MICRO BUBBLE DRAG REDUCTION

Hull skin friction reduction via micro-bubble lubrication

MICRO BUBBLES - Introduction

The resistance of a ship's hull is one of the most important factors in energy consumption, typically diesel fuel oil. A major factor in ship drag is skin friction resistance. A reduction in the frictional resistance depends on the wetted surface area of a ship and the fluid flow around it. 

Micro bubble injection is one promising technique to lower frictional resistance. The method has been employed on high speed torpedoes and proposed for attack SWATH surface craft such as the Ghost (Juliet Marine) and Charc (Lockheed Martin) and high speed fighter submarines (Predator) traveling underwater at speeds between 60-200 knots.

Injected air bubbles modify the energy inside the turbulent boundary layer by introducing a less dense medium, and thereby lower the skin friction. It follows that the percentage and distribution of the air bubbles will have a significant effect on any reduction in hull drag. 

Ship resistance reduction has been one of the major targets of research and development by naval architects for a long time. Resistance characteristics are principal aspects of the ship design spiral, coupled with speed and fuel economy and, consequently, the operating cost efficiency of the vessel.

Micro bubble deployment has been shown to reduce hull skin friction resistance. Ways are being tested where it may be possible to reduce the friction of a given ship design without the need for radical alteration. The micro-bubble method reduces the surface friction by a variation of the viscosity of the fluid around the ship, modifying the structure of the turbulent boundary layer:

A. Documented drag reduction techniques include electrolysis induced micro-bubbles was reported by McCormick and Bhattacharyya. The survey of Latorre and Bablenko showed that the reduction in local skin friction is sensitive to the bubble orientation on the surface.

B. Madavan et al. carried out an experiment using the boundary layer of the test section wall of a water tunnel with injection of air from a porous plate. The result showed that with injection of micro bubbles in the turbulent boundary layer of a flat plate that drag may be reduced by between 15-80% according to application. Such tests are promising but do not take into account practical difficulties.

Bubble size and location of the injection points are important parameters of drag reduction. The relation between the bubble size and drag reduction was examined by Kato et al. The results showed that the decrease in the bubble size according to the increase in the main flow velocity causes a larger reduction rate of skin friction. 

Experiments by Watanabe & Shirose and Takahashi et al. indicated that air lubrication does not persist over length/time scales. Micro bubble drag reduction for flat plate and low speed vessel has been investigated by many researchers, reaching similar conclusion.

Kato used a tanker model for experiment and showed that bottom air film escapes around the hull sides without the use of longitudinal air guards set at the bilge, reducing the effect. For tankers and barges with moderate length to beam (L/B) hulls, the bottom air covers a large percentage of the wetted surface simplifying deployment. Latorre and Miller investigated micro bubble influence on a fast catamaran type boat and concluded that drag reduction of around 6% was achieved. The purpose of this review is to identify the effect of injected micro bubbles in reducing total resistance to enable designers to calculate any advantage for any boat/ship design.

METHODS OF DEPLOYMENT

Three general approaches are  :

• Injection of air bubbles along the hull
• Air films under the hull
• Air cavities in the bottom of the hull

Several projects were started in the Netherlands in 1999. The PELS project has studied the capabilities on theoretical and numerical grounds with 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 focused on inland ships and coastal ships and both projects include a full-scale test with a demonstrator ship. We look here at 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, sea keeping and maneuverability using both model scale and full scale experiments is discussed.

BACKGROUND

The 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.

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 minibubbles 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 hull lengths 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.

The behavior of the bubbles in the boundary layer (insofar as they could be seen) showed that bubbles did not attach to the hull.

Although air lubrication by minibubbles can show a decrease of frictional resistance for ships, the results are not always convincing.

It can be concluded that for some setups the power required for air injection exceeds the power reduction by air lubrication - giving a net increase in energy required for movement through water. That defeats the exercise and designers need to know at what point micro bubble inclusion becomes counter productive.

Comparison of ship resistance with and without micro-bubble is as a function of the drag coefficient and Froude number. Test revealed that the location of micro bubble injection behind the mid-ship is the best location to get the most effective drag reduction, and the drag reduction caused by the micro bubbles could achieve of 6 - 9%.

MICRO BUBBLE TESTING CONCLUSIONS

With a good setup, the net power reduction of the quoted series of experiments was consistently measured at 0.6%, i.e., an increase in hull performance, for both fresh water and salt water conditions. This figure falls far short of that required by cargo and navy fleet operators to be able to comply with the latest directives and is disappointing.

As an example, in the United Kingdom the Royal Navy are looking for an 18% reduction in fuel costs by 2020. Current spending on fuel is 2.4% of their budget, which is estimated to rise to 3.9% by 2015 and 7% by 2020. Clearly, micro bubble lubrication alone cannot deliver the reduction that this navy are seeking. We would suggest looking to implement a range of measures, to include ships that emit no sulphur or nitrogen oxides - as a means to offset the lack of reduction from other vessels. The Navy should consider alternatives if they plan to deliver effective defense for the nation at comparable levels to that today.

Because of the IMO's regulation on air pollution, the example given for the Royal Navy, applies to all NATO forces and fleet operators, save only that military vessels are exempt when deployed during hostilities. 

Consider that not to take a progressive and positive stance now, while other navies around the world fund R&D companies for the same solutions, is in effect to allow those other navies to gain ground.

The SolarNavigator team hope to be able to run tests on the above and to report those findings in the years ahead. Meantime, a patent is in progress concerning a formula for zero pollution craft.

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