HPCI magazine vol.3

High-precision Fluid
Flow Simulations
to Revolutionize
Manufacturing
One of the crucial final steps in designing ships and cars is to produce a scale model ora prototype to evaluate real-world performance. These tests often require large facilities
to reproduce the environments in which the vessel or vehicle will actually be used. If
those tests could instead be carried out by computer simulations, it could make the design
process much faster and less expensive. At present, however, both of the simulation
software and the processing speed of even supercomputers are not enough to run such
simulations with the required accuracy. As the principal investigator on a project under the“Program for Promoting Research on the Supercomputer Fugaku”,*1 Chisachi Kato
intends to overcome these limitations. He has achieved simulations that could alter theperformance testing conventionally done in ship design, and that can run within a feasible
time during the design process.

Chisachi Kato

Center Director / Professor
Center for Research on Innovative Simulation Software,
Institute of Industrial Science, The University of Tokyo

The Simulations Neededfor Ship Design
Manufacturing vehicles or watercraft begins
with a “concept design” that determines thebasic shape, functions, and so forth of theproduct to be made. This is followed by a“basic design” that lays out the basic
specifications of the product, a “detail design”
that refines the detailed specifications, andfinally performance tests using ship models orprototypes (Fig.1). Explaining the significanceof his team's performance test simulations,
Kato says, “a large ship model is built andtested in a big long ‘towing tank’ filled withwater. The testing for a single ship design
costs about 10 to 20 million yen and takes
about a week. Our goal is to replace thisreal-world testing with computer simulations.
If we can run simulations instead of the01

towing-tank tests, we can reduce the costs
because we won't need to build a model
ship to test. And if we find a performance
problem, we can immediately modify thehull design to address it, which also saves
time.”
There are two main performance tests forships: resistance testing and self-propulsion
testing. In the resistance tests, a modelFig. 1ship is towed, without the propeller
installed, to ascertain what resistance thehull encounters from the surrounding
water. In the self-propulsion tests, themodel is towed, with the propeller
installed, to evaluate the interference
effects between the hull and the propeller.
A computational method known as “finiteelement analysis” is used to reproduce on
computer the flow of water around the hull

Expanding the use of simulations in manufacturing.

The design of a ship or automobile progresses through several stages: concept design, basic design, detaildesign, and finally, performance testing with models and prototypes. If a problem is detected in the performance tests, the designer must step back through the previous stages and start over, sometimes all the way
back to the concept design stage. So far, simulations have mainly been used at the detail design stage.
Replacing the prototyping and testing stages with simulations also could save time and money. Moreover, it
could dramatically reduce the overall design time, since required changes detected at the performanceand propeller in simulations that aspire tocompletely replace the resistance andpropulsion tests. In this method, the wateraround the hull is divided into a number ofsmall segments called “elements”, and theflow of water through these elements iscomputed according to the laws ofphysics.
To replace the towing-tank tests, thehydrodynamic modeling must be highly
precise and accurate. This means it isnecessary to replicate swirling eddies *2
less than 1 millimeter in size near the hull
exterior. With conventional computations,however, the number of elements aroundthe hull was at most tens to hundreds ofmillions, not enough to reproduce suchsmall eddies. Kato and his team ran simulations 1,000 times more detailed, using

testing stage could immediately be reflected in the design.

Scope ofsimulationsat present

Concept design

Basic design

Detail design

add

Scope ofsimulationsin the future

Testing*1 The “Research and Development of Innovative Fluid-dynamics
Simulations
forAerodynamic/Hydrodynamic
Performance
Predictions by using Fugaku (Principal Investigator: Chisachi Kato)”
research project consists of five research topics:• Topic 1: “Realization of Numerical Towing Tanks and Improvements in
Propulsion Efficiency with Energy Saving Devices”• Topic 2: “Wall-Resolved Large Eddy Simulation of Internal Flow in a
Multi-Stage Centrifugal Pump with Narrow Gaps”• Topic 3: “Direct Analysis of Compressor Surge”• Topic 4: “Prediction of Real-World Automotive AerodynamicPerformance”• Topic 5: “Real-World Automotive Aerodynamic Sound Prediction”*2 Includes a range of eddy sizes in temporally and spatially irregularflows.

