# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 3 Numerical Studies
### CFD Simulations on ACDR  
[PERSON] et al. [59], [PERSON] et al. [60], [PERSON] et al. [61], [PERSON] et al. [62], [PERSON] et al. [63], [PERSON] et al. [64] and [PERSON] et al. [65] all performed CFD simulations based on the RANS model to evaluate the air cavity ventilation. [PERSON] et al. [59] performed simulations with a ship free to heave and pitch. Key parameters like drag, lift, running draft, dynamic trim angle, and wetted area are analysed and compared with experimental data and a semi-empirical method. The CFD utilizes the k\(-\varepsilon\) turbulence model to consider the effects of turbulent flow. The paper also discusses the wake profile and reattachment location in relation to the stepped hull design. [PERSON] et al. [60] aim to assess the effectiveness of the RANS CFD approach in studying such ships. The study is divided into two phases: the first phase without air injection, aimed at evaluating the ship's drag curves, and the second with air injection to study air distribution under the hull. It employs a Shear Stress Transport k-\(\omega\) turbulence model to simulate the interaction between air and water and assess the impact of injected air on the ship's drag and air distribution under the hull. The results are compared with experimental data to validate the CFD approach. [PERSON] et al. [61] employ the k-\(\varepsilon\) turbulence model for the study of turbulence, simulating different conditions by varying the diameters of air bubbles introduced into the system. These simulations aim to assess the impact of these bubbles on the frictional drag and overall hydrodynamic efficiency of the flat plates, which are a key component in the design and operation of ACS. The research provides insights into the optimization of the air cavity system for enhancing the performance of these specialized ships. [PERSON] et al. [62] investigate the impact of longitudinal rails on the performance of a single-stepped planing hull with forced air ventilation on the bottom. The study includes a comparison between towing tank tests and CFD analyses. The study focuses on evaluating the effects of these rails on the drag and wetted surface (Figure 18), using the SST k-\(\omega\) model equations with multiphase models for high-resolution interface capture between air and water. The simulations were performed across a range of velocities and air flow rates. This analysis aims to understand the influence of longitudinal rails on hull performance in terms of drag reduction and air-water interaction. Figure 18: Wetted surface in three different conditions: no air and injection of the air to the minimum tested flow rate (Ref. [62] reproduced under a CC BY license). [PERSON] et al. [64] explore the effectiveness of air cavity technology in reducing drag and its impact on the heeling stability of planing hulls. The study employs the three-dimensional Reynolds-averaged Navier-Stokes equations coupled with the Volume of Fluid method (RANS-VOF). This approach is well-suited for analysing the fluid dynamics involved in the formation and maintenance of air cavities under the hull. The turbulence model used in the study is the SST k-\(\omega\) model. The air cavity's influence on drag reduction is predicted across a range of ventilation rates and heel angles. Various cavity forms, from meniscus growth cavity (MGC) to bottleneck stable cavity (BNSC), are evaluated to understand their effectiveness in drag reduction. The study identifies critical ventilation rates that optimize drag reduction, noting that excessive ventilation can lead to inefficiencies by expanding the tail air leakage opening, which negatively affects the air cavity's stability. Drag reduction effectiveness diminishes at a heel angle of about 2 degrees, where the air cavity shape starts to degenerate, impacting the drag reduction benefits. The air coverage provided by the cavity influences the ship's heeling stability. As the heel angle increases, the restoring moment (which helps keep the ship upright) decreases significantly, thereby reducing heeling stability.