# A Review of Advanced Air Lubrication Strategies for Resistance Reduction in the Naval Sector
## 3 Numerical Studies
### CFD Simulations on ALDR
[PERSON] [56] study the efficiency of different cavity designs in reducing drag on a flat plate. It uses large eddy simulation (LES) to model turbulence and the Volume of Fluid (VOF) method for two-phase flow simulation, focusing on the air-water interface. The study evaluates the impact of injector direction and front wedge height on air layer  
Figure 16: Influence of air cavity depth on air layer distribution (Ref. [53] reproduced under a CC BY license). formation and drag reduction. It finds that wedge height significantly affects air layer thickness and drag reduction, with simulations showing improved stability and uniformity of the gas layer when reducing the wedge height (Figure 17). [PERSON] et al. [57] detail a study on the application of the steady Coanda effect for controlling ship air flow, focusing on drag reduction and air wake manipulation using large eddy simulation (LES). The study employs the wall-adapting local eddy (WALE) viscosity model and enhances the flow control capabilities of a baseline Chalmers ship model (CSM) by introducing Coanda surfaces and injection slots at strategic locations on the ship. The baseline ship model is adapted by adding Coanda surfaces and jet slots along its edges (roof and sides), aimed at manipulating the air flow around the ship. The numerical model used is large eddy simulation (LES), specifically employing the wall-adapting local eddy (WALE) viscosity model. This approach is chosen for its ability to handle large-scale instabilities and turbulent structures, which are crucial for accurately simulating air flow control around a ship model. LES is particularly effective for fluid dynamics problems where capturing vortices and fluctuating phenomena is essential. The numerical methods are validated with experimental data from tests conducted on the unmodified baseline model. Four configurations were tested: a baseline no-jet case, and three jet configurations with varying locations of jet activation (roof, sides, and combined). The simulations reveal that the roof jet, in particular, significantly enhances the Coanda effect, more effectively directing air flow towards low-speed areas on the deck, reducing drag and modifying air wake characteristics. The findings suggest that the manipulation of air flows using Coanda surfaces and jets can significantly impact ship aerodynamics by reducing drag and stabilizing air wake, which are critical for operations such as helicopter landings on ship decks. The study concludes that jets, especially from the roof, provide substantial benefits by directing high-momentum flows towards critical areas, thereby enhancing overall flow characteristics around the ship. Figure 17: The effect of modified wedge block height on the air layer shape (Ref. [56] reproduced under a CC BY license). [PERSON] et al. [58] investigate the air-layer drag reduction (ALDR) mechanism using direct numerical simulations (DNS). It explores different air injection rates and examines the stability and mechanism of ALDR, comparing the effects of high and low air injection rates. The study also includes a theoretical investigation of air layer stability using the Orr-Sommerfeld equations. The research aims to understand the role of the air injection rate on the stability of the air layer and its influence on drag reduction, concluding that the mean reattachment length of the flow is influenced by the presence of the air layer, indicating changes in the flow characteristics due to the air injection.