Thermal Science & Engineering Vol.25 No.3 (2017)

Progress in DNS and Laser Diagnostics of Turbulence andTurbulent Combustion
Mamoru Tanahashi†, Masayasu Shimura†, Yuki Minamoto†

Abstract
In the community of turbulence and turbulent combustion research, results obtained from direct
numerical simulations and advanced laser diagnostics are regarded as forming the fundamental basis ofphysical insights and mathematical models of the highly non-linear, multi-physical phenomena. Over the
decades, we have contributed in these research areas by means of these numerical and experimental
approaches, and tried to ellucidate relevant physical mechanisms. The scale of these simulations or laser
diagnoctics has improved over the years thanks to the advancement of high performance computing,numerical schemes, and laser and optical systems. The insights based on these numerical and measured
resutls are then applied to develop mathematical models to describe microscale non-linear acitivities
incorporating theories, which could be used for computational fluid mechanics of major combustion
devices.
Key Words: Turbulence, Turbulent combustion, Direct numerical simulations, Advanced laser diagnostics
1.

INTRODUCTION
The first author of this paper (Mamoru Tanahashi:

versity, Japan. Without their strong recommendations,MT) would like to give a huge thanks to the Heat Trans-

cial thanks to Professor Emeritus Toshio Myauchi, Tokyo

fer Society of Japan for selecting him to be the Nukiya-

Institute of Technology, Japan. MT’s research has been

ma Memorial award recipient in 2016. The Nukiyama

initiated by Professor Toshio Miyauchi from the under-

Memorial award has been established in 2012 on thegraduate research in 1989. He has led that MT shouldoccasion of the 50th anniversary of the foundation of theextend direct numerical simulation (DNS) of turbulence

Heat Transfer Society of Japan. The name of this award

to turbulent combustion field in the future. After that, hehas been commemorating great contributions by Profes-

supervised MT’s master and doctoral degrees and has

sor Shiro Nukiyama who discovered the boiling curve

been a senior gentle colleague. MT also appreciate Pro-

(so-called Nukiyama curve). MT appreciates the interna-fessor Kunio Hijikata, Tokyo Institute of Technology,tional evaluation board by selecting him for this award

Japan who passed away in 1997. He was the supervisor

even though MT’s research areas, which are turbulence,of MT’s master degree since Professor Toshio Miyauchi

turbulent heat/mass transfer and turbulent combustion,

was absent in Japan and he had been encouraged MT as aare far from the boiling research.

great professor in the heat transfer society. MT also ex-

MT were not bestowed this award. MT expresses spe-

MT makes cordial acknowledgments to Professor

presses thanks to also all smart students who had been

Koichi Hishida, Keio University, Japan, Professor Emer-

part of his laboratory and conducted research work ex-itus Godfrey Mungal, Stanford University & Dean attensively, and all friends in this society.

Santa Clara University, USA, Professor Andreas Dreizler,

In this paper, MT’s research works are summarized

TU Darmstadt, Germany, Professor Sangmin Choi,with focusing on DNS and laser diagnostics of turbu-

KAIST, Korea, Professor Yasuyuki Takata, Kyushu Uni-lence and turbulent combustion. In section 2, the pro-†

Received: June 30, 2017, Editor : Katsunori Hanamura
Department of Mechanical Engineering, Tokyo Institute of Technology, Ookayma 2-12-1, Meguro-ku, Tokyo 152-8550, Japan- 27 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

gress in DNS of turbulence and turbulent combustion isare explicitly resolved both in time and space, thereby

discussed. In section 3, recent developments in laser di-naturally including any relevant non-linear fluid flow

agnostics of turbulence and turbulent combustion are

phenomena. This advantage of DNS is well represented

shown. Finally, in section 4, a short summary is present-in the study of “coherent fine scale eddies”, which de-ed.

termines the structure of smallest eddies prior to their
dissipation. Several examples of such smallest eddies areshown in Fig. 1 (Tanahashi et al., 2004a; Wang et al.,

2. STATE OF THE ART—DIRECT NUMERICAL

2007). Numerical approaches used for many industrial

SIMULATIONS

applications, such as Reynolds averaged Navier-Stokes(RANS) or Large Eddy Simulations (LES), are not suita-

2.1 Fundamentals of DNS and HPC
The “first principle” numerical simulation method forble for this kind of studies since any representation of

turbulent flows is called direct numerical simulations

such small eddies are masked by mathematical models.

