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OCR for page 80
TOWARD VIRTUAL REALITY
BY COMPUTATIONAL PHYSICS
H.Miyata (University of Tokyo, Tokyo)
Abstract
Twenty years' research works by the
author and his colleagues at the University of
Tokyo are reviewed and it is described and
discussed that the research is moving toward
virtual reality technology by use of
computational physics, especially
computational fluid dynamics techniques.
Computer technology is first utilized for the
purpose of elucidating physical phenomena,
especially nonlinear ones such as turbulence
and shock waves and then for the purpose of
designing or inventing new systems including
ships.
1. Introduction
Naval architecture plays one of the most
important parts of moving technology and has
made great progress in the 20th century. In the
last 20 years of the 20th century the computer
technology had accelerated the progress of
industrialized world and it had been also true
in the field of navel hydrodynamics. The
computer has become 10,000 times faster or
cheaper in the previous 15 years. The ability of
a supercomputer in 1985 is that of a personal
computer, which we can purchase with 2000
US dollars in 2000.
The purpose of engineering research may
be classified into two, that is, to elucidate
physical phenomena that influences the
mechanism of engineering products and to
invent some new system of advanced
availability.
In the field of naval hydrodynamics we
had two major physical phenomena to which
elucidation of the structure and mechanism
was of substantial importance. They are
nonlinear waves including wave breaking and
turbulent flow including large-scale separated
flow. Since both are typically nonlinear
phenomena, the theoretical fluid dynamics
could not make important contribution. And
noticeable attack was started at the end of
1970's. A great amount of efforts have been
devoted for the elucidation of such physical
phenomena especially in the flow about a
ship hull. Although experimental work has
made important contribution, the technology of
computational fluid dynamics has made much
more important contribution with the aid of
advanced computer technology.
After significant progress was made for the
elucidation of physical phenomena the
technology of CFD has been used to facilitate
design of ships hull-form and other artifacts.
At the beginning the purpose was to develop a
numerical towing tank. This was achieved by
solving the steady flow about a ship with free-
surface. In parallel with the advance of
computer technology this technique has been
extended to the simulation of ship maneuvering
and motion in waves. The CFD solution is
combined with the solution of the equation of
motion. Since arbitrary sea condition can be
realized in the numerical towing tank, the
motion simulation technique can be developed
to a more advanced technology of realizing all
motions of a ship in actual sea conditions.
It may be safe to say that we are working
towards a virtual reality technique based on
computational physics. All attitudes, forces and
moments as well as all physical phenomena are
realized in the computer with a simulation
system composed of our application software.
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Representative terms from entire chapter:
physical phenomena
In this article 20 years trend, from the
elucidation of physical phenomena to the
challenge for the realization of a kind of virtual
reality technique, is described with the outputs
I have derived with coworkers from 1979.
2. Free-surface shock wave
In the last two years of five years
experience as a designer at the Ship Initial
Design Office of IHI I was in charge of the hull-
form design of FUTURE-32 standard design
bulk carrier (32,000DWT). The speed
requirement was very severe and the fulfillment
of the guaranteed trial speed written on the
specification was supposed to be marginal.
However, the trial result of the first ship was
quite contrary. She was faster than the
guaranteed speed by 0.6 knot on the ballast
condition, which means the propulsion
horsepower was 15% excessive. The 6 cylinder
engine would have been installed instead of the
7 cylinder one.
This experience gave me a motivation for
the research of nonlinear ship waves. The cause
of such discrepancy between experiment and
trial results was supposed to consist in physical
phenomenon of ship waves, partially because
the discrepancy is much more prominent on the
ballast condition with breaking wave
phenomenon. Immediately after the delivery of
two FUTURE-32 ships, I was invited to the
University of Tokyo, where one of the prominent
works there was wave pattern picture analysis
to know the wave system. My research work
for nonlinear ship wave was started with the
observation of ship wave pattern pictures. The
first insight was an analogy to the shock wave.
The nonlinearity of the bow wave
phenomenon was exaggerated by a model ship
with an extreme bow shape as shown in Fig.1
t13~23~3~.
Fig.1 Free-surface shock wave about a wall-
sided model with a blunt bow, at
En = 0.12, 0.16, 0.24, 0.26.
Contrary to the theory of linear dispersive wave
the wave pattern is dependent on Froude
number and hull-form configuration. The
variation of the apex angle of bow eve is rather
systematic as shown in Fig.2 and the steepness
of the wave slope obviously exceeds the
limit of the linear wave system.
