Pressure drag (i.e., form resistance) and wave resistance are frequently optimised using computational fl uid dynamics but, the total wetted surface area remains a given. Reducing the frictional resistance by air lubrication is attractive and any reduction of the local skin friction leads to a decrease of the resistance and hence fuel costs, provided that the power needed to inject air under the vessel remains smaller. As the ship speed increases, the wave resistance becomes progressively larger, and the eff ect of air lubrication on the total resistance is expected to decrease. Laboratory results of micro-bubble
injection by Madavan et al. (1983) showed that a reductions of the frictional drag up to 80% is possible. At very low speeds, around 1m/s, micro-bubbles can cause a 10% decrease in resistance at only 1 volume percent of air in the boundary layer (Park & Sung, 2005), but the eff ect lessens with increasing speed. Still, this reduction far exceeds the effect of density reduction; note that viscosity actually increases for an air-water mixture. However, micro-bubbles are difficult
to create in a laboratory, let alone for a ship. As the bubbles increase in size, so do their tendencies to deform and oscillate in turbulence of the fl ow and bubbles no longer remain spherical. For current ship applications bubbles are on a millimeter scale and the term micro-bubble no longer applies; these bubbles are actually mini-bubbles on the order of tenths to several millimeters. Watanabe & Shirose (1998) used a 40m
plate to test the persistence of air bubble lubrication at a flow speed of 7m/s. Skin friction sensors indicated that the skin friction reduction diminished downstream from the injection point. However, aſt er 20m, the eff ect of lubrication had nearly vanished.
Making a splash; testing out the air lubrication system onboard Till Deymann.
Sanders et al. (2006) performed experiments in a recirculating water tunnel with a large fl at plate of 13m length for fl ow speeds of up to 18m/s with bubbles ranging from 0.1 to 1.0mm at Reynolds numbers that were not previously tested. T e experiments showed that near bubble-free liquid layer was formed near the wall aſt er a few meters and the eff ect of air lubrication almost disappeared. Van Gils et al. (2011) used a Taylor-Couette setup consisting of two counter-rotating cylinders where the 0.2 to 3.0mm bubbles remained trapped and could not escape from the shear in the fl ow. At high Reynolds numbers a 50% drag reduction was measured at a 4% volume fraction. The above experiments indicate that bubble drag reduction is a boundary layer eff ect and that air lubrication will not persist over long length or time scales when
bubbles can escape. T is is refl ected in ship trial results. The full-scale test vessel Seiun Maru
showed a 2% power decrease for a limited speed range only, with an increase in required power for other speeds, notwithstanding resistance decreases measured at model scale (referred to in Kodama et al. 2002). Tests on the cargo ship MV Filia Ariea fi tted with air injection devices did not show a change in shaſt power aſt er the air supply was switched on (Belkoned, 2008).
Resistance and propulsion of Till Deymann Simultaneous with most of the research outlined above, a consortium of industrial companies and research institutes initiated the EU-funded project SMOOTH. The
shipyard logistics and handling
INDUSTRIE COMETTO S.p.A. Italy
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The Naval Architect April 2011 building for the heaviest duties 45
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