Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
63

CFD on Cavitation around Marine Propellers
with Energy-Saving Devices

CHIHARU KAWAKITA*1

REIKO TAKASHIMA*2

KEI SATO*2

Mitsubishi Heavy Industries, Ltd. (MHI) has developed energy-saving devices that improve
the propulsive performance and fuel consumption of ships, including reaction fins for low-speed
full ships and stator fins for high-speed slender ships. These devices have been installed on manyships. While designing these devices, we developed computational fluid dynamics (CFD)
technologies that analyze and evaluate the cavitation occurrence characteristics of propellers
equipped with energy-saving devices as a unit, including the hull and rudder. This enables adetailed understanding of the flow-field characteristics, such as pressure distributions around thepropeller, which require many hours of effort to analyze experimentally. The result is an improved
degree of optimization of propellers and energy-saving devices.

|1. Introduction
The world’s first unified regulation that controls CO2 emissions in the field of international
marine transportation and requires improved fuel consumption for newly built and existing ships
has been established. This regulation will be applied to ships for which building contracts areconcluded on January 1, 2013 or later and will be tightened in four steps. As a result, significant
reduction of CO2 emissions, i.e., significant improvement of fuel consumption, will be required forships built in the future.
Various energy-saving technologies have been developed because improved fuelconsumption contributes to economical efficiency for ship owners through cost reduction. We havedeveloped the Mitsubishi Heavy Industries, Ltd. (MHI) bow, which has an optimized shapedepending on the type of ships; this is a long-proven product that reduces the wave-making
resistance, one of several types of hull resistance. Also, we have developed the Mitsubishi Air
Lubrication System (MALS), which covers a hull with millibubbles to reduce the skin-friction
resistance; no other effective skin-friction resistance reduction method has been proposed for many
years, even though skin friction constitutes approximately 50 to 80% of the hull resistance. The
world’s first MALS-equipped newly built ship has attained an energy savings of approximately
10%.1
In contrast, various energy-saving technologies have been developed for propulsion
efficiency over many years. In response to recent demand for energy saving, different ship yards
have released installation samples of various energy-saving devices. We have employedenergy-saving devices that improve propulsive performance on actual ships since the 1980s. We
have developed reaction fins, which consist of multiple fins installed in front of the propeller toimprove efficiency through recovery of rotational flow, for low-speed full ships, such as oil tankers
and liquefied petroleum gas (LPG) carriers, and stator fins, which are installed on the rudder behindthe propeller, for high-speed slender ships such as car carriers. Various benefits have been obtained
due to these energy-saving devices.
Typically, when an energy-saving device is installed around the propeller, inflow to thepropeller will become more complex, making it difficult to predict the occurrence of cavitation. Ifcavitation occurs on a propeller, it causes problems such as performance decrement, erosion, and*1 Research Manager, Nagasaki Research & Development Center, Technology & Innovation Headquarters*2 Nagasaki Research & Development Center, Technology & Innovation Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)64

noise. Thus, when designing or evaluating a propeller equipped with an energy-saving device, it isnecessary to adequately examine and verify its cavitation characteristics in consideration of such adevice. Theoretical methods or model experiments have been used to predict the occurrence ofcavitation on a propeller. However, theoretical methods require various assumptions and are lessreliable when there are significant changes in the flow conditions, while model experiments are
more expensive and require time for model building, especially when multiple models are required.
To eliminate these disadvantages, we used computational fluid dynamics (CFD) to estimate
the occurrence of cavitation by performing a flow analysis around a propeller that rotates at the rearof a hull, while considering the hull, rudder, and energy-saving device simultaneously. By using theresults of these analyses to design new propellers, we have developedenergy-saving-device-equipped propellers that are reliable and efficient and have a low risk oferosion occurrence.

|2. Principle of energy-saving devices
Energy-saving devices are based on energy-saving technologies and are installed around apropeller to improve propulsion efficiency. Typically, energy-saving devices recover the wasteenergy of a propelled ship in some way and are classified depending on the recovered energy asfollows.
(a) Devices that recover propeller rotational flow(b) Devices that reduce the propeller load(c) Devices that diffuse the propeller hub vortex
The reaction and stator fins described above are included in (a). We have also developed andmade practical the Mitsubishi Advanced Propeller (MAP), which is a low-revolution
large-diameter propeller included in (b), and the Hub Vortex Free Cap (HVFC), which is includedin (c).
Reaction fins, which consist of multiple fins installed in front of a propeller, as shown inFigure 1, swirl inflow to the propeller in the opposite direction of the propeller rotation. Stator fins,which are installed behind a propeller, cancel the rotational flow generated by the propeller. A
reaction fin itself creates resistance; thus, it is applied to low-speed full ships, for which theresistance is small compared to the effect of rotational flow recovery because of the slower flow
speed at the stern. Reaction fins are not applied to high-speed slender ships, for which theresistance is large because of the higher flow speed. A stator fin itself produces thrust; thus,energy-saving effects on high-speed slender ships can be expected.