Fig.2 Free-surface shock wave about series
models of various half-entrance
angle 5. <,10- <,15.
This nonlinear wave system appeared in the
near-field of a ship advancing in deep water
was named "Free-surface shock waves". The
typical characteristics are 1) the formation of
lines of discontinuity, 2) the steepness and
unsteadiness of wave front, 3) the satisfaction
of shock condition, 4) the systematical change
of the angle of wave front, 5) the nondispersive
propagation and 6) the dissipation of wave
energy into momentum less far behind. These
ware clarified through a series of physical
experiment t4], see Fig.3.
Linear dispersive wave is observed in the
far-field but nonlinear waves are prominent.
Both linear dispersive and nonlinear dissipative
phenomena coexist.
3. Thin, long-protrudent bulb
Analytical explanation of this nonlinear
phenomenon was supposed to be made only by
numerical analysis technique. Special
computational method must be developed for
such objectives, which needs remarkable time.
However, sound understanding of the physical
phenomenon by itself can be useful to the hull-
form design. The free-surface shock wave
(FSSW) is ruled by the Froude number based
on draft (or equivalent draft). Therefore, when
the Froude number based on draft exceeds 1.0,
the FSSW becomes dominant with strongly
interacting breaking wave. For the reduction
of FSSW at high Froude numbers the entrance
angle at the bow should be small. With the
decrease of the bow apex angle the angle of
shock decreases and consequently the wave
resistance value is reduced. This design
criterion is simple and straightforward as well
as analogical to the design criterion for a
supersonic body. This idea was first applied to
the hull-form of "USUKI PIONEER"
(26,000D WT bulk carrier) designed in 1983,
Fig.4, which was equipped with two sails for
the fuel saving purpose.
Fig.3
Results of detailed wave profile
measurement, wedge model of half-
entrance angle 20° , Fd~draft-
based)= 1. 1.
Fig.4 "USUKI PIONEER" 26000 D WT bulk
carrier the first ship designed with
a thin, long-protrudent bulb in
1983.
Thin, long-protrudent bulb could reduce wave
resistance by 10 to 20% in comparison with a
design with a conventional bulbous bow. In the
1980's this design was disseminated in the
world. The thin, long-protrudent bulb is
especially useful to ships of which service
Froude number is smaller than 0.22.
Nonlinear waves like FSSW also occur
near the stern, especially that of high-speed,
ships as shown in Fig.5, which is the case of
Sutorechia-maru passenger boat (Lpp=lOOm,
Fn=0,30~. A similar idea with the thin, long-
protrudenbt bulb is also effective for the
reduction of nonlinear stern waves. Due to
many reasons and restrictions the installation
position of such stern bulb is near the waterline
as shown in Fig.6. About 5% of energy saving is
attained by this Stern-End-Bulb (SEB) and
applied mostly to car carriers, truck ferries and
container carriers for which the draft at the
stern does not remarkably varyt5~6~.
Fig.5 Wave picture from airplane, En ·a
0.30, nonlinearity of waves is
observed especially around the stern.
Fig.6 Stern-End-Bulb for a car ferry.
4. TUMMAC
The research work to develop a finite-
difference method for the computation of the
flow with free-surface about a ship in steady
course was started in 1979 and four years was
needed until the TUMMAC-tW method was
completed in 1983 t7] t83.
The key algorithm is based on the MAC-
method and the technique to implement the
nonlinear free-surface condition is an extension
from the SUMMAC-method. Most efforts were
focused on the implementation of the body-
boundary condition in the framework of the
rectangular grid system. The free-slip condition
could be successfully satisfied in the body-
boundary cells of arbitrary configuration. The
computed wave pattern agreed very well with
the measured result, especially when wave
breaking is scarcely observed. The comparison
of wave pattern is made of a tanker hull on
ballast condition in Fig.7, which indicates that
some discrepancy is present and it is mostly
attributable to the fact that the TUMMAC-tW
method cannot cope with the breaking wave.
However, the agreement was of the satisfactory
level and was also applied to the diffraction
wave problem of a wedge-shaped bow model and
a tanker bow shape. The diffraction wave shows
phase-dependent variation of the FSSW
formation as shown in Fig.8~9~.
Fig.7 Wave contours about a bow of a tanker
model at ballast condition, measurement
(above) and computed results by
TUMMA C- tw code (below).
Fig.8 Computed wave view and unsteady
free-surface shock wave in regular
incident waves.
