air bubble lubrication

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The efficacy of air-bubble lubrication for decreasing friction resistance

1. introDuCtionFor a number of years, air lubrication has been under investigation at MARIN as a means of reducing the frictional resistance of ships. Three general approaches are identified

Injection of air bubbles along the hull

Air films under the hull Air cavities in the bottom of

the hullSeveral projects were started up

in the Netherlands in 1999. The PELS project has studied the capabilities on theoretical and numerical grounds and by extensive model tests (Thill et al., 2005). The positive conclusion spurred two follow-up projects: PELS 2, focusing on air cavity ships and the Eu-funded SMOOTH project, focusing on air-bubbles and air-film lubrication. Both projects are focused on inland ships and coastal ships and both projects include a full-scale test with a demonstrator ship. This paper presents the results of model scale and full scale tests within the SMOOTH project. The effect of air lubrication by bubble injection on resistance and propulsion, seakeeping and maneuverability using both model scale and full scale experiments is discussed.

2. baCKgrounDThe frictional resistance is the dominant resistance component for low-Froude-number ships. Pressure drag (i.e., form resistance) and wave resistance are frequently optimized using Computational Fluid Dynamics (CFD) but the total wetted surface

remains a given. Reducing this frictional resistance by air lubrication is attractive. The power needed to compress air and inject it under the vessel should be less than the alleged power reduction due to the air lubrication.

For displacement ships, any reduction of the local skin friction leads to decreases of the resistance and commensurately fuel savings. As the Froude number increases and the wave resistance becomes progressively larger, the effect of air lubrication on the total resistance expectedly decreases. The injection of air requires constant pumping power and if the ship sails too slowly it represents a significant part of the propulsive power. Therefore air injection is expected to be suited for moderately fast ships with a target speed range of Froude numbers between 0.05 and 0.15.

This paper focuses on air-bubble lubrication only. If bubble lubrication is effective, it requires only a small change to the hull compared to an air-cavity ship.

Laboratory results of micro-bubble injection by Madavan et al. (1983) showed reductions of the frictional drag up to 80%. These micro-bubbles are very difficult to create on a ship scale. As the bubble increases in size, so does its tendency to deform in the shear and turbulent fluctuations of the flow and it is no longer a spherical micro-bubble. Bubbles are on a millimeter scale for current ship applications; the term micro-bubble is no longer applicable. As the term

micro-bubble is used ambiguously, a distinction between (mini-)bubble drag reduction and micro-bubble drag reduction is required.

At very low speeds, around 1 m/s, bubbles with a diameter of only a few Kolmogorov length scales of the flow can generate a 10% decrease in resistance at only 1 volume percent of air in the boundary layer (Park & Sung, 2005). At more realistic flow speeds of 5 to 15 m/s, this viscous length scale drops rapidly, enforcing a small bubble that is difficult to produce in large quantities. Moriguchi & Kato (2002) used bubbles between 0.5 and 2.5 mm and reported up to a 40% decrease in resistance for air contents over 10%. Shen et al.

(2005), using smaller bubbles between 0.03 and 0.5 mm, found a 20% drag reduction at an air content of 20%. No appreciable influence of bubble size was found here, but Kawamura (2004), using bubbles from 0.3 to 1.3 mm scale, found that larger bubbles persisted downstream longer and were more effective at reducing the resistance. As larger bubbles showed less dispersion this may have been an effect of concentration only (Harleman et al., 2009).

The mechanisms by which mini-bubbles reduce friction are as yet unclear. Mini-bubbles affect the density and viscosity of the flow; viscosity actually increases for small amounts of air, but at high Reynolds

number the turbulent stress is more important than viscous stress. A decrease of the density outside the viscous sublayer may be more important. Kitagawa et al. (2005) found that bubbles deformed with a favorable orientation with respect to the flow, reducing turbulent stress as the flow field around the bubble is more isotropic, although other mechanisms have been proposed, such as compression (Lo et al., 2006) or bubble splitting (Meng & uhlman, 1998).

