# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on MBDR
In the endeavour to diminish the hydrodynamic drag experienced by a ship's hull at cruising speeds of 15-20 knots, the research spearheaded by [PERSON] et al. [26] has culminated in the inception of a novel energy-conserving apparatus (Figure 2). This apparatus harnesses small bubbles, generated via the Kelvin-Helmholtz instability phenomenon. The device is ingeniously designed, comprising strategically angled hydrofoils equipped with air introducers, which are adeptly positioned beneath the air release vents on the hull's surface. The primary objective of this advanced technology is to substantially mitigate the turbulent momentum transfer, fundamentally targeting the reduction of skin friction along the hull's surface. This is achieved through the strategic injection of diminutive bubbles, each measuring less than 0.5 mm in diameter, directly into the turbulent boundary layer. Despite the precise scientific mechanism behind the drag reduction elicited by these minuscule bubbles remaining a topic of ongoing academic exploration, historical empirical observations have inferred that injecting such bubbles into the turbulent domain effectively attenuates the Reynolds shear stress [27]. This discovery suggests that the ideal condition for reducing drag is attained by maximizing bubble density at the point of peak Reynolds shear stress, usually located between the viscous sublayer and the buffer layer within the turbulent boundary layer [28]. Furthermore, it has been documented that small bubbles exhibit a natural propensity to accumulate within the turbulent shear layer, persisting for extended durations within the turbulent boundary layer and thereby countering the effects of turbulent diffusion [29]. More precisely, these small bubbles are prone to reside in regions of high entropy, characterized by negative values of the second invariant of the velocity gradient tensor [30]. In the context of a ship in motion, this region coincides with the layer most densely populated with active turbulent eddies, hence enabling the small bubbles to endure within the turbulent boundary layer for prolonged periods, effectively resisting turbulent diffusion. A distinctive aspect of the system, referred to as the Winged Air Sensor Tube (WAIP) (Figure 3), is the incorporation of the aerofoil section NACA 653-618, which is positioned below the surface level and set at an angle of attack of 12 degrees. During the ship's forward motion, the hydrofoil redirects the water flow, generating a low-pressure area above the foil, which in turn draws atmospheric air into the water. Figure 2: Air vent arrangement and the pressurization chamber of the foreship. The image was reproduced from [26]. This setup not only reduces the pressure above the hydrofoil but also accelerates the water flow above it while slowing it down beneath, thereby creating vortices downstream of the foil and assisting bubble formation both at the interface between gas and water, and behind the trailing edge. The test model ship, developed specifically for evaluating the WAIP system, comprises four sections, identified as A, B, C, and D in Figure 4.  
Figure 4: Recirculating water flow (shown in **(a)**) (West Japan Fluid Dynamics Engineering Laboratory Co. Ltd., Nagasaki, Japan); microscopic bubbles produced by the hydrofoil system (shown in **(b)**) (Ref. [26] reproduced under a CC BY-NC-ND 3.0 DEED license). Figure 3: Dimension and Kelvin–Helmholtz action of the WAIP system (**a**) and schematic illustration of the WAIP cross-section (**b**) (Ref. [26] reproduced under a CC BY-NC-ND 3.0 DEED license). The combined drag force experienced by sections B and C (notated as Dt) was assessed using a singular load cell. Concurrently, the drag impact on section B alone was independently gauged by a different load cell (marked as Do). Thus, the drag exerted on the 2.6 m long acrylic flat panel (referred to as Db) was calculated using an equation (Equation (1)) from these data points.