Because the TUMMAC-twmethod has
the advantages that the effort of grid generation
is not required and that the accuracy is of
satisfactory level, the code was distributed to
major shipbuilding companies and employed as
a tool of hull-form design. The wave resistance
value obtained by integrating the surface
pressure distribution dose not qualitatively
coincide with the experimental value. But, hull-
form designers soon noted that the accuracy
was good in the relative relation of wave
resistance value due to the modification of hull-
form. It is often the case that the hull-form is
designed by the succession of improvement.
This code is extended to the version for
catamaran and the advantage of small wave
dissipation in the far-field is available for the
wake-wash problem. An example of wave
pattern of a catamaran is shown in Fig.9.
5 — ~ computed by TM4DFM modeled "non, 2001
_ win ~e god da. generated by ~ new cell generator, eal200 1 )
0.5
O
0 1
X
A - mmatrical hull Neumann be - C on SSTH70NS1300
beam ins~.helr outelddar- 15 10
atFn-0646
wave height {Intents 0 0025)
K-15
2
Fig.9 Wave contours of a catamaran fast
ship computed by TUMMAC- IV.
5. Wave breaking simulation
One of the most nonlinear fluid-
dynamical problems is wave breaking. Since the
CFD technology is based on the Navier-Stokes
equation, it can cope with any problem of high
nonlinearity, although the resolution of the
micro-mechanics and very fast phenomenon
necessitate top-end ability and capacity of
computer.
2D wave breaking simulation was
achieved with relatively small amount of efforts.
The deforming free-surface configuration is
represented by a succession of segments and
then not only overturning motion but also
impingement of the wave front on the forward
free-surface below is well simulated as show in
Fig.10 t111.
One application example of the 2D wave
breaking simulation is a problem of a circular
cylinder placed horizontally in the vicinity of
the free-surface. The complicated flow field of
vortices strongly interacting with the free-
surface is realistically simulated as show in
Fig.11 L121
Fig.10 Simulation of 2D wave breaking in
front of a steadily advancing
rectangular body.
Fig.11 Contours of vorticity about a circular
cylinder set horizontally and
advancing steadily near the free-
surface.
Quite different technique was necessary
to achieve 3D wave breaking simulation. The
water surface of three-dimensionally breaking
motion cannot be treated by a succession of
segment. Therefore, based on the idea of the
VOF (volume-of-fluid) method a new technique
was developed to implement free-surface
condition on an interface of complicated
topology. We called it density-function method,
for which it was revealed later that a similar
method called level-set method was being
developed almost simultaneously.
The first application of the TUMMAC
method with the density-function technique on
the free-surface was a flow about a vertically
placed rectangular cylinder t13~. When Froude
number is high the fluid flow is completely
nonlinear with wave breaking, vortex shedding,
spray and air-entrainment, Fig.12. The air flow
above the water surface is simultaneously
computed and some features of spray and air-
entrainment are observed as in Figl3. When
constant values of physical parameter such as
density, Reynolds number and so on are varied,
a variety of two-phase flow can be treated, such
as an oil flow on water layer, a water flow on a
liquefied sand bed, that is scoring problem, and
so forth.
Fig.12 Contours of wave height about a
vertical cylinder (0.lm long set in
a uniform stream 0.9 second after
start of acceleration.
Fig.13 Side view of Fig.12, on the
longitudinal plane on the side of
the pillar.
The density function technique was later
employed in another code of finite-volume
method for viscous and wave flow problem in
the framework of boundary-fitted curvilinear
coordinate system. One successful simulation
result is show in Fig. 14 for a bow wave problem
of a semi-planning boat t141. The 3D breaking
waves are well realized showing the difference
of wave formation depending on the Froude
number. Some features of FSSW about a wedge Fig 16
model were also simulated with the same free-
surface condition in t15] t16], and discontinuous
and dissipative features are realized as shown
in Figs.14 and 15.
Fig.14 Overturning wave features
simulated with the density-function
method, Fn=1.0 (above) and 0.6
(below).
Fig.15 Velocity vectors on the free-surface; g
discontinuity of flow is present on
the free-surface.
Bernoulli constant value across the
bow wave crest on the lines parallel
to the centerline and the disturbed
free-surface.
Fig.17 Simulation of continuous casting of
iron plate production process.
Vorticity contours of the flow in the
Sagami bay influenced by the
Kurosio current computed by
TUMMAC- v! code.