Watanabe & Shirose (1998) tested a 40 m plate at 7 m/s to test the persistence of air lubrication. Skin friction sensors indicated that the skin friction reduction diminished

from the injection point onward; after 20 m, the effect of lubrication was nearly gone. Sanders et al. (2006) performed experiments with a large flat plate of 13 m length for speeds of up to 18 m/s. This experiment allowed for tests with bubbles ranging from 0.1 to 1.0 mm at Reynolds numbers that were previously not tested. The experiments showed that the bubbles were pushed out of the boundary layer a few meters behind the air injectors, against the direction of buoyancy. An near bubble-free liquid layer was formed near the wall and the effect of air lubrication almost vanished. It is hypothesized that the lift force experienced by a bubble in the boundary layer is more than sufficient to overcome the buoyancy of the bubble. The experiments by Watanabe and Sanders indicate that air lubrication will not persist over long length or time scales. This indicates that for model testing with bubble injection a strong Reynolds-scale effect is present and that tests using full-scale ships will not yield the expected resistance reductions as found during model tests.

For example, the full-scale demonstrator vessel Seiun Maru showed a 2% decrease at only a limited speed range, notwithstanding resistance decreases measured at model scale (reported in Kodama et al. 2002), with an increase in required power over most of its speed range. The Filia Ariea has been fitted with so-called Wing-Air Induction Devices, i.e. a slot in the hull fitted with a small protruding wing over the slot inducing a natural low-pressure region; tests with the Filia Ariea fitted with these air injection devices did not show a change in shaft power after the air supply was switched on (Belkoned, 2008).

Although air lubrication by mini-bubbles can show a decrease of frictional resistance for ships, the results are not always convincing. In order to gain more experience with air lubrication, a consortium of industrial companies and research institutes initiated the Eu-funded

abstraCtReducing the frictional resistance by air injection below the ship in combination with special coatings is an active area of research; anecdotally, performance gains are usually large. The paper gives an overview of some model scale and full scale measurements results of ships with one type of air lubricationair bubble lubricationperformed by MARIN. The experiments were performed for the EU-SMOOTH project. The first series of experiments focused on an inland shipping vessel that was tested both on model scale and on full scale, with and without air lubrication. A second series of tests consisted of maneuvering and seakeeping tests with a model painted with different coatings and with and without air lubrication. No appreciable effects of air bubble lubrication were found during the resistance and propulsion tests at either model or full scale and no significant effects of air bubble lubrication on maneuvering and seakeeping model tests could be determined.

FIGURE 1 Top and side view of the Till Deymann. The accented areas are the location for air injection, consisting of one array of strips in the bottom plating and one area at the downstream end of the forward tunnel thruster opening in the bow. The bottom right photograph shows this area prior to fitting the porous medium. The locations of the forward and aft azimuthing thrusters are indicated.

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project SMOOTH. The project focuses on air lubrication and its effects on resistance, propulsion, maneuvering, and sea keeping of ships. This paper presents the results of the resistance and propulsion tests on model scale and full scale, and the results of maneuvering and seakeeping at model scale.

3. resistanCe anD ProPuLsion oF tHe tiLL DeYMannThe test vessel is the 109.8 m inland-shipping vessel Till Deymann, see Figure 1. The vessel has a semi-twin-hull bow with two openings in the sides fitted with two 1050 mm azimuthing thrusters rotated 15 degrees outboard and two nozzled azimuthing 1300 mm thrusters are fitted aft. The air is injected at the far wall of the recesses in the bow and a strip in the bottom through a porous medium with a 20 m pore size. The four thrusters were all powered and instrumented during the tests. The aft thrusters were fitted on a 6-DOF measurement frame with both propeller thrust and torque measured in the propeller hub, while the forward thrusters were rigidly mounted and only propeller torque was measured.

From full-scale trials was known that each thruster received equal power, but the forward thrusters could not deliver that power over the entire speed range without overloading. The forward thrusters were set to 60% of the power of the aft thrusters during all tests.

3.1 MoDeL sCaLe resuLtsAn 11.8 m model of the Till Deymann at a scale ratio of 1:9.286 has been constructed. Note that even for such a large model, the propeller diameters were small, making the test less suitable for extrapolation to full scale. The model was well suited for comparative tests between fully-wetted and air-lubricated conditions.

