Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
50

Application of Simulation Technology to
Mitsubishi Air Lubrication System

CHIHARU KAWAKITA*1SHINSUKE SATO*2

TAKAHIRO OKIMOTO*2

For the development and design of the Mitsubishi Air Lubrication System (hereafter MALS),
which targets energy saving in ships, the utilization of simulation technology is essential. This
paper describes the key technology for the development of MALS, which is CFD-based prediction
technology for air bubble distribution around a hull, and also presents the frictional dragreduction effect. It also presents a prediction technology for the pressure fluctuation of a propellerthat rotates in the bubbly flow, as well as a related technology for the prediction of gas-liquid
separation in a sea chest that is an engine cooling water suction port. The prediction accuracy ofthese technologies has been verified and enhanced through verification in actual ship tests andmodel tests, and therefore they are used as effective development and design tools. Also in the
future, MHI will continuously obtain data from the measurement results of actual ships and utilize
it for the performance enhancement of MALS.

|1. Introduction
The development of energy-saving ships is greatly anticipated by the shipping industry as acountermeasure against the surging prices of resources including oil, and environmental issues suchas CO2 emission regulations for international shipping operations. The air lubrication method,which reduces frictional drag by mixing air bubbles of millimeter order into the flow around thehull, has attracted attention because it can be expected to result in an especially significant
energy-saving effect among other energy-saving technologies. In April 2010, MHI completed
Yamatai, a module carrier ordered by NYK-Hinode Line, Ltd., which was the world’s first newly
built ship equipped with the Mitsubishi Air Lubrication System (MALS) advanced air lubrication
method.

Figure 1

Image of ship equipped with MALS

The bottom of the ship is covered by air bubbles released from the air injectors.

Figure 1 shows a conceptual image of a ship equipped with MALS. The ship achieved anenergy-saving effect of more than 10% attributable to MALS during actual ship tests in a sea
trial(1). In December 2010, MHI completed Yamato, Yamatai’s sister ship, which was also
equipped with MALS. Using Yamato, the energy-saving effect was verified in a sea trial, the flowof air bubbles around the ship bottom was observed with a towed underwater vehicle as shown in
Figure 2, and the local frictional drag was measured by shear stress sensors installed on the bottom*1

Chief Staff Manager, Nagasaki Research & Development Center, Technology & Innovation Headquarters*2

Nagasaki Research & Development Center, Technology & Innovation Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
51of the ship. As a result, actual ship data valuable for the enhancement of the frictional dragreduction effect of MALS were acquired. As four years have elapsed since the module carrier went
into service, MALS is still working without any major failures. MHI is now aiming at expanding
MALS to ferries, bulk carriers, and large passenger ships(2).
This paper describes the main simulation technology utilized for the design of MALS. First,
the flowchart of the energy-saving effect prediction attributable to MALS will be presented, andthen the key technology for the estimation of the energy-saving effect resulting from MALS, which
is CFD-based prediction technology for air bubble distribution around a hull, will be explained
together with the frictional drag reduction effect. MHI is working hard on the enhancement of theprediction accuracy of this technology by obtaining data acquired in actual ship tests and model
tests. In addition, a prediction technology for the pressure fluctuation of a propeller that rotates inthe bubbly flow, as well as a prediction technology for gas-liquid separation in a sea chest that is anengine cooling water suction port, will be introduced as related technologies.

Figure 2

Towed underwater vehicle and air bubble flow released from air injector

The flow of air bubbles around the ship bottom was observed with a towed underwater vehicle
containing a camera (left photograph) to verify that the air bubbles were released uniformly from the
air injector (right photograph) and that air bubbles did not leak to the side of the ship.