Since relatively smaller effort is required
for the grid generation the TUMMAC method
is applied to other problems, which includes
complicated 3D configuration. The TUMMAC-
tW technique can be applied to a variety of fluid
flow problem which contain free-surface and a
body of 3D configuration in any engineering
field. One example is an application to the
continuous casting process of steel plate
manufacturing as shown in Fig.17. The
complicated flow field made of liquid iron jet,
3D vorticities and free-surface wave are well
simulated t101. The versatility of ()FD technique
is thus demonstrated. A case of oceanographical
flow near a Sagami bay is shown in Figl8. It is
noted that the ocean current of Pacific ocean
(Kuroshio) often causes 3D flow field at the
interface with the bay.
Similar two-phase flow technique with
density function treatment was recently applied
to a flow with bubbles L171. 108 micro bubbles
are placed in the turbulent boundary layer and
the mechanism to reduce frictional resistance
was investigated by numerical simulation as
show in Fig.19. This application indicates that
such CFD technique with density function
method in the framework of rectangular grid
system has broad possibility of application to
various environmental problems.
Fig.19 Air bubbles in a turbulent boundary
layer with vorticity contours of the
How.
6. Separated flow
The problem of turbulent flow is of global
importance and the turbulent flow is often
accompanied by flow separation. In the
problems of moving body the emphasis is put
on not only the fluid motion in the boundary
layer but also that in the wake region. In the
problems of naval hydrodynamics such viscous
fluid flow region is also connected to the moving
interface.
Since a turbulent flow is intrinsically of
time-dependent nature, the time-marching
simulation method such as that of the MAC
method must be employed. First method is
developed in 1985 for the solution of viscous
flow with free-surface in the framework of a
finite-difference method with a curvilinear,
boundary-fitted coordinate system [181. The
viscous fluid motion at the stern of a ship is
accompanied by separation phenomenon and a
part of this structure is called wake, which is
important for the propulsion efficiency. First
output of such effort appeared in 1987 by a
finite-difference method as shown in Fig.20.
Fig.20 Vorticity contours about a Wigly hull
advancing steadily, wave and
viscous flow are simultaneously
solved.
It was soon noted that the finite-
volume method is more suitable for this
problem mostly from the viewpoint of
robustness. A number of researchers attacked
this problem with special efforts of choosing and
tuning the turbulence model. It is the consensus
we have that the grid number is still insufficient
for such high-Reynolds number flow with free-
surface and some tuning or compromise is
required in the choice of grid spacing and
turbulence model. However, the simulation of
viscous flow about a ship can be successfully
used for practical design purpose.
The mechanism of flow separation and
the structure of a separated flow about a more
blunt body were not well elucidated. The
separated flows about a circular cylinder and
that about a sphere were important target of
scientific research. Because they are 3D and
unsteady phenomena, experimental approach
found substantial difficulties. From rather
scientific viewpoint the separated flows about
a sphere and a body of revolution with conical
after body are studied by use of TUMMAC
method and a finite-volume method,
respectively. The computer-graphic pictures of
the flow are shown in Figs.21, 22 and 23~19~20~.
Fig.21 Isosurfaces of second derivative of
pressure of a flow past a sphere.
Fig.22 Isosurfaces of pressure at the after
- end of a body of revolution with
conical afterbody.
Fig.23 Same as Fig.22.
The most interesting thing is that the structure
of a separated flow contains some common
features independent of the object configuration
and furthermore some part is also common to
the structure of a turbulent boundary layer. The
separated flow past a blunt body is composed
of ring-shaped vortices with mostly by lateral
vorticity component and horseshoe vortices
with mostly longitudinal vorticity component.
These two types of vortices are periodically
shed. The presence of horseshoe-type vortex is
quite common to all turbulent flow.
One application of the simulation of
separated flow is the case of automobile ~21~.
With the understanding that small-scale
vortices are not resolved due to the limited
number of grid points numerical tests were
conducted whether the method, which resolves
only middle-scale vortices (5% of automobile
length and greater), can discriminate
automobile configuration of smaller drag
coefficient. Since grossly 80% of aerodynamical
drag is by the pressure distribution made by
the time-dependent vortex shedding behind the
body, the resolution of the separated from field
is important, see Fig.24.
Fig.24 Streamlines behind an automobile
model with critical geometry.
The relative relation between the
estimated drag by CFD and the measured drag
for some series of automobile configuration was
very good. Furthermore the lift was well
estimated with same level of accuracy and the
similar technique was successfully applied to
the aerodynamical noise problem, in which it
is elucidated that one of the causes of wind noise
is breakdown of vortices ~22~. The time-marching
technique of the MAC-type algorithm provides
good resolution of the periodically- repeated
viscous phenomenon of vortex shedding. It may
be said that the accordance with physics is, in a
sense, better for the separated flow of thick
boundary layer and it is a little more difficult
and sensitive to have good agreement in the case
of thinner boundary layer accompanied with
relatively gentle separation as the case of ship
flow.