(DNS). In this method, Navier-Stokes equations as well

While DNS of non-reacting turbulent flows haveas other relevant scalar equations are solved numerically

been already standard method to study fundamental

without using a mathematical model except for thephysics of turbulence for around 30 years, direct numer-

chemical source terms if the flow field involves chemi-

ical simulations of turbulent combustion using a sin-cally active scalars. The advantage of the use of DNS is

gle-step chemistry model with variable density havethat all the fluctuation scales of turbulence and scalars

started being used for two- and three-dimensional do(b)

(a)

(c)

Fig. 1 Typical axis distribution of coherent fine scale eddies in homogeneous isotropic turbulence (a), in a turbulent
channel flow (b) and in a turbulent mixing layer (c).
- 28 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

14

10

1EXA
100P

Flops

1P
100T

12

10

HIT TCF TML

other countries
Japan
Turbulent Combustion

10

10

10T
8

10

1T
100G

6

10

10G

Number of grid points

10P

1G
100M
1960

4

1970

1980

1990
year

2000

2010

10
2020

Fig. 2 Development of the fastest supercomputer in the world and grid points of DNS of various turbulence and turbulent combustion.
mains since early 90s. However, as it is well known, as-grid points used in DNS of nonreactive turbulence aresuming single step chemical reaction is indeed oversim-

also presented. Here, the plots are split into DNS by Ja-

plification since the effect of localized radicals and in-pan and other countries for each fundamental turbulent

termediate species on the major species and global flame

field (homogeneous isotropic turbulence: HIT, turbulent

structure could be substantial during the process of tur-channel flow: TCF and turbulent mixing layer: TML).

bulence-flame interaction. The first three-dimensional

The number of grid points used in DNS of turbulence

DNS with a detailed chemistry including all radicals andincreases at the same rate of the #1 SC. Figure 2 suggests

intermediates including temperature-dependent transportthat the K computer will realize DNS of turbulence withproperties has been performed for hydrogen-air premixed

1012 grid points. The trends of the #1 SC and DNS also

combustion at Tokyo Tech and reported in 1999

suggest that DNS with 1014 grid points is projected to be(Tanahashi et al., 1999; Tanahashi et al., 2000; Tanahashi

reported in 2020, if the trend continues.

et al., 2002; Poinsot and Veynante 2005). This has been

In numerical simulations of turbulent combustion

then followed by many more DNS with complex or de-with complex or detailed chemistry, one need to solve

tailed chemical kinetics, which are nowadays standard

multiple species conservation equations together withpractice in the turbulent combustion research community.

mass, momentum and energy conservation equations.

This is mostly due to the recent understanding of im-

The number of species is around 10 for hydrogen andportance of such chemical kinetics and advancement of

about 50 for methane or propane related combustion. For

high performance computing.

gasoline, several hundred species should be considered

Figure 2 shows a trend of the fastest supercomputerin DNS to include the influence of important chemistry

in the world (hereafter, #1 SC) (http://top500.org/). As of

on turbulent combustion. The reaction rate of each spe-

June 2017, the #1 SC has about 93 PFlops. The fastest

cies is determined from the complicated pathways ofsupercomputer in Japan is Oakforest-PACS (about 13.5

elementary reactions. The number of elementary reac-

PFlops) at Joint Center for Advanced High Performancetions also depends on fuel: around 30 for hydrogen;

Computing of the University of Tokyo and Tsukuba

around 300 for methane or propane; and several thou-

University (not K computer). Recent improvement rate

sand for gasoline. These reaction “layers” need to beof #1 SC is 10 times in 3.5 years. If this rate is sustained,

explicitly resolved in DNS, and the overall reaction zone

1 EXAFlops supercomputer will be realized in 2020,

thickness is typically much less than 1 mm, within which

18

where EXA means 10 . In Fig. 2, developments in total

all of these reaction layers are located at different posi- 29 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

Unburned

BurnedFig. 3 Distributions of heat release rate in high Reynolds
number turbulent premixed flame (Re = 120).