vol.3 - High-precision Fluid Flow Simulations to Revolutionize Manufacturing

02

from tens of billions to 100 billion elements,
which could reproduce eddies less than 1
millimeter in size. Their simulations
produced results that very closely matchedthe towing-tank test results (Fig.2).
Kato explains, “We had, in fact, done thesame simulations before on the K computer.
But they were so intensive that, evenusing around 25,000 of its compute nodes,
the computations took about two days tocomplete. That's not fast enough for theactual design process. So, we've been
doing research and development work on
Fugaku with the goal of executing thosecomputations in less than 1/30 th the time.”

Testing a model ship

Fig. 2(circled in red)

The precision needed forcomputer simulations toreplace towing-tank
performance tests in shipdesign.
When testing a ship hull design, a scale• Tank size: 400 x 18 m
• Water depth: 8 m• Model length: 6 m• Max. towing speed:
15 m/s (54 km/h)
Image courtesy: Shipbuilding Research Center of Japanmodel of the ship is towed in a long watertank, as shown in the photo at the top. The
three graphs below compare waterflow

Flow velocities at ship rear
(Vertical cross-section)

velocities near the stern as obtained with:resistance testing in the water tank (top),
conventional simulations (middle), andTowing tank test resultsthe new Fugaku simulations (bottom). The
conventional simulations weren't able toreproduce the small turbulent eddies. But
the new simulations use grids with 1,000
times as many grid points, allowing them
to closely replicate the test results down
to the small eddies. This is the level ofprecision that will be needed for simulations to replace the towing-tank performance
tests.

Conventional simulation
results
Grid points (elements):
Tens to hundreds of millions

Simulation resultsin this study
Grid points (elements):
Tens of billions to 100 billion

vol.3 - High-precision Fluid Flow Simulations to Revolutionize Manufacturing

03

Faster Data Transfer Rates Speeded UpComputations
Kato and his team are developing a “flow solver” software basedon the finite element method. It is called “FrontFlow/blue” (FFB).
After completing his master's degree program in 1984, Kato
joined a manufacturing company, where he wrote the softwarethat served as a prototype for FFB. Since moving to a positionin academia, he has been upgrading the FFB software, withthe support of projects sponsored by MEXT, to achieve ever-better
performance as supercomputers have grown more powerful.
Taking advantage of the distinctive design features of Fugaku,
his team devised yet another way to reduce FFB's run time.
They leveraged the faster rate at which data is transferred
between the memory and the arithmetic logic unit in a CPU.
FFB's computations require large amounts of data to be readfrom memory. The improved data transfer rate means fasterexecution.

“One of our innovations was to arrange thedata as carefully as possible when placing
data into memory. This makes it possible
to quickly retrieve the data to be used incomputations. Another is that we havedevised an algorithm with a modified finite
element method (Fig. 3). Conventionally,
element values are computed in order byelement. But we decided to computeelement values by node, in the order of thenodes that delineate the elements.”

vol.3 - High-precision Fluid Flow Simulations to Revolutionize Manufacturing

Fig. 3A new algorithm utilizing Fugaku’s fast memorytransfer.

Conventional finite element algorithms compute in order by element. The value for
Element 1 is computed first, then Element 2, and so on. FFB's new algorithm computes
in order by node. All the elements sharing a common node are computed at once, andthen those resulting values are used to update the value for that particular node, and soon. Changing the basic execution order of these computations takes full advantage of theperformance offered by Fugaku's state-of-the-art CPU design.