7. Moving technology
The moving technology is to complete a
moving system by implementing factors of
substantial importance, they are, support and
structure, power and speed, stability and control.
Overall performance is given by synthesizing
these factors. In the case of ships for sea
transportation the weight is hydrodynamically
supported, the power is transmitted to the
hydrodynamical propulsion system and the total
performance including stability, speed, ride-
comfort, maneuverability, are mostly determined
by hydrodynamical mechanism. Therefore the
total system is hydrodynamically designed.
Due to the progress of CFD both wave and
viscous fluid flow are well elucidated and
resultant forces can be utilized in the design
process. The simulation of
a flow about a ship advancing in steady straight
course provides better hull form of smaller
resistance and when a model for propulsion
system is introduced the propulsion performance
can be estimated.
A new challenge was pursued in the latter
part of 1990's towards the technique to cope
with maneuvering and motion simulation. This
can be achieved by combining the CFD
technique with the solution of the equation of
motion. The forces and moments are obtained
by integrating the surface pressure and they
are put into the equations of motion. For the
introduction of the motion into the CFD
computation two methods are supposed to be
most successful. One is a method in which the
grid system is fixed to the ship and the grid
system moves giving accelerations due to
motions to the Navier-Stokes equation as
external forces. Another method is to use the
moving grid system, in which the grids are
deformed in accordance with the ship motion
in six degrees of freedom.
For more complicated system like
catamaran or hydrofoils these method was still
useless within the 20th century, when we worked
hard to develop new ship systems operating at
high-speed, one is super-slender twin-hull
(SSTH) system (Fig.25) and the hydrofoil
catamaran (HC) system in the 1990's(Fig26~.
Fig.25 Fast catamaran ferry with super-
slender twin-hull (SSTH).
Fig.26 Experiment ship "Exceller" for
hydrofoil catamaran system.
8. Racing yacht design
When the author was assigned to be a
technical director and chief designer of Nippon
Challenge for the 30th Americas Cup yacht race,
I decided to fully utilize the CFD techniques
for the development and design, Fig.27. The
experience of sailing boat design is poor in
Japan and this must be compensated by the
information technology especially CFD. In
parallel with the formation of a strong design
team a new system of digital design by CAD
and CFD is developed for the special sailing boat
as shown in Fig.28. The design system for the
international Americas Cup Class sailing boat
was composed with the newest CAD and CFD
softwares. Each element of the boat. that is.
.. . . ... . .. . . .
, ,
sails, keel with bulb and rudder are modeled
by equations derived from experiments and
CFD computations and the characteristics of
these elements are put into the equation of
motion for the PPS system and then CFD
computation is made only for the hull.
A new simulation technique called
PPS(performance prediction simulation) is
developed using the moving method by grid
deformation. The sailing boat is set in an O-O
type boundary-fitted coordinate system with
six-degrees of freedom. The motion of the boat
is treated by the grid deformation except that
the steady advance motion is treated
at the outer boundary and the hull surface is
allowed to slip for the rolling motion as seen in
Fig.29.
Fig.27 America's cup yacht during the race
i n 2000
| Initial Condition
~ Initial GRID Generation
{~ ~ I ; ~ , . .
it.'.': '< i.: ~ 'a ~'1 ~ ,
~ ill ~~ Time LevelT-0 ~ . ~ i ~ i.
Reed Restart Data
Update Density Function and Generate Wave I
1
| SOLVE EQiJATIOIS OF MOTION
I I I GRID Generation
Move Water Surface?
i,
SOLVE
NS-| EGUATION
Fig.28 Block diagram for the simulation of
ship motion.
Lo: - ~
~ ; ~ ~ ~
Fig. 29 Grid system for a racing boat.
1
In spite of the small grid number of 45,000 this
simulation could provide very useful results
t243. For the design of racing yacht the 1%
difference of resistance is meaningful. The
~ .. . .. .. . .
accuracy ot the relative magn~tucte clue to small
modification was 70% correct in case the
experimentally verified difference of resistance
was 1%. The difference of 0.1°trim angle due to
the difference of hull form was rigorously
estimated which is very important information
because such small difference of attitude
influences on the balance and maneuverability
of the yacht. Since the equation of motion and
the Nervier-Stokes equation are simultaneously
solved in the time-marching procedure, not only
the performance in a steady motion but also
that of unsteady motion can be simulated by
this PPS technique. Actually a simulation of
course changing maneuver can be made with a
simple control system as shown in Fi~.30.
rat ~" . · ~
O
the etI~ect~veness of our design
method was not wholly verified bv the race in
_ ... . ..