Fig. 4 Radical fingering in high intensity turbulent premixed flame.

tions and variations. To fully resolve flame inner reaction
structure, each of these reaction layers need to be re-that the number of grid points of this DNS is not so huge,solved by approximately 5-10 mesh points, which easily

while this DNS is the highest Reynolds number case foroverwhelms the resolution requirement imposed byturbulent combustion, as far as we know. Although

Kolmogorov length scale for turbulence. In the next few

Reynolds number of turbulent combustion fields in real

subsections, we will summarize key combustion DNS

applications is higher than that of this DNS and seems toperformed at our and other groups in the past 30 years

be higher than Re =300, more than 10 years are required
for the realization of such kinds of DNS. Note that actual

2.2 DNS of High Reynolds Number Turbulent Pre-

Reynolds number in real combustors is not known be-

mixed Flames
From the first 3D DNS of turbulent combustion withcause detailed experimental data are not enough.
In the above paragraph, the expression of ‘higha detailed kinetic mechanism, around 20 years has

Reynolds number’ is used to represent turbulent combus-

passed. Nowadays, 3D DNS becomes a powerful tool intion phenomena. However, this expression is not always

turbulent combustion research. Similar to the research

appropriate to characterize the turbulent combustion be-

trend in DNS of non-reacting turbulence, DNS of turbu-

cause that consists of two phenomena: turbulence andlent combustion is used for investigation of turbulent

flame, and both of them have independent representative

flame structure which is hardly obtained by experimental

scales. Reynolds number only represents physical state

studies and development/ validation of turbulent com-of turbulence side. Even for the same Reynolds number,bustion models. However, Reynolds number in DNS is

if the characteristic scale ratios of two phenomena arevery low compared with that in many combustors in realdifferent, characteristics of turbulent flame are signifi-

applications even for state of the art DNS and then DNS

cantly affected. It should be noted that the product ofat high Reynolds number has been required to understand the turbulent combustion phenomena in real com-velocity scale ratio (u’rms/SL) and length scale ratio (l/F)
represents Reynolds number based on the integral lengthbustors. Figure 3 shows a result of our resent 3D DNS ofscale. For the cases in which velocity scale ratio is high,turbulent premixed flame (Shim et al., 2011). Visualized

we call this state ‘high intensity turbulent premixed

quantity is heat release rate which is rate of energy con-flames’. Since these flames are hardly attained in labor-

version to heat from chemical bond. Reynolds number

atory scale burners and measurements in this condition isbased on Taylor micro scale (Re) of this DNS is aboutquite difficult, detailed flame structures have not been

120. Higher Reynolds number case (Re = 220) is also
conducted by using about 1.8 billion grid points. Com-

clarified.

pared to DNS of non-reaction turbulence, one may feeldrogen-air turbulent premixed flame is shown (Shim et

In Fig. 4, one example of DNS of high intensity hy-- 30 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

(a)

(b)

Fig. 5 DNS of turbulent jet premixed flame (Re = 97.1).
Instantaneous contour surfaces of temperature (T= 1400 K, clear yellow) and eddies (bronze), andFig. 6 Pressure effects on flame structure in turbulent

distribution of OH mole fraction on a typical x-y

V-shape premixed flame. Distributions of temper-

plane.

ature at (a) 0.1 MPa, (b) 0.2 MPa.

al., 2013). In this condition, since time scale of elemen-

flame (Shimura et al., 2012). In this case, flame is sus-tary reaction and that of coherent fine scale eddies be-ained by surrounding burnt gas. In this geometry, sincecomes in comparable order, inner structure of local flame

velocity difference between unburnt mixture and burnt

elements reflects effects of turbulent motion and showsgas makes mean shear, flame structures are affected bothparticular behaviors. In Fig. 4, concentration of one im-by the mean shear and turbulence, and their statistical

portant radical, HO2, which is shortly created in the pro-nature is very important for the developments in turbu-

cess of hydrogen combustion, is presented. In laminarlent combustion models used in numerical analyses offlame, this radical is created in front of thin flame sur-real combustors.

face (unburned side). In high intensity turbulent flames,

In addition to the geometry of combustion fields,

however, that is transported in low temperature regions

pressure effects should be overcome in DNS of turbulent

where that radical hardly exists in general condition by

combustion to clarify characteristics of turbulent flames

the strong turbulent motion. The authors called this phe-in engineering applications. In general, combustors innomenon as ‘radical fingering’.

real applications are operated at several tens of atm (orseveral MPa). However, most of turbulent combustion