Node 2
Node 1

Computation
result 1

Computation
result 2

Element 1

Computation
result 3

Element 2

Element 4
Node 3Element 3
Node 4Computation
order

Node 5

Conventional finite element computation method

Element 9
Element 7
Computation
result 2
Computation
result 1

Element 6

Element 8

Node 2Computation
result 3

Element 10Element 1

Node 1
Computation
result 6

Element 5

Computation
result 4

Element 2

Computation
result 5

Element 4

Computation
order
Element 11

Node 3Element 3

Computation method devised in this study04This increased the memory data transferrate, nearing Fugaku's effective maximum
memory bandwidth. *3 The computations
were executed 70 times faster on Fugaku
than on the K computer. Computations
that had taken around two days with the K
computer were completed in less than anhour, a success far exceeding the team'sgoals.
*3 The actual maximum memory bandwidth performanceaccording to benchmark software, rather than a theoreticalmemory bandwidth according to memory specifications.

Fig. 4

Simulations to evaluate waterflow interference effects near the propeller.

Image courtesy: Shipbuilding Research Center of Japan.

Working with End Users to
Make Simulations more
Practical
Kato and his team have run simulations,with detail down to small eddies, on several
ship hull designs. They check the waterflow around the hull during resistancetesting, the interference effects between
the hull and propeller during self-propelled
testing (Fig.4), and the resistance caused
by waves. The results have closely matchedthose of the towing-tank testing. The simulations are getting to a level where they canreplace real-world tests.
Speaking about future plans, Kato says,

vol.3 - High-precision Fluid Flow Simulations to Revolutionize Manufacturing

The flow of water around the hull and the rotation of
the propeller cause interference, which affects thepropulsion performance of the ship. Self-propelledship models are used in water tank tests tomeasure these interference effects. But thesetowing-tank tests are limited in how much detailed
data they can provide. The new simulations, however,can provide much more detailed data, which isexpected to lead to improved ship hull and propeller
designs. Pressure is shown in color, and waterflow
is shown in black and white.

“Our main goal is putting these simulations
to actual use. So, we want to move forwardwith computations for various ship designs
and other conditions to demonstrate thatthese simulations are usable in the designstages.”
But this is not limited to ships. The “Programfor Promoting Research on the Supercomputer Fugaku” project that Kato oversees
aims to make the use of supercomputersimulations feasible in the design of automobiles, pumps, compressors, and more.
To facilitate industrial adoption, they haveformed a consortium of 150 people from54 organizations. They include the makersof ships, turbo-chargers, and hydraulic05devices, as well as hardware and software
vendors, who will work together to solve
problems that manufacturers might
encounter when implementing thesecomputer models.
Kato has been working on software for
fluid dynamics modeling since the dawn ofsupercomputers in Japan. The day when
his efforts toward the practical use ofsimulations will bear fruit is just around thecorner.

About theResearcherChisachi Kato says as an elementary
school student, he had a paper route toearn money to buy plastic models. Atuniversity, he majored in MechanicalEngineering and ran real-world experiments.
But when he encountered computer-based
simulations in his workplace after graduating,
he became fascinated and has worked withsimulations ever since. Having picked up

organizational skills while working part-time
as a mover and a party organizer, and withhis attention to grammatical detail whenpreparing English manuscripts, computer
simulation work seems to be his dream job.
The research work doesn't seem to cause
him stress. For fun, he plays golf withfriends. He looks forward to playing again,
once the COVID-19 crisis has ended.

Associated Research Projects• “Research and Development of Innovative Fluid-dynamics Simulations for Aerodynamic/Hydrodynamic Performance Predictions byusing Fugaku (Research and Development of a Turbomachinery Design Simulation System)” (hp200133)
Principal Investigator: Chisachi Kato, Institute of Industrial Science, The University of Tokyo
2020 ACM Gordon Bell Prize finalist for Research Findings

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HPCI magazine FUGAKU HYAKKEI vol.3

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