HPCI magazine vol.8

High-Fidelity
Simulation of Airflow
Ushering in a New Roundof Innovation in
Aircraft Design
Air and water constantly flow around us. Although we are not usually aware of it, technologies andproducts that make good use of these flows make our lives more convenient and prosperous. A
typical example is airplanes. Recently, Prof. Kawai and his team succeeded in numericallysimulating the airflow around an aircraft using a computer with overwhelmingly higher fidelity thanever before. This achievement was realized by combining the performance of the supercomputer
Fugaku with the fundamental research that Prof. Kawai and his team have conducted over manyyears, and is expected to bring about a revolution in aircraft design.

Soshi KawaiProfessor, Department of Aerospace Engineering,Graduate School of Engineering, Tohoku University“I Was Able to Realize Partof a Dream I Have Had Since
My Graduate School Days.”Prof. Kawai discussed the history and current
status of aircraft aerodynamic design asfollows. “Computational fluid dynamics simulations
have been used in aircraft aerodynamic
design since the 1980s. Before that, until theBoeing 767, design relied on wind tunnel testing,which was costly and time-consuming. The
introduction of numerical simulation has
revolutionized aircraft design, but simulation
can still only predict the flow around the aircraftwith smooth geometry and underless-turbulent cruise conditions. Therefore,
aircraft manufacturers expect the second
round of innovation to occur when it becomes
possible to simulate the flow around the entire
aircraft (whole-aircraft simulation) during
01

takeoff and landing, which involves
complex geometries and highly turbulent
flow.”

The flow follows the Navier-Stokes
equations, which are solved numericallyin Large Eddy Simulation (LES), where
the space around the object is divided bya grid and the eddies (turbulence) with large
energy are calculated directly on the grid.
Prof. Kawai began researching LES in 2002,
when he entered his doctoral program. In
2005, when he received his doctorate, hedecided he wanted to use LES to analyze
the flow around an entire aircraft. At thetime, there was no one working on LES in
the aerospace field in Japan. Since then,Prof. Kawai and his collaborators haveconducted fundamental research andvol.8 - High-Fidelity Simulation of Airflow Ushering in a New Round of Innovation in Aircraft Design(a) JSM (JAXA Standard Model) was used as the aircraft
geometry model, and the flow was calculated withapproximately 12 billion grid points around the aircraft. The
minimum grid spacing is 250 µm, and it took about two days toobtain the results of flow using 3,456 nodes of Fugaku for thiscalculation. The flow eddies (turbulence) around the wing andthe longitudinal vortex from the engine nacelle are well
reproduced. (b) The blue circle shows the lift near themaximum lift obtained from this simulation. It is in good
agreement with the experimental data.

32.5

Lift

Since an aircraft raises its nose to obtain
more lift during takeoff and landing, it is
crucial to predict the relationship between
the aircraft’s inclination (angle of attack)and lift in aircraft design. In particular, themaximum lift value significantly affects
takeoff and landing performance, such as
how much runway is required, and safety.
Therefore, it is expected that a whole-aircraft
simulation that can obtain lift values withhigh accuracy and can replace actual
flight tests can be developed. However,this is not easy to achieve.

(b) Comparison of lift

Example of aircraft simulation
Fig. 1
using FFVHC-ACE

2

1.5

1

Experimental data
Numerical simulation results
05(a) Simulation processes

1015
Angle of attack

20

25

Automatic
grid generation

Geometry data

LES analysis

Flow around wing with high lift devices

Flow around engine nacelle02

developed the “FFVHC-ACE” software,and recently succeeded in a high-fidelity(physically more accurate) simulation of
the flow around an entire aircraft byusing this software on Fugaku (Fig.1).
Furthermore, the maximum lift obtained
from these simulation results agreed wellwith experimental data. The results are
beginning to meet the expectations ofaircraft manufacturers.

