Numerical Study on Air Cavity Flows
Author Zverkhovskyi, A., Kerkvliet, M., Lampe, A., Vaz, G. and Terwisga, T.J.C. van
Title Numerical Study on Air Cavity Flows
Conference/Journal 18th Numerical Towing Tank Symposium (NuTTS), Cortona, Italy
Month September
Year 2015

One of the most promising viscous drag reduction techniques for ships is so-called air lubrication. Particularly, a stable air layer created on the bottom reduces the wetted area and therefore reduces the resistance of a ship due to the negligibly small viscous drag of air compared to water. The air cavity and air chamber concepts are considered as the most effective and practical ways to form a stable air layer on the bottom capable to reduce the total drag by 10-20% for cargo ships. The air cavity concept is based on injecting air behind a small obstruction that separates the flow which is called a cavitator. The cavitator is extended in the span-wise direction and typically has a rectangular or triangular cross section with a sharp edge. In the developed stage the length of an air cavity is limited to the length of a half the gravity-wave length and it should be restricted by skegs/keels on the sides. The air chamber is created by injecting air into a recess formed in the bottom. Provided the recess has sufficient depth, the free surface is not limited by the wave length and can have a multi-wave profile. The shape of the free-surface at different flow conditions and air injection rates is of interest.
For cargo ships, a series of air chambers or cavities behind each other have to be created on the bottom separated from each other in longitudinal direction. The effectiveness of such a system with periodically changing condition from no-slip (on wetted part) to slip (along the air cavity/chamber) is difficult to be modelled numerically. This work is focused on the numerical study of two problems: the drag reduction prediction by air cavities with known size and the free-surface simulation of air chambers and cavities. All the results presented here are obtained by using ReFRESCO. ReFRESCO is a viscous-flow CFD code that solves multiphase (unsteady) incompressible flows with the RANS equations, complemented with turbulence models, cavitation models and volume-fraction transport equations for different phases.

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