MICRO BUBBLE DRAG REDUCTION
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.
• Injection of air bubbles along
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.
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
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
The mechanisms by which minibubbles
reduce friction are as yet
unclear. Mini-bubbles affect the
density and viscosity of the flow;
Kitagawa et al. (2005)
found that bubbles deformed with
a favorable orientation with respect
to the flow, reducing turbulent stress
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
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.
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.
MICRO BUBBLE TESTING CONCLUSIONS
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.
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