Fundamental Researchand Fugaku Made Difficult
Simulations Possible
Why has it been so difficult to perform
high-fidelity simulations of the flow aroundan aircraft? One reason is that theReynolds number of the flow is very high,about 10 million. The Reynolds number isthe ratio of the flow effect (inertial force)and viscous effect (viscous force) of theflow. When this number exceeds a certainvalue, eddies (turbulence) occur in theflow; the higher the value, the smaller theeddies. “LES is a method that simulates
turbulent phenomena almost directly, so
the prediction accuracy is high. However,to calculate small eddies, the grid sizemust be reduced, so the number of grid
points required for the calculation becomesenormous, and the calculation cost jumps,”
Prof. Kawai says.
In particular, the vortices are smaller near
the walls of the aircraft, so Prof. Kawai hasdeveloped a physical model that accuratelyreflects the effects of small vortices near
the walls (Fig. 2a). “Only very close to thesurface of the aircraft, we replaced the LES
calculations with calculations based on thismodel. This reduced the computation timeby a factor of about 10,000. Without thismodeling, we would not have been able tocalculate for the entire aircraft even with all
the nodes in Fugaku,” Prof. Kawai says,
explaining the significance of the modeling.
Another reason why high-fidelity simulations
are difficult is that the flow around theaircraft must be treated as a compressible
flow. Computations of compressible flow
are prone to instability, so existing software
stabilizes the calculations with mathematicalvol.8 - High-Fidelity Simulation of Airflow Ushering in a New Round of Innovation in Aircraft Design03

manipulations that provide artificialdiffusion. However, this manipulation
causes turbulence to decay in a waythat is contrary to physics, and thus
does not yield correct results.
To counter this problem, Prof. Kawai andhis colleagues established a calculation
method named the KEEP (Kinetic-Energyand Entropy Preserving) scheme in theirfundamental research. In compressible

Fig. 2flow simulations, the flow is usuallyc a l c u l a t ed under the condition that its a t i s fies the mass conservation law,
momentum conservation law, and energy
conservation law. In addition to them, thekinetic energy equation and entropy conservation law are satisfied when calculating with
the KEEP scheme (Fig. 2b). The KEEP
scheme allows researchers to conduct stablecalculations without adding artificial diffusion
and to obtain more physically correct results.

Two fundamental researches thatenable high-fidelity simulations(a) The thickness (δ ) of the turbulent boundary layer that develops on the surfaceof the aircraft is about 3.5 mm at 10% of the position from the leading edge of thewing. An area less than one-tenth of this thickness (shown by the red shading) ismodeled, and the rest is calculated using LES. This results in a speedup of about
10,000 times faster than when all of the boundary layer thickness is calculated withLES. (b) The usual compressible flow simulation is performed under the condition
that the mass conservation law, momentum conservation law, and energy conservation law are satisfied, but in this calculation, a discretization method is devised tosatisfy the kinetic energy equation and entropy conservation law. This enables
stable and high-fidelity LES calculations.

“In flow simulation, which requires large-scalecalculations, the use of supercomputers has
always been promoted, but Fugaku is very
easy to use, and I think it is a good computerin terms of smoothly conducting large-scalecomputations. I believe that the results ofthis project have come out of a w e l l -ti me dencounter between the fundam e n t a lresearch we have been conducting for manyyears and Fugaku, and the multiplication ofthe two,” Prof. Kawai says.

(b) KEEP (Kinetic Energy and Entropy Preserving) scheme

Governing equations forcompressible flow
(Euler equation without viscous term)

Equations for secondary
physical quantities

Analytical
Mass conservation law

Kinetic energy equation
Discrete(existingmethods)

Momentum conservation law

Entropy conservation law(a) Wall-modeled LES
Discrete(KEEPscheme)
Outer layer turbulence → calculated with LESTurbulent
boundary layer(thickness = δ )Energy conservation law

Inner layer turbulence (thickness < 0.1δ )→ calculated with physical model
Discretization is devised so that the equations for
kinetic energy and entropy are also satisfied.

vol.8 - High-Fidelity Simulation of Airflow Ushering in a New Round of Innovation in Aircraft Design04