,,
2()uo, although Nippon was the second at the
end of Round Robins and assumed to be one of
the fastest boats. The experiment after the race
shows that our design was superior to the boat
of the defender Team New Zealand, as shown
in Fig.31. On almost all close-hauled conditions
our boat must have been faster when same sails,
and boat maneuver were given. Two boats
designed by our new system are now sailed by
the UK team, Fig.32.
Fig.30 Computer-graphic view of an IACC
. . . .
,
class yacht In course keeping motion.
Ct (Roenianoo codfic~en~h~
UPRIGHT condition
tot
067 .
006
0~3
002
0~
o
5
7 t tl '3
V'~o'
|—°~4 o 4 - - ~ - h 't |
1—N=4O ~
Fig.31 Resistance coefficient curve of the
race boat of Nippon 2000 (blue) and
of Team New Zealand.
Fig.32 GBR52 Idaten~ex.JPN52)
won the 150years Jubilee Regatta
in August 2001.
9. Ships in waves
Another method of simulating ship
motion is to use a grid system fixed to the ship.
The grid system moves giving accelerations by
the motions to the Nervier-Stokes equation as
external forces as shown in Fig.33. For the grid
system an O-H type boundary-fitted grids are
used so that the wave making is more
conveniently made.
The first simulation was made for a
Series 60 hull advancing in regular heading
waves, see Fig.34.The hull is set free to pitching
and heaving motions. For the motion problem
the robustness is of significant importance
because the high pressure caused by motion
may locally break the conservation law locally
and subsequently break the solution. The
regeneration of grids at each time step is
carefully made and the density-function method
Is emptoyect. ~ ne simulation shows that this
technique can cope with the pitching motion
very close to the slamming motion and the
motion amplitude agrees well with the
experimental results.
~ Ace. : ,~r~
Region of computation
O-H grid system
:: ~ ~ ~ :
Fig.33 Ship-fixed coordinate system in a
space-fixed coordinate system for
the simulation of ship motion.
Fig.34 Series 60 model in heaving and
pitching motion in heading waves,
wave contours (left) and pressuure
distribution~right).
For the rolling motion special technique
is devised. The free-surface mane rotates
instead ot the grids nor the hull, and clustering
are made near the free-surface at each time
step. Motion simulation with three degrees of
freedom pitch, heave and roll is performed for
an IACC sailing boat advancing in oblique(15013)
regular heading waves.
It is noted that the coupling motion of rolling
and pitching gives quite different polar diagram
due to the difference of wave length. However
the IACC boat is not equipped with sails, keel
and rudder, the motion is not realistic, see
Fig.35.
When the model equations and data of
the sails and appendages are added the
performance turns to be much more realistic
due to the motion damping effect of the four
lifting surfaces as seen in Fig.36. Considering
that the motion experiment of sailing boat with
sails is very difficult, such a "virtual" sailing
simulation seems to be very useful.
Nonlinear motion often occurs in the
following wave conditions the extreme case is
broaching, which may be caused by nonlinear
(often sudden) change of hydrodynamical forces.
By generating waves by a numerical wavemaker
of acceleration type set at the exit boundary,
the motion simulation is made in the following
wave condition with three degrees of freedom,
that is, pitch, roll and heave as seen in Fig.37.
When we can increase the degree of freedom of
the motion, more realistic motion will be
observed in the computer together with the
interesting time history of a lot of physical
values in the near future.
Fig.35 PITCHING,HEAVING and ROLLING
motion of AC boat in oblique
incident wave.
Fig. 36 PITCHING,HEAVING and ROLLING
motion of AC boat with KEEL and
RUDDER in oblique incident wave.
Fig.37 PITCHING,HEAVING and ROLLING
motion of Series 60 model in oblique
following wave.
10. Design by virtual reality
The performance of a ship has been
classified into four, that is, resistance,
propulsion, maneuver and motion in waves.
However all these characteristics are going to
be examined by CFD simulations. They are
judged by the forces, moments, attitude and
motions, which are evaluated by the numerical
simulations. A designer, which uses a set of CFD
simulation codes, can be a specialist of all of
resistance, propulsion, maneuver and motion
In waves.
