The Subtle Effects of Air Lubrication: Mirco-Bubbles and Enhanced Control of Ship Dynamics

With the advent of terms like “globalization” and “export driven economy”, international trade is clearly an important part of the world’s economic landscape – trade that travels, in significant part, across the world’s oceans.  Even though maritime shipping is efficient, making it more effective is crucial to reducing global transportation costs and the shipping industry’s environmental impact.  Consequently, numerous studies have investigated the “air lubrication” of the ship’s hull in which air bubbles are introduced along a ship’s wetted surfaces in an attempt to reduce the drag on the ship.  Significant power reductions (between 4-12% net) have been achieved with some air lubrication systems.  Almost completely absent in these studies, however, is the bubbles’ impact on the propeller’s thrust.  Since changes in thrust, coupled with the modified drag, can have a significant impact on both the ship dynamics and the overall effectiveness of an air lubrication system (ALS), our initial focus has been to design and conduct model tests in a water channel that attempt to capture this effect.  During these tests, bubbles were injected near the propeller of a remote controlled boat, and the resulting thrust was measured.  Combinations of trials with and without bubbles were run, and initial results indicate a 4.5% reduction in thrust when bubbles are present.  Alongside this experimental work, theoretical investigations into the nature of the bubble/water interaction have also been undertaken.  This work has led to the development of creeping flow equations describing flow fields in both very small bubbles and the water that surrounds those bubbles.  The results of these experimental and theoretical studies have been applied to a general ship design to determine the effect these air bubbles have on the ship dynamics and how they may be used to assist with the control of the vessel.  In this presentation, the preliminary experimental results for the propeller tests, the theoretical solutions for shear flow around micro-bubbles, and the bubbles’ ability to influence the control of a vessel will be presented.