Influence of Water Pressure on Bubble Induced Friction Drag Reduction

Abstract The tightening of environmental regulations by IMO (International Maritime Organization) has emerged as a significant challenge for the maritime industry, the air lubrication system (ALS), a friction drag reduction technology on ships, have emerged as one of the promising solutions. This study investigated the influence of water pressure on the friction drag reduction under the actual operating conditions of ALS. Experiments were conducted on a 4.0-meter-long flat plate installed in the LCT (Large cavitation tunnel) in KRISO (Korea Research Institute of Ships & Ocean engineering) to evaluate the impact of key parameters of air lubrication including water pressure, water speed and air flow rate, on friction drag reduction. The Friction drag was measured using a load cell connected to the flat plate. The results of the study indicate that the key parameters affect the air lubrication performance. The friction drag reduction gradually decreased with increasing water pressure when water speed and air flow rate were held constant. At constant water pressure and air flow rate, an increase in water speed also resulted in a gradual decrease in the friction drag reduction. In contrast, increasing the air flow rate under constant water pressure and water speed resulted in a gradual improvement in friction drag reduction. By analyzing these experimental results, the study showed that maintaining a consistent level of drag reduction is achievable by adjusting the supply air flow rate to compensate for air volume reduction under pressurized conditions, as described by the ideal gas law. To support these adjustments, the concept of tALC was introduced to calculate the supply air flow rate. Additionally, regression analysis was conducted to evaluate the relationship between tALC, water speed, and frictional drag reduction of the flat plate. These findings provide valuable insights for calculating the required air flow rate of ALS.