It may be safe to say that the information
technique of CFD is a kind of techniques of
system integration. The complicated design
process for better performance of a ship has
been simplified by use of CFD simulation, which
provides all hydrodynamical properties
simultaneously.
This seems to be a great tread of design
technology. The performance of the designed
vehicle is examined through the simulation in
the computer and a lot of information is fed back
to the designer. A designer can have a great
number of design experiences, learn a lot of
things and become a talented expert one by this
system assisted by the cheap computers in a
relatively short period.
The ultimate goal of CFD research is to
establish a system of virtual reality, which
provide not only excellent image by computer-
graphics, but also all performance
characteristics of a designed object to a
designer. A very simple procedure is as shown
in Fig.38. Every possibility is examined and
evaluated through the succession of CFD
computation.
When ship motion and added resistance
which determines the fuel consumption in the
life-cycle are of special importance, the system
shown in Fig.39 will be useful, in which
simulation of ship motion is employed to
estimate the added resistance.
A significant number of ships encounter
serious accident and often suck into the deep
sea on stormy condition. The performance of a
ship in an extreme condition, such as the
survival condition with 30m wave height and
70knots wind speed, is very difficult to predict
but the structural design must be made with
sound understanding of the characteristics of
a ship in such condition. With the virtual reality
technique by the computational physics this will
be done in the future. The extreme sea condition
can be realized in the computer and the
designed ship is set on this sea surface and all
characteristics are derived from the simulation.
~ r — ~
: | WAVE SPECTRUM RESPONSE FUNCTIONS | RESISTANCE |:
~ . ~ . ·:
I .,
ADDED RESISTANCE I
(PROBA8lLITY) l
:: :: : :: ~ ~ '1,:
- j FUEL CONSUMPTION in Me LIFSCYCLE .~ :~ ~~ . ~ ~~ . .
.. ~ WIN PROBABILITY in the LONG-RUNNING RACE
~ i ~ i ~ ~ ~~ ~ ~ . ~ ~ ~
~ 1 1 1 ~ 1 1 ~ 1 ~ 1 1 ~
1 ~ 1 1 ~ ~ 1 ~ ~ 1 1 _
11 1 11 1 _11 11 1 ~
Fig.38 Hull-form design procedure by
Fig.39 Flow chart for life-cycle oriented
hull- form design.
Concluding remarks
Seventy years from 1980 to 2000 will be
said a special period in the long history of
technology. The progress of computer technology
influenced a lot onto a wide variety of the
engineering field. It was also true in our field of
naval architecture. We must still continue to
fully digest the new technology and this may
contrarily reveal that the technology of system
engineering such as naval architecture is very
valuable thing, for which we must keep making
good progress.
References
[1l T. Inui, H. Kajitani and H. Miyata ~
Experimental investigations on the wave
making in the near-field of ships, J. Kansai
Soc. Nav. Archit. Jpn. 173 (June 1979), 95-107.
[2] H. Miyata, T. Inui and H. Kajitani: Free
full use of CAD and CFD simulation. surface shock waves around ships and their
effects on ship resistance, J. Soc. Nav.
Archit. Jpn. 147 (June 1980), 1-9. Nav.
Archit. Ocean Engng. 18 (1980), 1-9.
t3] N. Kawamura, H. Kajitani, H. Miyata and
Y. Tsuchiya: Experimental investigation on
the resistance component due to Wee surface
shock waves on series ship models, J. Kansai
Soc. Nav. Archit. Jpn. 179 (Dec. 1980), 45-55.
t10] N. Suzuki and H. Miyata: Practical use
of a fluid flow simulation with solidification
in the design of continuous casting
processes, Industrial and Environmental
Applications of Fluid Mechanics, ASME-
FED Vol. 145 (1992), 97- 101,
t4] H. Miyata and T. Inui: Nonlinear ship
waves, Advances in Applied Mechanics 24, t11] H. Miyata: Finite-difference simulation
Academic Press (1984), 215-288. of breaking waves, J. Computational
Physics, 65-1 (July 1986), 179-214.
t5] H. Miyata, Y. Tsuchiya and T. Inui:
Resistance reduction by stern-end-bulb (first
report), J. Soc. Nav. Archit. Jpn. 148 (Dec.
1980), 10-16.
