Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)
53Development of Technology to Estimate
the Flow Field around Ship Hull Considering
Wave Making and Propeller Rotating Effects

MAKOTO KAWABUCHI*1

MASAYA KUBOTA*1SATORU ISHIKAWA*2

As can be seen from the application of energy efficiency design index (EEDI)-relatedregulations the enhancement of propulsive performance in the design phase of merchant ships havebeen increasing in recent years. Mitsubishi Heavy Industries, Ltd. (MHI) develops hull forms
precisely and efficiently using enhanced CFD technologies. The company improved the estimation
accuracy of flow fields around the vessel by eliminating simplification of models as much as
possible, by reproducing conditions close to the actual phenomenon. This paper presents thedeveloped CFD tool and examples of its application to product development.

|1. Introduction
The common practice in evaluating the propulsive performance of a merchant vessel is toseparate it into the hull resistance, the self-propulsion factor. Self-propulsion factor is theinterference effect between the hull and propeller and the propeller open water characteristics.
These parameters can be obtained by resistance tests, self-propulsion tests and propeller open-water
tests, respectively. These performance parameters vary while interacting with each other according
to changes in the shape of the hull, the appendages and the propeller. It is necessary for thedevelopment of a vessel shape superior in energy-saving performance to accurately evaluate theresistance, the self-propulsion factor, and the propeller open water characteristics to determine
whether the performance is good or bad.
Improvement in the estimation accuracy of numerical parameters related to propulsive
performance requires enhancement in the estimation accuracy of flow fields. Accordingly, MHI
utilized an ultrahigh-speed mesh generator1 to precisely reproduce the shape of an analysis object
and developed large-scale analysis technology using meshes with the mesh density required for theprecise reproduction of the phenomenon occurring around the hull.
We developed an analysis technique with higher versatility and higher estimation accuracy offlow fields compared to the conventional analysis technique2 by improving the self-propulsion
analysis technique|2. Establishment of analysis technique
2.1Summary of conventional technique

The conventional analysis technique focuses on the analysis time and the improvement of thestability and uses, particularly in the estimation of self-propulsion factors. The simplification of theanalysis model regarding the two phenomena of a free surface effect and a propeller rotation effectas shown in Table 1. Specifically, a two phase flow model that assumes the free surface effect tobe small and gives a vertical symmetric condition to the boundary corresponding to a still waterinterface is adopted. The propeller model simulates the flow generated from the propeller by
providing the effect of the propeller rotation and the suction flow calculated by the UQCM
(unsteady quasi-continuous method) as a body force into the disc region defined in the calculatingarea.
*1 Fluid Dynamics Research Department, Research & Innovation Center*2 Senior Manager, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)
54

Table 1Treatment of stern shape
Grid type
Number of grids
Propeller model inself-propulsion calculation
Handling of free surface inself-navigation calculation
Turbulence model
Number of parallel CPUs

2.2

Comparison of analysis techniques
Conventional CFD
Complex shape isreproduced precisely
Unstructured hexahedral grid
Several million to 10 million
Body force model
based on UQCM

Latest CFD
Complex shape isreproduced precisely
Unstructured hexahedral grid
25 million to 50 million
Sliding meshbased on actual propeller geometry

Still water interface

Interface capturing (VOF)

SST k-ω (two equations)
16 to 64

SST k-ω (two equations)
64 to 256

Improvement of analysis technique

We tried to improve the analysis accuracy by precisely modeling phenomena that aresimplified by the conventional techniques. In other words, the analysis of a free surface using a
VOF (volume of fluid) model in self-propulsion analysis, reproduction of the blade shape and amodel for rotating the propeller shape using sliding mesh methods are adopted. Figure 1 shows amesh arrangement of the analysis grid around the propeller for a Japan Bulk Carrier (hereinafterreferred to as JBC), which is a bulk carrier for which the vessel shape and tank test results arepublically available. It is known that three-dimensional separation of vortexes occur at the stern ofa full ship such as a bulk carrier. The mesh is subdivided in order to capture the inception of thevortex separation and suppress the numerical attenuation of the vortexes. The mesh refinement isprovided for the area covering distances from the central part of the ship (midship) to the stern end
in the longitudinal direction, half of the vessel along the width and from the Base Line of the vesselto the still water interface in the height direction.
Figure 2 shows the isosurface of the second invariant of the velocity gradient tensor.3 This
figure indicates that the inception of vortexes occurs in the refined mesh area and that the vortexes
flow into the propeller surface without attenuation.

