# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies
### Tests Performed on MBDR
RBI maintains frictional drag reduction by generating bubble swarms, even with a low mean void fraction of injected bubbles into the turbulent boundary layer. The mechanism of enhanced drag reduction and the effectiveness of the RBI approach were thoroughly investigated by measuring the temporal variation of local wall shear stress, the velocity vector field of the liquid phase, and the gas-liquid interface in turbulent horizontal channel flows. The experimental setup included a horizontal rectangular channel made from transparent acrylic resin, using silicone oil as the working fluid. Two 4 MHz ultrasonic transducers were attached to measure the internal flow structures. The RBI approach was examined at Reynolds numbers greater than 1200. To diminish the frictional drag during a ship's progress, the introduction of small amounts of bubbles, as studied by [PERSON] et al. [20], can be chosen. In the primary experiment, conducted in a 25 m long recirculating water channel, the focus was on understanding the reduction of viscous drag in a turbulent boundary layer over a flat plate, influenced by the presence of microbubbles. These microbubbles, having an average diameter similar to the local Kolmogorov length scale and a bulk void fraction of approximately 0.1%, were studied. The frictional drag of a 6 m long flat plate with microbubbles, generated through electrolysis, was initially measured in a towing tank. To assess the streamwise and wall-normal components of the liquid phase's velocity, image analysis techniques and a Feature Tracking (FT) algorithm were employed. The void fraction of the bubbles was estimated by calculating the projected bubble volumes within the measurement volume. The distribution of bubbles in the near-wall region of the boundary layer was also analysed [21]. An alternative method to enhance the efficiency of frictional drag reduction using bubbles, studied by [PERSON] et al., involves using a high-speed vessel model for experiments in a test basin. The basin had dimensions of 50 m in length and 40 m in width, with a constant water depth of 2 m.  
The experiments encompassed a Froude number reaching as high as 0.65. Central to the setup was the ship model, which was affixed to a load cell transducer above the baseline. This arrangement permitted unobstructed vertical movement of the model. Throughout the range of Froude numbers tested, the total drag experienced by the model was recorded for each trial. A wire rope was used to tow the ship model, enabling the measurement of the total longitudinal force exerted on the model at different speeds. During these measurement trials, the ship model was allowed the freedom to both pitch and heave [22]. The authors [PERSON], [PERSON] et al. [23] investigate skin friction reduction using large air bubbles in turbulent flow within a horizontal channel. They experimentally studied the relationship between local skin friction and the interfacial structure of the bubbles, employing relatively large air bubbles compared to the boundary layer thickness. They found a negative correlation between local skin friction and local void fraction, with significant skin friction reduction at the rear of large individual bubbles. This study provides new insights into the bubble-induced skin friction reduction process and suggests that bubbles of a certain size may be more effective in reducing friction. Another significant study on the effect of skin friction and its applicability to ships was conducted by [PERSON] et al. [24]. This research explored the use of microbubbles in a water tunnel to analyse how they can reduce skin friction on ships [25]. The authors measured the friction reduction and investigated the relationship between local void fraction and friction reduction. The article also examines the impact of ship sizes on the effectiveness of microbubbles, laying the groundwork for future research on the applicability of this technology to real-sized ships.