Model experimental research of the air injection drag reduction system without air maintenance devices for a 2600 DWT bulk carrier

Air Injection Drag Reduction (AIDR) holds significant potential for energy savings and emission reduction in maritime vessels. Current AIDR systems depend on Air Maintenance Devices (AMDs), such as air cavities, which are challenging to install and exhibit high appendage resistances. This study presents a novel AIDR system for a 2600 DWT bulk carrier that eliminates the need for AMDs. The system employs rectangular slots in staggered positions on the ship's bottom, and its performance was evaluated through a model test in a circulating water channel (CWC) and numerical simulations, with airflow rate determined by nominal air layer thickness (T). The results showed that at T = 3.521 and 8.621 mm, the air-water mixed flows were divided into three regions: an air layer at the front, an air layer tearing in the parallel middle body, and air escaping at the ship's sides. As ship speed increased, the beneficial air layer length first increased and then decreased, with the air escape path tending to angle toward the stern, reducing the disturbances of the air on the free surface. At varying airflow rates, AIDR exhibited three forms similar to those observed on flat plates: bubble drag reduction (BDR), transitional air layer drag reduction (TALDR), and air layer drag reduction (ALDR). A large air coverage area and an air emerging position near the stern demonstrated high drag reduction effects. Reducing the exit widths of air injection decreased the air spreading angle, but required a higher airflow rate to establish an effective air layer. Under conditions of high speed, shallow draft, and low airflow rate, narrow slots positioned away from the ship's sides exhibited reduced spreading angles, effectively suppressing air escape. The air-water mixed flow was sensitive to the ship's attitude. Heeling and stern trimming reduced air coverage area and increased air escape, while bow trimming aided airflow spread. Therefore, a collaborative design of the AIDR system and the ship's attitudes is crucial when planning AIDR installation on new vessels. The experimental data will inform the development of numerical methods used for ship AIDR system without AMDs and guide design of the system.