The porous medium is not scaled and has a 20 m pore size as on the real ship. In order to properly scale the

effect of the atmospheric pressure, the model was tested in the MARIN depressurized towing tank. Nitrogen instead of air was injected from a pressurized gas canister where the nitrogen was allowed to expand and was subsequently heated before being injected. The gas-volume flow was measured and controlled by Bronkhorst EL_FLOW mass-flow controllers calibrated for nitrogen gas. The 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 10 kts with the air-volume flow rate set at 0 (reference), 3, and 6 L/min. Figure 2 shows the results of the bare-hull resistance test. A 1% increase of the resistance was measured (although this falls well within the measurement accuracy) while most model tests show decreases. The amount of air may have been insufficient to have any effect while the air injection may disturb the boundary layer too much. It was observed that the air from the openings for the forward thrusters immediately rose to the surface. The exact cause of the resistance increase is unclear and further tests are planned with an increase in the air volume flow rate.

3.2 FuLL sCaLe resuLtsThe full-scale trials with the Till Deymann were performed with the same draught as for the model tests. These tests were performed in the Netherlands, and in both fresh and salt water as the coalescent behavior of bubbles is known to depend on salinity. The ship was fitted with an anemometer (horizontal plane), a six-DOF accelerometer, shaft torque and rpm sensors (strain gauge and optical sensor respectively), a boroscope placed aft of the air injection array and fitted with an image intensifier capable of a frame rate of 200 Hz, and two GPS antennae to determine the course with 0.5 accuracy.

The tests consisted of sailing in 10-minute intervals in each direction (track length permitting) and taking

the average of six of these runs per measured point. Several 11 kW compressors were used for air injection. The weather conditions were very good with wind condition mostly at Bft 1 and occasionally up to Bft 3. The repeatability of the testswithin 2%is considered to be high. Although the measured trend is constant and consistent, the effect of air lubrication is not significant. The power required for air injection was measured. It can be concluded that for the current setup the power required for air injection exceeds the power reduction by air lubrication. From the model tests it was hypothesized that the wake of the forward thrusters aimed partially at the rear wall of the hull openings may benefitat least locallyfrom air lubrication. The full scale trials did not show any effect when the

air lubrication at that location was activated.

4. seaKeePing anD ManeuVering at MoDeL sCaLeThe influence of air lubrication and coating type on seakeeping and maneuvering characteristics was investigated using two wooden models of a tanker. Different coatings were applied to the bottom of the model by means of interchangeable pre-coated plates. The first model was previously segmented and had the plates bolted to the model, hence the later need for a second model where the plates were mounted flush with the bottom using recesses. The models were instrumented with electrical drives, thrusters, and an air supply system. Three types of plates were used: uncoated, coated with Intersleek

900, and coated with an experimental highly-hydrophobic coating designed by AkzoNobel.

The following model data were collected:

1. Position, rotation, accelerations2. Actual mass flow in air supply

system3. Rotation rate, torque and thrust

of propellers4. Steering angle of thrusters5. undisturbed wave height (clear

of the model)As with the resistance and

propulsion tests, nitrogen gas was used to produce the bubbles and the

same mass flow controllers were used to measure and control the mass flow rate through small-pore sintered steel inserts and then through 1 mm holes in the steel plate, resulting in large bubbles (0.52 mm). Air injection points are placed from the bow and every 20% of the model length, except at 100%, numbered 1 to 5. The air injection point at the bow is only a third of the beam of the ship. The following tests were performed:

1. Powering runs to determine propeller rate versus speed

2. Zig-zag tests3. Combined Turning-Circle/Pull-

Out4. Roll-decay tests5. Seakeeping runs in irregular

bow-quartering and stern-quartering seas

underwater video observation was used to verify the performance of the air layer. Experimentation with the volume flow rate of nitrogen was performed and the proper flow amount was determined based on the general appearance of the bubbles. The performance was highly sensitive to the exact fitting of the plates; even minor alignment problems of the plates caused nitrogen not distribute evenly and to collect in certain locations.. A constant flow of bubbles along the bottom of the plates was observed. A representative view is given in Figure 4.

4.1 tested configurationsThe tested configurations and associated names are indicated in Table 1.