2.3 DNS of Turbulent Premixed Flames in Complex

DNS have been conducted at atmosphere pressure condi-

Geometry
Recent DNS of turbulent combustion has been ex-tion (0.1 MPa). In high pressure conditions, Reynolds

tended to relatively complex combustion geometry suchcrease of kinematic viscosity. Pressure effects appear notas jet flames (Sankaran et al., 2007; Bell et al., 2007;

only in flow fields but also in flame structures. For the

Hawkes et al., 2009; Shimura et al., 2012), V-shapeflame side, flame thickness at the high pressure becomes

flames (Bell et al., 2005; Minamoto et al., 2011; Fuku-

thinner even for the same fuel, which results in an in-

shima et al., 2013), swirling flames (Tanaka et al., 2011;

crease of grid points in DNS compared to the condition

Luo et al., 2011; Wang et al., 2011; Aoki et al., 2014;

of 0.1 MPa. From these reasons, DNS at high pressure

Minamoto et al., 2015), flame propagation in a constantconditions have been difficult. In Fig. 6, temperaturevolume vessel (Yenergdag et al., 2014), etc. Figure 5

distributions obtained from DNS of turbulent V-shapeshows an example of DNS of turbulent jet premixed

flame at different pressures are shown on a typical crossnumber of the flow field further increases due to the de-- 31 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

cal vortical structures in the vicinity of inlet and finescale eddies develops in downstream region. These vortical motions distort flame geometry, and results in spatial fluctuations of heat release rate. Furthermore, DNS
of turbulent flame in a combustor enable us to analyze
heat loss from wall, which is significant information forimprovement of thermal efficiency of combustors. These
sorts of physical quantities are influenced by pressure
oscillation in the combustor. From the DNS of turbulent
flame in the cuboid combustor, relations between acoustic field and heat release rate in three dimensions can berevealed (Aoki et al., 2014), which will be beneficial tounderstand experimental data on pressure fluctuations
and heat release rate in lower dimensions.
Ignition and flame propagation mechanism are keys
Fig. 7 DNS of turbulent swirling premixed flame in afor development of high efficiency spark ignition engines.

cuboid combustor. Eddy structures are visualized

Figure 8 shows examples of DNS of flame propagationby contour surface of the second invariant of ve-

in a confined vessel (Yenerdag et al., 2014). Flame islocity gradient tensor (gray). Volume renderedignited by a hot spot at the center of the vessel andheat release rate is shown in orange region. Heat

propagates in isotropic turbulence. Flame front is dis-

flux on side walls are shown by the rainbow color

torted by the turbulence motion and the flame wrinklingmap.

increases with increase of pressure in confined vessel.
This DNS will contribute to the clarifications of effects

section. In Fig. 6 (a), temperature distribution for 0.1

of fine scale eddies in turbulence on ignition and flame

MPa (Minamoto et al., 2011) is shown. In Fig. 6 (b), that

propagations, pressure effects on flame propagation infor 0.2 MPa (Fukushima et al., 2013) is presented for

turbulence, and flame-wall interactions.

comparison. DNS at 0.2 MPa is just running by using

These simulated DNS data sets are very useful toabout 1.5 billion grid points which is 3.4 times larger

elucidate underlying physics of turbulent combustion, as

than those of DNS at 0.1 MPa. Only from the increase of

they include all the relevant information. Also, these data

grid points, one may understand the difficulty of DNS atbases are extensively used for the development ofhigh pressure conditions. In these cases, flame is sus-

mathematical models that describe the non-linear phe-tained by a high-temperature rod and V-shape flame isnomena without explicitly resolving all the scales (Mi-created behind the rod. Figure 6 suggests that turbu-

namoto et al., 2015; Hiraoka et al., 2016a; Hiraoka et al.,lence-flame interactions strongly depend on the pressure,

2016b).

and those induce complicity in turbulent flame structures.
It should be noted that more detailed investigations will
be needed to clarify the turbulent flame structures at high

2.4 New Extensions of DNS of Turbulent Combustion
Recently, as a new extension of DNS of turbulent

pressure conditions.

combustion, many research works have been reported

In a lot of gas-turbine combustors, swirling flow isabout the investigation of HCCI (homogeneous charge

utilized for flame stabilization. To clarify characteristics

compression ignition) combustion. Since HCCI combus-of flow and flame structures in gas-turbine like combus-

tion has an instinct nature of low emission and high effi-

tors, DNS of turbulent swirling premixed flames in a

ciency, this will be one of promising combustion tech-cuboid combustor has been conducted (Tanaka et al.,nology applying in gas engines in local power generation2011). Figure 7 shows an example of the DNS with swirl

system and in IC engines for automobiles. However,number of 1.2. Swirling flow generates large-scale heli-difficulties in the control of ignition prevent the engi
- 32 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