However, Prof. Kawai is not satisfied withthis achievement. “We will continue ourresearch to predict not only takeoff andlanding performance, but also high-speed
flight limits, aerodynamic noise during thelanding approach, and flutter phenomenon(severe vibration of the wings causedby f l o w ) , w i t h t h e a i m o f u s i n g t h e
FFVHC-ACE in aircraft design,” he says
enthusiastically.
Prof. Kawai has a dream beyond this kind of
success of the FFVHC-ACE. “Aerodynamicvol.8 - High-Fidelity Simulation of Airflow Ushering in a New Round of Innovation in Aircraft Design

Fig. 3simulation. I would be happy if we can turnthe ‘imagination’ of having an airplane of adesired geometry into an actual ‘creation’.”(Prof. Kawai used the Japanese word
souzou , which means both “imagination” and“creation.”)Industrial applications of FFVHC-ACE

The Mitsubishi Heavy Industries Group (Mitsubishi Heavy Industries and Mitsubishi Aircraft) applied FFVHC-ACE to geometry
data of the aircraft SpaceJet and conducted a simulation with Fugaku. Although Prof. Kawai was not directly involved in thecalculations, only giving advice, the simulation was successful. The maximum lift was in good agreement with the actual flight test
compressible flow LES analysis software that can withstand use indata, demonstrating that FFVHC-ACE is a high-fidelity compressib
Mitsubishi Heavy Industries Group.
academic research to industrial applications. Image courtesy of Mitsu

Comparison and validation withactual flight data

0.5

Prof. Kawai and his colleagues have applied
FFVHC-ACE to several aircraft geometry
models, and have confirmed that themaximum lift and other parameters agree
well with experiments. “We are proud thatFFVHC-ACE is the world’s most powerfulcompressible flow LES analysis software,”he says. “Moreover, industrial users only
need to prepare geometry data to perform
complex geometry LES calculations withhigh speed, ease, and high quality. In fact,
the Mitsubishi Heavy Industries Group has
successfully conducted a simulation using
FFVHC-ACE and the geometry data of theSpaceJet and obtained good agreement withactual flight test data (Fig.3).”

design of aircraft until now has reliedheavily on experience, so it has been risky
to develop an aircraft that looks different
from past aircraft,” he says. “But that will
become possible when various performanceof an aircraft is accurately predicted throughLift trimmed

A Dream that Extends
Beyond the Sky

Actual flight data
Numerical simulation results
5 [deg]Angle of attack05

He also hopes to release FFVHC-ACE as abasic software that can be used for all kindsof compressible flow analysis, not restricted
to aircraft, so that it can be used widely fromacademia to industry. The engineering fields
involving compressible flow range fromhigh-speed railroads and turbines for power
generation to nozzles and diffusers, jetengines, and rockets. Since compressible
flow simulations can directly determine thesound, which is the coarse and dense waves
of air, simulations can also be used to findways to reduce noise, which is a common
problem in the industry.
The dream Prof. Kawai had 20 years ago is
now becoming a reality through steadyfundamental research and the advent ofFugaku. And now, it is about to expand
beyond the framework of aircraft to theindustrial world and bring about majorinnovations.

About theResearcherProf. Kawai entered the Department ofplace he was supposed to go as a postdoctoral

Aerospace Engineering at his university because

researcher in the U.S. had to be changed due toof his childhood dream of becoming a pilot.

the 9/11 terrorist attacks. “I have been blessed toBecause of this, he enjoyed his student life someet many people, including a researcher I met atmuch that when he was assigned to a laboratory

the new place and developed compressible flowin his fourth year, he was astonished at thecalculation methods and a wall-modeled LES,” helimitations of his knowledge. To become asays. Prof. Kawai loves spending time in nature

responsible member of society, he studied hard andand frequently visited national parks when he wastook a doctor’s course, where he was fascinated bystudying abroad in the U.S. Now that he is in

the fun of research and became a researcher. His

Sendai, he regrets that he does not have much

path was not smooth, however. For example, thetime to spend in Tohoku's nature.

Associated Research Projects“Leading Research on Innovative Aircraft Design Technologies to Replace Flight Test” (hp200137, hp210168, hp220160)
Principal Investigator: Soshi Kawai, Graduate School of Engineering, Tohoku University

vol.8 - High-Fidelity Simulation of Airflow Ushering in a New Round of Innovation in Aircraft Design06

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