4.2 Maneuvering behaviorZig-zag tests and combined turning circle/pull-out tests showed small differences in the results. Most of these differences were erratic and fall within the confidence interval of maneuvering tests. All observed differences have little or no influence on ship operation. The difference between coatings

was observed to be larger than the difference between fully-wetted and air lubrication conditions.

4.3 influence of air configurationThe influence of air lubrication was limited to the following aspects:

1. In the turning circle test, the stable turning diameter increased by approximately 5% with an increasing air flow rate (Figure 5, top).

2. In the turning circle test, the pivot point moved 5% forward from midships with an increasing air flow rate (Figure 5, center).

3. In the turning circle test, the maintained speed increased by about 5% with an increasing air flow rate (Figure 5, bottom).

4.4 influence of coating typeAs the different coatings were applied to interchangeable plates, the measured changes were influenced by small geometric differences between the plates (size, paint thickness, accuracy of fitting them to the model, etc.). Only minor differences between plates were measured and as a difference of a few percent can be expected from one set of plates to the next, no significant influence on the maneuvering performance could be attributed to the coating type itself. Additionally, no consistent trends

FIGURE 2 Measured resistance of the model. A 1% increase is measured (except for one outlier).

FIGURE 3 Measured shaft power onboard the Till Deymann with and without air lubrication both for salt water and fresh water conditions.

FIGURE 4 Air bubbles under ship model

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could be observed.

5. seaKeePing beHaViorThe model was tested in the irregular wave conditions in bow and stern quatering wavesi.e. 315 and 135 headingwith a signigficant wave height of 1.5 m and wave peak period of 9 s for all configurations as presented in Table 1.

These conditions were repeated from the PELS project and are considered representative for what an inland ship can encounter. It should be noted that these conditions do not excite the ship in its natural frequencies of motion and therefore the measured motions are small. Measurements with a small magnitude are more susceptible to disturbances and therefore reduce the accuracy of these results. The results show the root mean square (RMS) of the motion divided by the RMS of the undisturbed wave height. Roll and pitch are considered to be the most relevant ship motions.

5.1 influence of air configuration and coating typeFigure 6 shows the obtained

results for roll and pitch in bow- and stern quartering seas. Note thatexcept for the extra large roll in bow-quartering waves for uNCOATED FuLL AIR A, following from a single measurementall the measurements show little variation between fully-wetted and air-lubricated conditions.

When considering the accuracy level in the tests one must conclude that the differences between these results of each air configuration fall within the confidence interval of these tests for all coating types. Also, no consistent trends can be seen. Therefore, it is concluded that no influence of air lubrication and coating type on seakeeping performance could be found.

ConCLusionsExperiments have been performed with ships with and without air-bubble injection at model scale and full scale. The results of model scale experiments showed a small increase in resistance and a small increase in propulsion efficiency, both around 1-2%. A trial with the ship with air lubrication at full scale showedat besta 2% reduction in required propulsive power with air lubrication (roughly

2.6%). The net power reduction was consistently measured at 0.6%, i.e., an increase, for both fresh water and salt water conditions. The behavior of the bubbles in the boundary layer of the full-scale ship (insofar they could be seen) showed that bubbles did not

attach to the hull.Maneuvering and seakeeping tests

showed very small differences between air-lubricated and fully-wetted hulls for maneuvering and no differences for seakeeping. Therefore, any vessel with air bubble lubrication does

90% 92% 94% 96% 98% 100% 102% 104%

UNCOATED

INTERSLEEK 900

HYDROPHOBIC

DSTC [-]

Turning Diameter (2nd model; relative to UNCOATED NO AIR)

80% 85% 90% 95% 100% 105%

UNCOATED

INTERSLEEK 900

HYDROPHOBIC

Non-dim pivot point [-]

Non -dimensional pivot point (2nd model; relative to UNCOATED NO AIR)

75% 80% 85% 90% 95% 100% 105%

UNCOATED

INTERSLEEK 900

HYDROPHOBIC

Vstc/V0 [-]

Maintained speed fraction(2nd model; relative to UNCOATED NO AIR)

STERN MAX AIR MAX AIR FULL AIR B NO AIR

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

UNCOATED (1st model)