Figure 1 Mesh arrangement around JBC
stern

Figure 2 Isosurface of the second invariantof the velocity gradient tensor for JBC

Figure 3 compares the flow rate contours in the direction of the main flow field to thepropeller surface without the propeller between the flow field PIV (Particle Image Velocimetry)measurement results in the conventional analysis technique and the improved analysis techniquefor JBC. This figure indicates that the improved analysis results are closer to the experimental
values than the conventional analysis results, such as the distribution profile of the low-speed
region shown around the width direction coordinate of -0.01 and the height direction coordinate of-0.04, and the distribution of the low-speed region shown around the width direction coordinate of-0.01 to 0 and the height direction coordinate of -0.05 to -0.04.
Figure 4 compares the estimation results of self-propulsion factors in the analysis of JBC.
The self-propulsion factor parameters, 1-t (where t is the thrust deduction factor(Note 1)), 1-wm
(where wm is the wake fraction t(Note 2)), and er (relative rotative efficiency(Note 3)) are compared.
This figure indicates that the estimation accuracy of self-propulsion factors has increased
significantly in comparison with the conventional technique due to the improvement of the flow
field estimation accuracy and the enhancement of the propeller model.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)55
Note 1: The ratio of difference between the propeller thrust in the self-propulsion of a ship at a certain
speed and the hull resistances occurring in the towing of the same ship at the same speed against
the propeller thrust
Note 2: The ratio of difference between the inflow velocity to the propeller and the ship advancing speed
against the ship advancing speed
Note 3: The ratio of the propeller efficiency behind the ship, which is the efficiency of the propeller
operating in a non-uniform flow field at the stern against the propeller open-water characteristics ,which is the efficiency of the propeller operating in a uniform flowFigure 3

Comparison of wake fraction in the direction of main flow on propeller surface

Figure 4 Comparison ofself-propulsion factor obtained from experiments andCFD(new and old models)|3. Application examples of analysis technique
MHI utilizes the analysis technique for which the self-propulsion factor analysis accuracy
has been enhanced in comparison with the conventional techniques for the development of a hull
form and additional appendages around the propeller. This chapter presents two examples.

3.1

Improvement of shape of rudder, propeller and energy-saving device

MHI uses highly accurate flow field estimation results to apply a new analysis technique toimprove the shape of the propeller, rudder and energy-saving devices. The accuracy of theestimation is strongly influenced by the flow generated by the propeller for designing the crosssectional shape and the installation angle of the rudder and energy-saving devices.
Figure 5 shows unsteady integrative analysis situations where the shapes of the rudder,energy-saving device and a propeller are reproduced. Based on the highly accurate analysis ofinterference between wakes from the hull and flows at the energy-saving device, the propeller andthe rudder our development does not aim the individual optimization for maximizing each effect,
but overall optimization of the propulsion system including the hull, device, propeller and rudder.
As an example of such improvement, Figure 6 shows an analysis case where the blade angle of anMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)56

energy-saving device was changed. Before the improvement, there was separation at the suction
side of the blade that was assumed to cause the increased device resistance. After the improvement,however, no separation occurs on the suction side or the pressure side of the blade at the sametime low resistance and a higher device impact can be expected.

Figure 5

Integrative analysis situation of rudder, energy-saving device and propeller

Left figure: Arrangement of energy-saving device, propeller rudder assembly and limiting flowline of object surface
Right figure: Indication of flow line within the space at the stern and a check of the flow field
improvement effect of the energy-saving device

Figure 6 Comparison of flow on energy-saving device blade surface before
and after changing the blade angle

3.2

Consideration of submergence at transom stern

Due to the precise reproduction of the stern shape and the increased grid resolution close to
the free surface wave making at the stern, which is important for the reduction of hull resistancecan be improved using CFD analysis.
For ensuring the stability of container carriers and other ship types that require higher
stability performance at a relatively high speed, it is necessary to enlarge the water plane area.
Figure 7 shows an example case where wave making was improved by lowering the transom sternheight. After the improvement the transom stern is set at a lower height, the wave upsurge justbehind the transom stern is reduced and a broader water plane area can be kept.

Before improvement
Transom stern height: High; Submergence: Low

Figure 7

After improvement
Transom stern height: Low; Submergence: High

Change in wave height by changing height of transom stern

The stern wave height can be controlled by adjusting the height (submergence) and the sectional shape of thetransom stern.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)
57|4. Conclusion
MHI improved the analysis accuracy considering the free surface effect and propeller
rotation effect in the self-propulsion condition of a ship by using an analysis model that reproduces
the actual flow field accurately. In addition to enhancement in the accuracy of the optimum designof the hull itself, efficient improvement in the shape of appendages such as the rudder orenergy-saving devices around the propeller is also made possible. This leads to the reduction of thenumber of tank tests. It also became possible to consider the unsteadiness caused by the rotation ofthe propeller. In the future, we will utilize these technologies to develop devices and rudders withgreater energy-saving effects in terms of robustness against changes in the flow field.

References
1.

Tanaka, M. et al., Product Innovation by applying the Latest Technology of Numerical Simulations,
Mitsubishi Juko Giho Vol. 48 No. 1 (2011)
2. Nishigaki, M. et al., Application of Advanced CFD Technology to Energy-Saving Hull form Development，Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (2013)
3. Hino, T. et al, FLOW SIMULATIONS OF JAPAN BULK CARRIER USING OVERSET GRIDAPPROACH, Tokyo 2015 A Workshop on CFD in Ship Hydrodynamics, Proceedings, Volume III, (2015)，
P. III-443-III-447