Feature 2 | CAD/CAM
project focused on air lubrication and its effects on resistance, propulsion, maneuvering, and sea keeping of ships. Te first full-scale vessel tested was the 109.8m inland-shipping vessel Till Deymann, see Figure 4. Te tests focused on air bubbles only. The vessel has a semi-twin-hull bow
with two openings in the sides fitted with two 1050mm azimuthing thrusters rotated 15degs outboard and two nozzled azimuthing 1300mm thrusters fitted aſt. Te air was injected at the far wall of the recesses in the bow and a strip in the bottom, through a porous medium with a 20mm pore size.
Model scale results An 11.8m model at a scale ratio of 1:9.286 has been constructed. Even for such a large model, the propeller diameters are small, making the test less suitable for extrapolation to full scale, but well-suited for comparative tests between fully-wetted and air-lubricated conditions. Te porous medium is not scaled and has
a 20mm pore size as on the real ship. In order to properly scale the effect of the atmospheric pressure, the model was tested in MARIN’s depressurised towing tank. Nitrogen instead of air was injected from a pressurised gas canister where the nitrogen expanded and was subsequently heated before being injected. Mass-flow controllers calibrated for nitrogen gas maintained a constant supply. Te model was set at the correct draught and trim with chambers filled with nitrogen gas. The model was tested at a ship speed
range from 5 to 10knots with the air-volume flow rate set at 0 (reference), 3, and 6L/min. Figure 1 shows the result of the bare-hull resistance test. A 1% increase of the resistance was measured—although this falls within the measurement accuracy—while most model tests wit air lubrication typically show decreases. Te amount of air may have been insufficient to have any effect while the air injection may have disturbed the boundary layer too much. The propulsion tests, however, showed a decrease of about 2% of the required shaſt power.
Full scale results Te full-scale trials for the Till Deymann were performed at the same draught as the model tests. Tese tests were performed in both fresh and salt water, as the coalescent behavior of bubbles is known to depend
46
on salinity. The ship was fitted with an anemometer, a six degree-of-freedom accelerometer,
strain-gauge shaſt torque
and optical rpm sensors, a boroscope placed aſt of the air injection array fitted with an image intensifier capable of a frame rate of 200Hz, and two GPS antennae to determine the course within 0.5degs. The tests consisted of sailing in
10-minute intervals in opposite direction (track length permitting) and taking the average of six runs per measured point. Several 11kW compressors were used for air injection. Te weather conditions were very good with wind condition mostly at Beaufort 1 and occasionally up to Beaufort 3. Te repeatability of the tests—within 2%—is good. Although the measured trend is constant and consistent, the effect of air lubrication is not significant and it is concluded that for the current setup the power required for air injection exceeds the power reduction by air lubrication; the optimum point was found at the break-even point. Te behavior of the bubbles in the boundary layer of the full-scale ship—insofar as they could be seen— showed that bubbles did not remain attached to the hull.
Conclusions Experiments have been performed with the ship Till Deymann with and without air-bubble injection at model and full scale. The results of model scale experiments showed a small increase in resistance and a small increase in propulsion efficiency around 1-2%. A trial with the Till Deymann with air lubrication at full scale showed a 2% reduction in required propulsive power with air lubrication, excluding pump losses. Te total average power reduction was measured at –0.6%, i.e., an increase, for both fresh and salt water conditions. Although, air lubrication by mini-bubbles
decreases frictional resistance for ships, the obtained power reduction is not significant. The problem is that effective resistance- reducing micro-bubbles cannot be produced on ship scale and that the bubbles that can be produced are forced away from the hull by the ship boundary layer, shortening their effective life span. Barring unforeseen effects of special coatings and other surface treatments, an ad hoc application of bubble injection for ship hulls is not expected to
yield any appreciable savings. When excessive amounts of air are injected, an air layer is formed that can have a resistance-reducing effect and conclusions with regard to the efficacy of bubbles no longer apply. Research within SMOOTH now continues with air-cavity ships, including extensive testing at both model and full scale for an inland- shipping vessel, where net power savings up to 15% have been confirmed during trials at both deep and shallow water conditions.
Acknowledgments SMOOTH is supported with funding from the European Commission’s Sixth Framework Programme with participation of MARIN, DST, SSPA, Damen Shipyards, Imtech, AkzoNobel, Bureau Veritas, Istanbul Technical University, Atlas Copco Ketting Marine Centre, New Logistics & Tyssen Krupp Veerhaven. NA
References Belkoned Marine Service b.v., Report No 973-A/08, 2008 Gils, D.P.M., Bruggert, G.W., Lathrop, D.P., Sun, C., Lohse, D., “The Twente turbulent Taylor-Couette (T3C) facility: Strongly turbulent (multiphase) flow between two independently rotating cylinders”, Rev. Sci. Instrum., 82 (2011) Kodama, Y., Kakugawa, A., Takahashi, T., Nagaya, S., and Sugiyama, K., “Microbubbles: drag reduction mechanism and applicability to ships”, 24th Symp. Naval Hydrodynamics, 2002 Madavan, N.K., Deutsch, S., and Merkle, C.L., “Reduction of turbulent skin friction by microbubbles”, Phys. Fluids, 27 (1983): 356-363 Park, Y. S. and Sung, J. H., Influence of local ultrasonic forcing on a turbulent boundary layer. Exp. Fluids, 39 (2005) 966–976. Sanders, W. C., Winkel, E. S., Dowling, D. R., Perlin, M. and Ceccio, S. L., “Bubble friction drag reduction in a high-Reynolds-number flat-plate turbulent boundary layer.” J. Fluid Mech., 552 (2006): 353-380. Till, C., Toxopeus, S, and Walree,wan, F., “Project Energy Saving air-Lubricated Ships (PELS)”, 2nd Int. Symp. Seawater Drag Reduction, Busan, Korea, 2005 Watanabe, O. and Y. Shirose. “Measurements of drag reduction by microbubbles using very long ship models”, J. Soc. Naval Architects Japan, 183 (1998): 53-6.
The Naval Architect April 2011
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