L6] H. Miyata, Y. Tsuchiya and T. Inui:
Resistance reduction by stern-end-bulb
(second report), J. Soc. Nav. Archit. Jpn. 149 t13] H. Miyata, M. Katsumata, Y. G. Lee and
(June 1981), 1-10. H. Kajitani: A finite-difference simulation
method for strongly interacting two-layer
flow, J. Soc. Nav. Archit. Jpn. 163 (June
t7] H. Miyata and S. Nishimura: Finite- 1988), 1-16.
difference simulation of nonlinear ship waves,
J. Fluid Mechanics 157 (Aug. 1985), 327-
357.
[12] H. Akimoto, M. Sugihara and H. Miyata
: Vortex motions and forces about a
horizontal cylinder advancing Beneath the
waves, J. Soc. Nav. Archit. Jpn. 170 (Dec.
1991), 253-263
t8] H. Miyata, S. Nishimura and A. Masuko:
Finite difference simulation of nonlinear
waves generated by ships of arbitrary three-
dimensional configuration, J.
Computational Physics 60-3 (Sept. 1985), 391- [15] A. Kanai and H. Miyata: Elucidation of
436. the structure of free surface shock waves
about a wedge model by finite-difference
method, J. Soc.Nav. Archit. Jpn. Vol.177,
t14] H. Orihara and H. Miyata Numerical
Simulation Method for Flows About a Semi-
Planing Boat with a Transom Stern J. Ship
Research, Vol.44, No.3, Sept. 2000, pp. 170-
185
t9] H. Miyata, M. Kanai, N. Yoshiyasu and Y. 147-159. (May 1995)
Furuno: Diffraction waves about an
advancing wedge model in deep water, J. Ship
Research 34-2 (June 1990), 105- 122.
[16] A. Kanai and H. Miyata Numerical
analysis of the structure of free-surface shock
wave about a wedge model, J. Ship Research,
Vol.40, No.4, Decl996, 278-287
t17] A. Kanai and H. Miyata,Direct numerical
simulation of wall turbulent flows with
microbubbles International Journal for
Numerical Methods in Fluids Int. J. Numer.
Meth. Fluids 2001; 35: 593-615
[18] H. Miyata, T. Sato and N. Baba:
Difference solution of a viscous flow with free-
surface wave about an advancing ship, J.
Computational Physics, 72-2 (Oct. 1987), 393-
421.
t1 9] H. Miyata and Y. Yamada: A finite
difference method for 3D flows about bodies
of complex geometry in rectangular co-
ordinate systems, Int. J. Numerical Methods
in Fluids 14 (1992), 1261-1287.
[20] H. Miyata, M. Zhu and O. Watanabe
Numerical study on a viscous flow with free-
surface waves about a ship in steady straight
course by a finite-volume method,J. Ship
Research 36-4 (Dec. 1992), 332-345.
[21] K. Matsunaga, H. Miyata, K. Aoki and
M. Zhu: Finite-difference simulation of 3D
vertical flows past road vehicles, 1992 SAE
Inter. Congress and Exposition,
Detroit,Vehicle Aerodynamics, SAE SP-908
(Feb. 1992), 65-84.
.,
[22] Y. Hanaoka, M. Zhu and H. Miyata:
Numerical prediction of wind noise around the
front pillar of a car-like body, 1993 SAE, 7th
Inter. Pacific Conf. and Exposition on
AutomotiveEngg., Phoenics, SAE 931895
(Nov. 1993), 1-11.
[23] Y. Sato, H. Miyata and T. Sato :CFD
simulation of 3-dimensional motion of a ship
in waves: application to an advancing ship in
regular heading waves, J. Mar. Sci. Technol.
(1999) 4:108-116
t24] H. Akimoto, Development and application
of CFD simulation technique for ships in 3D
motion, Ph.D thesis, University of Tokyo
1996(in Japanese).
L25] H. miyata: Time-marching CFD
simulation for moving boundary Problems, 21St
Symposium on Naval Hydrodynamics 1996,
Tronheim
DISCUSSION
Ki-Han Kim
Office of Naval Research, USA
How did you handle the free-surface "ridding in
your computations; surface tracking or surface
capturing technique?
AUTHOR'S REPLY
In most of the 3D cases the capturing technique
using density function method is employed. This
is similar to the level-set method.
DISCUSSION
Arthur M. Reed
Naval Surface Warfare Center, Carderock
Division, USA
Prof Miyata used the term "stability" several
times. However, it is not what is meant by
stability. Did the references refer to ship
stability, or was the reference referring to
computational stability?
AUTHOR'S REPLY
For computer simulation stability of the solution
is most important rather than computational
economy due to the advance of computer
technology.
We are now very close to capsizing simulation.
Therefore the ship stability problem is one of the
important targets of CFD simulation.