Figure 1

Principle of reaction and stator fins

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
65|3. Unified CFD analysis of hull, propeller, and energy-saving
devices
The governing equations used for our CFD analyses consisted of the incompressible
Navier–Stokes equations and continuity equation and were discretized through a finite-volume
method using an unstructured grid. The computational grid included areas fixed to a defined space(hull, rudder, and energy-saving devices) and an area fixed to the rotating propeller. At thejunctions, a sliding mesh method that fulfills continuity and conservation of physical values wasused. A two-equation turbulent flow model closed the set of equations.2 The cavitation model wasbased on the Rayleigh–Plesset equation, which describes the motion of a single bubble.3
In order to conform to the conditions tested at the large cavitation tunnel (HYKAT) at theGerman experimental tank institute HSVA (Hamburgische Schiffbau Versuchsanstalt GmbH), thecalculation targets, i.e., the hull, propeller, rudder, and reaction fins, were placed in a domain that
replicates the measuring points in the cavitation tunnel, as shown in Figure 2.

Figure 2 Calculation grid used to analyze the hull, propeller, and reaction
fins as a unit
Reproduction of experiments in a large cavitation tunnel (HYKAT, Germany) by numericalcalculations.

|4. Examples of CFD application to energy-saving-device-equipped
propellers
Application examples of the unified CFD analysis are shown below.

4.1

Reaction fins

Figure 3 shows the predicted inflow to the propeller for cases with and without reaction fins.
The results show that reaction fins generate flow in the opposite direction to the propeller rotation.
The fin-mounting angle can be optimized based on these results.

Figure 3 Flow-field predictions of the propeller surface inflow with andwithout reaction fins

The velocity distribution in the mainstream direction is shown as contours, where blue
indicates a higher velocity, and the rotational flow is shown as vectors. The flow field without
reaction fins is symmetrical. However, the flow field with reaction fins has rotational flow inthe opposite direction to the propeller rotation (clockwise), generated by the reaction fins.

Figure 4 shows the cavitation calculations for two different propellers, A and B, rotating in
the flow field shown in Figure 3. For propeller A, sheet cavitation occurs on the suction side (bowside), and face cavitation occurs on the pressure side (stern side). Typically, a propeller is designed
to prevent face cavitation because when it occurs, cavitation is eliminated on the blade surface,
increasing the risk of cavitation erosion. Propeller B prevents the occurrence of face cavitation andhas a low risk of cavitation erosion.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)66

Figure 5 compares the cavitation occurrence area of propeller B obtained from CFD with adrawing based on experimental data. The calculated sheet cavitation occurrence area on the backsurface was almost identical to the experimental findings. Also, the calculations reproduced thesheet cavitation caught in the tip vortex cavitation.

Figure 4 Comparison of cavitation occurrence for propellers
A and B
Propeller A has face cavitation. Propeller B has no face cavitationand has a low risk of cavitation erosion.

Figure 5 Comparison of cavitation
occurrence areas on the blade(back surface) of propeller B
The calculated cavitation occurrence
area is almost identical to theexperimental results.

These results demonstrate that the unified CFD analysis enables prediction of cavitation
generated by a propeller equipped with reaction fins, which previously had to be checked byexperimentation because of the complex inflow to the propeller. The unified CFD analysis iseffective for designing a reliable high-performance propeller without face cavitation and has
already been utilized for one installed on tankers.

4.2

Stator fins

Figure 6 shows the CFD results for a propeller equipped with stator fins. This figure shows
the surface pressure distribution obtained from the analysis of the hull, propeller, rudder, and stator
fins as a unit. The surface pressure distribution of the propeller blades, stator fins, and rudder
followed the same trends. There was a decrease in pressure on the negative pressure side of thestator fins and an increase of pressure at the leading edge of the rudder. Based on the predicted flow
field and pressure distribution around the stator fins, a highly efficient stator fin-equipped propeller
can be designed through optimization of the number and mounting angle of the stator fins installedbehind the propeller.

Figure 6 Pressure distribution around a propeller equipped withstator fins
Red indicates high pressure and blue indicates low pressure.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)67|5. Conclusion
The significance of reducing the energy consumption of ships will increase in response to theworld’s first unified regulation that controls CO2 emissions for international marine transportationand inflating oil prices. We will continue to develop fuel-efficient and highly reliable ships
designed using CFD-based performance-prediction tools. As an example, this paper introduced
CFD technology that predicts cavitation occurrence on propellers equipped with an energy-saving
device by analyzing the hull, propeller, rudder, and energy-saving device as a unit. By using thismethod, useful information, such as the flow field and pressure distribution around a propeller
under actual operating conditions, can be shown visually to designers; these data are difficult tomeasure experimentally. This will allow us to improve the optimization of propellers andenergy-saving devices.
We expect that CFD technologies will improve with time, and the application scope willexpand. In order to establish CFD as a reliable design tool, it is necessary to verify the accuracy ofthe predictions and check the application scope based on comparison with detailed experimental
data in every aspect. We plan to continue contributing to the development of fuel-efficient ships byimproving CFD technologies while increasing our technological level, without, of course,
neglecting our experimental measurement technologies.

References
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2.

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Kawakita, C. et al., Full Scale Bubbly Flow on The Ship Bottom and Effect of Frictional Drag Reduction
for The Ship with Air Lubrication System, Conference proceedings, the Japan Society of Naval Architects
and Ocean Engineers, Vol.12（2011）p.429-432
Kawakita, C. et al., Prediction of Unsteady Cavitation on Propeller in Consideration of Hull and Rudder
Using CFD, Conference proceedings, the Japan Society of Naval Architects and Ocean Engineers, Vol.13（2011）p.35-38
Sato, K. et al., Numerical Prediction of Cavitation and Pressure Fluctuation around Marine Propeller，Proceedings of the 7th International Symposium on Cavitation (2009)