# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 2 Experimental Studies  
### Tests Performed on MBDR  
To study friction reduction in the turbulent boundary layers of a liquid flow caused by the presence of bubbles, several experiments were conducted, the first of which was led by [PERSON] et al. [14]. In this study, a model was developed to predict the effectiveness of microbubble drag reduction techniques in a boundary layer on a flat plate. The experimental techniques included a system for injecting microbubbles through porous material and a resistance measuring system to assess the frictional drag of the flat plate both with and without injected microbubbles. These experiments were conducted in a wind tunnel and a towing tank, varying the flow velocity and air injection rate, to test the reliability of the proposed model's predictions. The conclusions drawn by [PERSON] et al. highlighted that the drag reduction effect predicted by the boundary layer mixture model is almost directly proportional to the density ratio between the mixture and water, and that there exists an optimal air flow rate for each velocity in the towing tank. In another study conducted by [PERSON] et al. [15], the reduction of bubble friction in a turbulent boundary layer on a high-Reynolds-number flat plate was investigated. The experiments were carried out in the US Navy's Large Cavitation Channel, using a 12.9 m long flat plate to create a turbulent boundary layer with a near-zero pressure gradient. Air was injected through 40 \(\upmu\)m sintered metal strips flush-mounted to the test model's surface at high volumetric rates. The surface shear stress measurements without air injection, conducted by [PERSON] et al., revealed a systematic underestimation of frictional drag compared to the expected value based on Schultz-Grunow's friction line [16]. [PERSON] et al. [17] conducted a series of experiments at the US Navy's William B. Morgan Large Cavitation Channel (LCC) to explore skin friction drag reduction in a turbulent boundary layer (TBL) at high Reynolds numbers and large scales. These experiments involved injecting gas (air) from a line source through the wall of a TBL with nearly zero pressure gradient, formed on a flat-plate test model that was either hydraulically smooth or fully rough. The study focused on two specific phenomena of drag reduction: bubble drag reduction and air-layer drag reduction. The experiments measured the longitudinal distribution of skin friction drag reduction using six drag balances, with downstream-distance-based Reynolds numbers reaching up to 220 million and test speeds of up to 20.0 m/s. In the initial test, variables such as free-stream liquid velocity, gas injection rate, injection location, background surface tension, and injector type were altered to assess their impacts. Another method of study was conducted by [PERSON] and [PERSON] [18], utilising an experimental model to conduct physical model experiments in a controlled environment. At the School of Mechanical Engineering's Centre of Excellence in Hydrodynamics and Dynamics of Marine Vehicles, Sharif University of Technology, a series of experiments were conducted. These tests involved a catamaran model, 70 cm in length, crafted from plexiglass. The model was designed to facilitate air injection at three distinct locations: the bow, midship, and stern areas. A critical aspect of utilizing the microbubble method effectively lies in accurately estimating the rate of air injection. Although there is no direct conventional method to determine this rate, it can be inferred from the indices of the boundary water flow. The experimental results revealed a trend: as the speed increased, the peak drag reduction effect diminished, suggesting that the microbubble drag reduction technique is notably more effective for ships operating at lower speeds. A different approach to improve the effectiveness of reducing frictional drag through bubble utilization, researched by [PERSON] et al. [19], involves repetitive bubble injection (RBI) instead of the conventional continuous bubble injection approach.