|2. Flowchart of energy-saving effect prediction attributable to
MALS
Although the propulsive power of a typical ship is estimated from the results of a tank test
using a model ship, it is difficult to estimate the propulsive power of a ship equipped with MALS
in this manner. This is because it is necessary to facilitate the flow of air bubbles of micrometerorder around the model ship hull (compared with air bubbles of millimeter order around an actualship) according to the model size, which is difficult to do by simple blowing air from the model
ship. Therefore the tank test, for which there is no choice but to use air bubbles larger than the scaleratio of the model, results in the underestimation of the frictional drag reduction effect and air
bubble distribution around the propeller. For this reason, it is necessary and important for theestimation of the energy-saving effect of a ship equipped with MALS to use a prediction
technology that combines a tank test and a CFD-based prediction method of the frictional dragreduction effect.
Figure 3 shows the assumed flow of the energy-saving effect attributable to MALS. First,
the air injector location and the amount of air blow-off volume are set, and then the air bubble
distribution (void fraction distribution) around the hull and the propeller is estimated using theresults of CFD-based simulation of the air bubble flow around the hull. Next, data of the void
fraction distribution in the vicinity of the hull surface is input and the simulation of the reduction in
frictional drag is performed using MHI’s proprietary frictional drag reduction model. Withreference to the void fraction distribution at the location of the propeller acquired from thesimulation of the air bubble flow around the hull, the efficiency of the propeller in the bubbly flow
and the change in the propeller pressure fluctuation that is necessary for the prediction of hull
vibration are estimated using a combination of experiments and simulation. Finally, the net
energy-saving effect is obtained by estimating the power needed for air blow and subtracting itfrom the energy-saving effect of the reduction in frictional drag. In addition, the amount of air
bubbles taken into the sea chest is estimated based on the void fraction distribution around the hullto determine the shape and internal structure that can perform gas-liquid separation successfully,
based on the gas-liquid simulation in the sea chest.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
52

Figure 3

Flowchart of energy-saving effect prediction attributable to MALS

|3. CFD-based prediction technology for air bubble distribution
around a hull and frictional drag reduction effect
The air bubble flow model used for CFD-based simulation of the air bubble flow around thehull consists of a continuous expression of the mixed phase, a momentum equation of the mixed
phase, a conservation equation of the air bubble void fraction and a translation motion equation of
air bubbles being the governing equation. The estimation of the frictional drag reduction effect uses
MHI’s proprietary frictional drag reduction model based on the corresponding air film thickness
acquired by integrating the air bubble distribution in a boundary layer around the hull in the
direction vertical to the solid wall. This frictional drag reduction model is the application of a wall
function against a rough surface. In this model, the frictional drag increases on a rough surface incomparison with a smooth surface and is reduced through the use of an air lubrication method (3).

3.1

Calculation flow

Actual calculation proceeds in the order of single-phase flow calculation without flowing air
bubbles, one-way calculation, two-way calculation, and three-way calculation. A summary of eachcalculation is as follows:(1) One-way calculation
Bubble flow calculation is performed using the density, viscosity and normal eddy
viscosity coefficient of water. The air bubbles are solved as a passive scalar. The flow of theliquid phase influences the gas phase, but the flow of the gas phase does not influence the liquid
phase. This calculation is known as a one-way calculation, and can obtain the air bubble
distribution.
(2) Two-way calculation
The result of the one-way calculation is read. The calculation is performed with thedensity and viscosity changing depending on the void fraction. The flow of the gas phaseinfluences the liquid phase, and therefore this calculation is known as a two-way calculation.
The hull resistance changes based on the influence of the changing density and viscosity, but
although both the frictional drag and the pressure resistance change, there is little frictional dragreduction effect because the influence of turbulence modulation caused by the air bubbles is notincluded. The void fraction distribution changes slightly due to the influence of density change.
(3) Three-way calculation
The result of the two-way calculation is read. The calculation is performed with theMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)53

aforementioned frictional drag reduction model activated. This calculation takes into
consideration the reduction in the frictional drag and all flow changes caused thereby.

3.2

Calculation example

Calculation for the aforementioned module carrier with a length of 162 m, for which anactual ship test was conducted, was performed in order to verify the calculation accuracy. Figure 4
shows the void fraction distributions around the hull and at the location of the propeller obtained
through the simulation of the air bubble flow around the hull. The air bubbles flowed along the hull
without leaking from the bottom of the hull to the side of the ship, and the air bubble flow observedin the actual ship test using a towed underwater vehicle was reproduced. The void fraction at thelocation of the propeller tended to increase in the vicinity of the bottom of the hull, and decrease
with distance from the hull. Figure 5 shows the calculation results of shearing force distribution on
the bottom of the hull with MALS activated and inactivated. It is found that air bubbles covered awide area of the bottom of the hull and the shearing force of the covered area decreased. Figure 6
compares the actual ship test results and the calculation results of the local frictional drag reduction
effect with the change in the amount of air blow-off volume. The tendency where the local
frictional drag reduction effect increases more when the amount of air blow-off volume becomeslarger could be evaluated by the calculation. It is found that the frictional drag reduction effect rises
to as much as 60% when the amount of air blow-off volume is large. As the calculation accuracy
also attained a quantitative result that was roughly consistent with the results of the actual ship test,
this method is sufficiently applicable to the design of a ship equipped with an air lubrication
system.

