surface area by 5%. Limited motion response tests in specific wave conditions suggest that the air cavity can induce nonlinear vertical motion behaviours, potentially affecting drag. The study by [PERSON] et al. [40] involves experimental tests and numerical simulations on a stepped hull model to explore its hydrodynamics. Conducted in a towing tank test, these tests focused on drag, dynamic sinking, and the trim angle of the model. Numerical simulations employed various mesh techniques to model hull motion and water flow. The primary aim was to understand the complex hydrodynamic flow phenomena generated by the stepped hull, especially in the unwetted area behind the step. The naval tank tests revealed complex vortical flow phenomena in the unwetted aft body area of the stepped hull, particularly at towing speeds above 2.36 m/s. These vortices, originating from air spillage at the sides of the step, exhibited complex three-dimensional behaviour. The \"down-thrust\" method used for the tests enabled precise data collection, crucial for analysing drag, dynamic sinking, trim, and the dynamic wetted surface of the hull. These findings provide significant insight into the hydrodynamic behaviour of stepped hulls. A significant contribution to the exploration of ACS systems is represented by the experimental study conducted by [PERSON] et al. [41]. This study focused on the towing tank test to evaluate different air cavity configurations in a planing hull (Figure 12).