(a)

(b)

(c)

(d)

Fig. 8 Temporal development of the local heat release rate at the flame front (orange-red) and turbulent vortices (gray)in the confined volume vessel. (a) t = 97.2 s, (b) t = 264 s, (c) t = 360 s, (d) t = 456 s.
neering applications of HCCI. To understand HCCI(a)

combustion mechanism, DNS of HCCI combustion has(b)been conducted (Chen et al., 2006; Fukumoto et al.,2011; Yoo et al., 2013; Zhang et al., 2013). Figure 9shows a result obtained from DNS of HCCI turbulent
combustion (Fukushima et al., 2014). In this DNS, methane-air mixture is used for fuel and a detailed kinetic
mechanism with 53 species and 325 elementary reactions
are considered. The pressure condition is about 50 atm

Fig. 9 DNS of HCCI turbulent combustion. Examples ofwhich is near those in real applications. In HCCI en-

results of fluid particle tracking analysis t = 0 s -gines, premixed gas is compressed rapidly up to very

60 s (a) and 300 s - 360 s (b). Red and blue
show high and low temperature.

high pressure and auto-ignition is enhanced. To reduce
drastic pressure raise due to auto-ignition, fluctuations oftemperature or equivalence ratio are introduced in gen-near future.

eral. However, real feature of HCCI combustion with

For the distributed power generation system and portabletemperature or mixture fluctuations has not been clari-power generation, micro combustion technology is in the

fied yet. To investigate local combustion characteristics

spotlight. Although combustion phenomena in microin HCCI condition, Lagrangian particle tracking havecombustors are in laminar state in general, detailed in-been conducted with DNS of flow and combustion field.

formation obtained from DNS gives great impacts on

From temporal development of physical and chemical

understanding of the micro combustion. The application

quantities on a particle, combustion regimes dependingof DNS to micro flow reactor (Miyata et al, 2015) is one

on local temperature and mixture condition at the initial

typical example.

stage have been clarified. This DNS is still limited in
two-dimension, while this will be extended for 3D in the- 33 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

2.5 Projection of Turbulent Combustion DNS
To show the possibility of the perfect simulation ofinternal combustion engines, a trend for gird points usedin DNS of turbulent combustion is shown in Fig. 2. DNS
of turbulent combustion plotted in Fig. 2 have been conducted for hydrogen fuel with a detailed kinetic mechanism including 12 reactive species and 27 elementary
reactions. From the estimation in Sec. 2.1, required
memory is about 5×103 times that for incompressible
turbulence. Figure 2 shows that the number of grid points
used in DNS of turbulent combustion is lower than thatof turbulence with this factor. The increasing rate of thegrid points used in DNS of turbulent combustion also
coincides with that of #1 SC similar to DNS of turbulence. If the K computer is used, 1010 gird points will be
used for DNS of turbulent combustion. For 1 EXA Flops,

Fig. 10 One of the typical time-series velocity vector

that with 1012 will be reported in 2020. Here, it should be

distribution

noted that this expectation is for hydrogen fuel. In manytime-resolved PIV system (Tanahashi et al.,combustors in engineering applications, hydrocarbon

2004b)

imagesobtainedbythefuels are used. As for heptane which is one compositionof gasoline, a detailed kinetic mechanism will be con-to 5 weeks. In addition to these numerical aspects, phys-structed from 544 reactive species and 2446 elementary

ical investigations are required for Soret effect, Dufour

reactions. In this case, the number of available grid

effect, pressure gradient diffusion, wall-flame interaction,

3

points is reduced to about 30 billion (3100 ) due to in-radiation heat transfer, etc.

crease in required memory. Here, based on the first three-

Even though the full simulation of IC engine is pro-dimensional DNS of turbulent combustion, CPU time

jected to be realized in 2020, those of large scale com-which is required for DNS of a cylinder of an automobile

bustors such as gas turbine in power generation plant are

IC engine (1600 cc) can be estimated. Even for the pre-still impossible. Therefore, for accurate numerical pre-sent #1 SC which has 93 PFlops, 3 years are required for

dictions of combustion phenomena in engineering appli-

the perfect simulation of IC engine. If the 1 EXAFlops

cations, high-accuracy turbulent combustion models

SC is realized, this CPU time is reduced to about 4

should be developed by including many turbulent com-months. This estimation suggests that DNS of IC engines

bustion mechanisms.