INTERSLEEK 900 (1st model)

UNCOATED (2nd model)

HYDROPHOBIC (2nd model)

rms/rms [deg/m]

Roll; bow quartering seas

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

UNCOATED (1st model)

INTERSLEEK 900 (1st model)

UNCOATED (2nd model)

HYDROPHOBIC (2nd model)

rms/rms [deg/m]

Pitch; bow quartering seas

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

UNCOATED (1st model)

INTERSLEEK 900 (1st model)

UNCOATED (2nd model)

HYDROPHOBIC (2nd model)

rms/rms [deg/m]

Roll; stern quartering seas

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

UNCOATED (1st model)

INTERSLEEK 900 (1st model)

UNCOATED (2nd model)

HYDROPHOBIC (2nd model)

rms/rms [deg/m]

Pitch; stern quartering seas

STERN MAX AIR FULL AIR B FULL AIR A NO AIR

not need a specially-trained crew to handle the ship.

In conclusion, no appreciable effect was found of the injection of air bubbles on resistance, propulsion, and maneuvering characteristics of a ship. 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 significant results. It should be noted that this conclusion does not apply to air lubrication by either air films or air-cavity ships.

6. aCKnowLeDgeMentsSMOOTH is supported with funding from the European Commissions Sixth Framework Programme with participation of MARIN, AkzoNobel, Bureau Veritas, Damen Shipyards, Istanbul Technical university, Atlas Copco Ketting Marine Centre, New Logistics, SSPA, DST, Thyssen Krupp Veerhaven, & Imtech.

7. reFerenCesBelkoned Marine Service b.v.,

Report No 973-A/08, 2008FuKADA, K., TOKuNAGA, J.,

NOBuNAGA, T., NAKATANI, T. and ISAWAKI, T., Frictional drag reduction with air lubricant over a super-repellent surface, J. of Marine Sc. And Techn., 5 (2000): 123-130

HARLEMAN, M., DELFOS, R., WESTERWEEL, J., TERWISGA, T. VAN, Characterizing 2-phase boundary layer flow, Wall turbulence conference, Lille, France, 2009

LO, T.S., LVOV, V.S. and PROCACCIA, I., Drag reduction by compressible bubbles, Phys. Rev., 72 (2006): 036408

KATO, H., Microbubbles as a skin friction reduction device, 4th symposium on smart control of turbulence, Tokyo, 2003

KAWAMuRA, T., FuJIWARA, A., TAKAHASHI, T., KATO, H., and KODAMA, Y., The effects of bubble size on the bubble dispersion and the skin friction reduction, 5th Symp. Smart Control of Turbulence, 2004

KITAGAWA, A., HISHIDA, A.

and KODAMA,Y., Flow structure of microbubble-laden turbulent channel flow measured by PIV combined with the shadow image technique, Exp. Fluids, 38 (2005): 466-475

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

MENG, J.C. and uLHMAN, J.S., Microbubble formation and splitting in a turbulent boundary layer for turbulence reduction, Intl. Symp. on seawater drag reduction, Newport, RI, uSA, ONR, Arlington, VA, 1998: 341-355

MORIGuCHI, Y. and KATO, H., Influence of microbubble diameter and distribution on frictional resistance reduction, J. Mar. Sci. Technol., 7 (2002): 79-85.

PARK, Y. S. and SuNG, J. H., Influence of local ultrasonic forcing on a turbulent boundary layer. Exp. Fluids, 39 (2005) 966976.

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.

THILL, C., TOXOPEuS, S, and WALREE,VAN, 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

FIGURE 5 Turning diameter, Pivot point, and maintained speed fraction in a turning circle test, maneuvering tests.

FIGURE 6 Roll and pitch in both bow quartering and stern quartering seas, seakeeping tests.

Mod

el

Flow [/min]

Air config per location Coating

1 2 3 4 5

1

Air A

Air A

11

33

33

33

33

Intersleek 900Intersleek 900

2

Air B

Air BMax Air

Stern Max

3_330

3_360

3_3150

3_31515

3_31515

Intersleek 900Intersleek 900HydrophobicHydrophobicHydrophobicHydrophobic

TABLE 1 Tested air configurations

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