ANALYSIS OF COMPRESSIBLE FLOW DURING THE WATER ENTRY OF A BODY EQUIPPED WITH AN AIR LUBRICATION SYSTEM

Sasan Tavakoli ^a, Spyros Hirdaris ^b

^a Marine and Arctic Technology Group, Aalto University, Espoo, Finland
^b American Bureau of Shipping, Global Ship Systems Centre, Athens, Greece

Presented at the 25th Numerical Towing Tank Symposium, Ericeira, Portugal – October 2023

ABSTRACT

The pressing need to reduce ship emissions has given rise to the emergence of new technologies and methods aimed at either decreasing carbon emissions from ships or reducing power requirements by minimizing ship resistance. The former primarily focuses on ship engines and power generation methods. The latter directly addresses fluid dynamics, particularly turbulent fluid motion around flat plates exposed to viscous fluid streams. The underlying concept behind the latter methods revolves around significantly reducing frictional stresses acting on the ship’s surface, which play a crucial role in turbulent fluid flow. To reduce frictional stresses, two commonly employed approaches exist. The first approach involves utilizing polymer covers capable of rendering the solid surface smoother, thereby resulting in reduced frictional stresses within the viscous sublayer. The other method entails gas injection. By introducing gas into the system, microbubbles or a gas layer can form beneath the
plate, effectively diminishing normal shear stress.

Since the 1970s, there has been a growing interest in studying the impact of gas injection on fluid dynamics around flat plates towed in water tunnels. Subsequently, further investigations explored the effects of gas injection on the resistance force of ships towed in tanks. It has been discovered that gas injection can reduce the frictional resistance of a flat plate by 80% and decrease ship resistance by 15% (Wang et al. 2022). However, it is important to note that frictional resistance is not the primary contributor to overall resistance, as other factors such as wave making resistance and form drag also play significant roles.