is possible in 2020.
To shorten CPU time required for DNS of turbulent

3. STATE OF THE ART—LASER DIAGNOSTICS

combustion, two numerical methods are introduced to

OF TURBULENCE AND TURBULENT COM-

full-explicit full-compressible DNS code. One is com-

BUSTION

pact finite difference filter to reduce spatial resolution
requirements and to eliminate higher spatial frequency

3.1 Fundamentals of Particle Image Velocimetry

oscillations than the spatial resolution, and another iswell-known point-implicit scheme to avoid quite small(PIV) and Direct Comparison with DNS
Coherent fine scale eddies are investigated and theirtime integration of the order of nanosecond for fully ex-statistical characteristics are found to be independent ofplicit DNS. Availability and accuracy of these numericalthe Reynolds number and flow geometry in previous

methods have been confirmed carefully for auto-ignition,

DNS studies (Jimenez et al., 1993, Tanahashi et al., 1997,

planar laminar flame and turbulent premixed flames (notTanahashi et al., 2001, Tanahashi et al., 2004a, Alamo etshown here). These modifications reduce the CPU timeal., 2006, Wang et al., 2007). The diameter and the- 34 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

3.0
(b)

<u>/uk

(a)

2.5
2.0
1.5
1.0
0.5
0.0
0

2

4

r /

6

8

10

Fig. 11 Distribution of second invariant and velocity vectors around the typical fine scale eddy (a) and mean azimuthal
velocity profile of the typical fine scale eddy (b).
maximum azimuthal velocity of coherent fine scale ed-

to the turbulent jet flow. Figure 11(a) shows distribution

dies are scaled by the Kolmogorov length ( and theKolmogorov velocity (uk). The most expected diameterof second invariant and velocity vectors around a typicaland maximum azimuthal velocity are close to 8 and
1.2uk except very near the wall (Tanahashi et al., 2004a).

is subtracted from the velocity field, and the color repre-

Moreover, these fine scale eddies travel with the velocity

tive Q and black does negative one. The angle betweenof the order of the velocity fluctuation u’rms, and the az-

the vorticity vector at the center of the eddy and theimuthal velocity of an intense fine scale eddy can reachmeasurement planes is 87.3 degrees. The asymmetric

up to 3 - 4 u’rms. To investigate the coherent fine scalecharacteristic of the fine scale eddy shown in Fig. 11(a)eddies, high spatial resolution time-series PIV system, ofagrees well with that obtained from DNS (Kang et al.,which time-resolution is potentially applicable from sev-

2006). Figure 11(b) shows mean azimuthal velocity pro-eral hundreds of Hz to several tens of kHz, has been de-file of the typical fine scale eddy. The radius and theveloped.

fine scale eddy.

The velocity at the center of the eddy

sents the second invariant. Here, white represents posi-mean azimuthal velocity in Fig. 11(b) are normalized by

To investigate the coherent fine scale eddies, and uk which are obtained from the present PIV. The

time-resolved stereoscopic PIV system, of whichradius of the fine scale eddy is defined as the distance

time-resolution is potentially applicable from several

between the center and the locations where the mean

hundreds of Hz to several tens of kHz, has been devel-azimuthal velocity shows the maximum or minimum

oped. The developed system was applied to flow field ofvalue. Thus, the diameter of this eddy is 11.4 and thea turbulent jet and fluid flow maps with about 2 kHz

maximum azimuthal velocity u,max is 2.5 uk.

time-resolution were demonstrated (Tanahashi et al.,2004b). Figure 10 shows one of the typical time-series

3.2 Quad-plane Stereoscopic PIV

velocity vectors images obtained by the PIV system. In

The dual plane stereoscopic PIV was used to investi-

addition, measurements with various time-resolution up

gate characteristics of turbulent eddies in a lot of studies.

to 26.7 kHz were also conducted to estimate the perfor-

Actually there is a disadvantage in evaluation of velocity

mance of the developed high spatial resolution

gradients in the out-of-plane direction. Although

time-series PIV system (Tanahashi et al., 2004b). FromDPSPIV extends the velocity distribution in another di-

the time-series PIV measurement with high temporal andmension, the number of data points in the third dimen-

spatial resolution, dynamics of the turbulent structuresion is inherently limited in two. Moreover, the computa-

were visualized clearly. This PIV system was extended totion of the out-of-plane velocity gradient is restricted todual-plane measurement by utilizing the difference of