Figure 4 Void fraction distribution on hull surface
and at the location of propeller

Figure 6
Comparison of local
frictional drag reduction effect

The upper figure shows the void fraction distribution of air
bubbles flowing on the hull surface from the air injector to
the stern. The lower figure shows a magnified view of thevoid fraction distribution at the location of the propeller on
the port side.

The local frictional drag reduction effects
based on the situation with MALS inactivatedwere compared. The lower the bar, the higher
the frictional drag reduction effect.

Figure 5

Prediction of frictional drag distribution on hull surface

The frictional drag distribution on the bottom of the hull on the port side viewed frombelow. The frictional drag decreased in a wide area of the bottom of the hull.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)54|4. Prediction technology for increase/decrease in the pressure
fluctuation of a propeller that rotates in the bubbly flow throughsimulation of sound pressure propagation
The air bubbles flowing around the hull are anticipated to flow into the propeller when a ship
is equipped with MALS. Based on knowledge acquired from a tank test, it is known to lead to adecrease in efficiency of the propeller and increase/decrease in the pressure fluctuation of thepropeller. An accumulation of air bubbles at the leading edge because of the pressure gradientcontributes to the decrease in the efficiency of the propeller. On the other hand, it is becoming clearthat the increase/decrease in the pressure fluctuation of the propeller is affected by the reflection,interference, and other phenomena of pressure waves at the air bubble layer between the propeller
and the bottom of the hull.
In the examination of MALS, the increase/decrease of the pressure fluctuation of thepropeller affected by the air bubbles is predicted through simulation technology for sound pressure
propagation using the finite element method and the cavitation test in bubbly flow with the model
propeller shown in Figure 7. The thickness of the air bubble layer and the void fraction distribution
between the propeller and the bottom of the hull are set with reference to the void fraction
distribution around the propeller obtained from the simulation of the air bubble flow around thehull. Then, the increase/decrease of the pressure fluctuation of the propeller corresponding to thepropeller blade frequency is estimated using a calculation model that has a point pressure source
simulating fluctuating cavitation on the propeller(4).
For the study of a mechanism that increases the pressure fluctuation of a propeller, Figure 8
shows the pressure fluctuation distributions for cases where an air bubble layer exists and where no
air bubble layer (only water) exists, together with the air bubble layer thickness and the frequency
which lead to the increase in the pressure fluctuation. In cases where the air bubble layer exists, thepropeller pressure fluctuation at the bottom of the hull decreases due to the reflection of pressure
waves at the boundary of the air bubble layer and the pressure damping effect in the air bubble
layer. However, the standing wave (antinode-node-antinode) occurs in the direction of the air
bubble layer thickness due to the interference of pressure waves in the air bubble layer andtherefore the propeller pressure fluctuation at the bottom of the hull increases at the frequency
where the standing wave occurs.

Figure 7 Cavitation test in bubbly
flow

Figure 8 Change in pressure fluctuation
distribution with and without air bubble layer

The propeller pressure fluctuation wasmeasured by multiple pressure sensors
embedded in the measuring plate immediately
above the propeller. Air bubbles flowed fromthe air bubble release device installed on the
upstream side.

These are contour figures of the distributions of the pressure
fluctuation propagating from the pressure source (red area)
where the pressure fluctuation is high.

The propeller pressure fluctuation reduction effect was researched while maintaining a
constant average void fraction in the air bubble layer and changing the air bubble layer thickness.
Typically, the air bubble layer between the propeller and the bottom of the hull tends to have ahigher void fraction when the layer is closer to the bottom of the hull. Therefore, with the average
void fraction kept constant, the void fraction distribution in the air bubble layer has a higher void

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fraction in the vicinity of the bottom of the hull when the air bubble layer thickness is thinner.
Figure 9 shows an example calculation result. This result represents no pressure fluctuation
reduction effect in the lower frequency region. In the higher frequency region, however, the thinner
the air bubble layer thickness becomes, the more the pressure fluctuation reduction effect increases.
For this reason, the condition for reducing the propeller pressure fluctuation is regarded as theestablishment of a thin air bubble layer with a high void fraction in the vicinity of the bottom of thehull. Assuming the development of a technology for controlling void fraction distribution at thelocation of the propeller, the disadvantageous risk of increasing propeller pressure fluctuation in aship equipped with MALS might become the advantage of reducing the pressure fluctuation of apropeller. Such technologies include controlling the air bubble flow by providing new air bubble
flow in the vicinity of the bottom of the hull from the upstream of the propeller or by collecting air
bubbles flowing along the bottom of the hull in front of the propeller and releasing them again in
the vicinity of the bottom of the hull on the upstream side of the propeller.

Figure 9 Change in pressure fluctuation of propeller depending on air
bubble layerThis shows the change in the pressure fluctuation of the propeller corresponding to thechange in the thickness of the air bubble layer between the propeller and the bottom of
the hull and the change in the propeller blade frequency.

|5. CFD-based prediction technology for gas-liquid separation in sea
chest
A sea chest is a cooling water suction port placed on the stern bottom. A ship equipped with
MALS runs the risk of a large amount of air bubbles flowing along the bottom of the hull being
drawn into the sea chest and then taken into the pump, resulting in the deterioration of the pump
performance and an adverse effect on engine cooling capacity. Therefore, the establishment of ashape and internal structure of a sea chest that can accurately separate gas from liquid is necessary.
For this reason, MHI prepared a design standard for the determination of the shape and internal
structure of a sea chest by utilizing the calculation results obtained from the simulation ofgas-liquid separation in a sea chest using CFD shown in Figure 10.
For the effective confirmation of the simulation of gas-liquid separation in a sea chest, theestimation accuracy of the prediction technology for gas-liquid separation in a sea chest wasverified through an experimental observation of the air bubble flow using a sea chest model.
Figure 11 gives example results of CFD calculation and experimental observation of the air bubble
flow. The flow taken into the sea chest moved up along the partition plate. There occurred
circulating flow at the top of the partition plate and air bubbles were separated from the liquid on afree-surface boundary between the gas phase and the liquid phase located on the circulating flow
path. The flow pattern and the flow velocity of the calculation results were roughly consistent withthose of the experimental results, and therefore this prediction technology was found to be effective
for the examination of the gas-liquid separation structure in a sea chest.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
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Figure 10 Simulation of gas-liquid
separation in sea chest

Figure 11 Flow velocity distribution and void
fraction distribution in sea chest

Air bubbles flowing along the bottom of thehull are drawn into the sea chest, where
gas-liquid separation occurs. This contour
figure of the void fraction shows the gas-liquid
separation.

This figure shows an experiment using a partial sea chestmodel and flow velocity distribution and void fraction
distribution in the sea chest obtained from a simulated
calculation result. The red line in the flow velocity distribution
indicates the circulating flow.

|6. Conclusion
For the development and design of MALS, which applies an air lubrication method that
reduces frictional drag by mixing air bubbles of millimeter order into the flow around the hull, theutilization of simulation technology is essential. A CFD-based prediction technology for air bubble
distribution around a hull and a prediction technology for the frictional drag reduction effect using
MHI’s own frictional drag reduction model are key technologies for the estimation of theenergy-saving effect of MALS. The estimation results are roughly consistent with the results ofactual ship tests, indicating that these simulation technologies are sufficiently applicable to design.
A prediction technology for the pressure fluctuation of a propeller that rotates in the bubbly flow
has the potential of turning the disadvantageous risk of increasing propeller pressure fluctuation ina ship equipped with MALS into the advantage of reducing the pressure fluctuation of a propeller.
This is assuming that a technology for controlling void fraction distribution at the location of thepropeller is developed in the future. A technology for gas-liquid separation in a sea chest can beutilized for the examination of the shapes and internal structures of a sea chest that are effective forthe prevention of air bubble intrusion into a cooling water pump. MHI will continue to utilize datafrom actual ships equipped with MALS in simulation technologies, aiming at the enhancement ofsimulation accuracy, further improvement in the performance of MALS, and the expansion of shiptypes to which MALS is applicable.

References
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2.
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Mizokami, S. et al., Experimental Study of Air Lubrication Method and Verification of Effects on Actual
Hull by Means of Sea Trial，Mitsubishi Heavy Industries Technical Review Vol. 47 No. 3 (2010)
pp.41-47
Mizokami, S. et al., Implementation of Ship Energy-Saving Operations with Mitsubishi Air Lubrication
System，Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (2013) pp.44-49
Kawakita, C., Estimation of Frictional Drag Reduction Effect for the Ship with Air Lubrication System,
Conference Proceedings the Japan Society of Naval Architects and Ocean Engineers (19) 2014A-JK-2
Kawakita, C., Study on the incerase or decrease of pressure fluctuation of marine propeller running inbubbly flow，Conference Proceedings the Japan Society of Naval Architects and Ocean Engineers (19)2014A-GS7-6