2nd order scheme, and the spatial resolution of the ve-polarization of lasers (Tanahashi et al., 2008a). The

locity and velocity gradient are determined by the laser

time-resolved dual-plane stereoscopic PIV was applied

light sheet thickness. Recently, the tomographic PIV- 35 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

camerafilterlensparallel beam
forming optics

PBS

mirrormirrorlaser sheet
dichroic
forming optics mirror

dumper

PBS
PBS

test section
-v

-v

PBS

532
560

-h

mirrormirror-h532

532 nmNd:YAG laser #1

half wave plate

parallel beam
forming optics

560

560 nmPBS
dye laser #1
Rhodamine 6G

half wave plate

dye laser #2
Rhodamine 6G

half wave plate

PBS
Nd:YAG laser #2

PBS mirror

mirrorswitchdelay pulse generator

Fig. 12 Schematic of the quad-plane stereoscopic PIV system.

50

(a)

40

40

30

30

20

20

10

10

x/η

x/η

50

0

(b)

0

-10

-10

-20

-20

-30

-30

-40

-40
-50

-50
-70 -60 -50 -40 -30 -20 -10

0

y/η

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10

70

0

y/η

10

20

30

40

50

60

70

Fig. 13 Instantaneous snapshots of Q (a) and ɛ (b) on the mid-plane at Re= 128.9; corresponding in-plane velocity
vectors are superposed. The every other velocity vectors are presented for readability: the visualized vector
spacing is 1.7.
technique, which provides 3D3C (three dimensional,

biguity of the particle position, but it requires further

three component) velocity field in the measurement

particle 3D reconstruction which is computationally ex-

volume, has been attracting attention for almost one

haustive. For these reasons, higher accuracy in calculat-

decade, whereas the spatial resolution is limited mainly

ing the full velocity gradient tensor is desired for turbu-

by the particle concentration: increasing the number oflence measurements under high Reynolds number condi-

particles included in one voxel generally ameliorates thetions. Hence, quad-plane stereoscopic PIV (QPSPIV) has

quality of cross correlation, but excessive particle con-been developed in our recent study (Naka et al., 2016).

centration would cause ghost particles which are a main

The QPSPIV is accomplished by the difference ofsource of error of this technique. The ghost particles canlaser wavelengths and polarizations. A schematic of thebe reduced by adding extra cameras eliminating the am-present QPSPIV system is shown in Fig. 12, and its de- 36 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

scription can be found in the literature (Naka et al., 2016).

with DNS. In this section, the progress of laser diagnos-

The advantages of the QPSPIV especially in the effect oftics for understanding turbulent combustion mechanism

the order of the finite difference schemes in theis presented.

out-of-plane direction was confirmed by resampling the

To investigate turbulent flame structures experimen-

velocity data in-plane directions so that the distance oftally, planar laser induced fluorescence (PLIF) of mole-the data points becomes comparable to the distance ofcules and radicals produced by chemical reactions suchthe light sheets which roughly corresponds to the 0%

as OH, CO, CH and CH2O are commonly used. Since

overlap. This can mimic the velocity distributions in the

OH radicals show high concentration in the burned gas,

out-of-plane direction, and is effective to check the spec-

OH PLIF is useful to distinguish the burned gas from thetral response.

unburned mixture. Although the edges of OH radical

Measurements were undertaken in the developed re-distribution may correspond to the flame fronts for low

gion of the turbulent jet with a spatial resolution suffi-

Reynolds number turbulent flames, there is a possibility

cient to capture the small scale structures. Figure 13

that flame fronts do not exist at the edge of OH radicals

shows the 2D field of Q and ɛ on the middle plane at Rein high Reynolds number cases in which flame front is=128.9. The axes x and y are normalized by . The pattern of the velocity vector looks to represent some pack-significantly distorted and multiply folded. On the otherets of swirling motions. The distribution of Q becomestics of the flame front in turbulence because CH radicals

positive near the center of such rotating motions. On theare produced at the flame front and have very narrowother hand, ɛ takes relatively large value slightly off fromwidth enough to represent the reaction zones. However,the center. It is noted that every other velocity vector is

it is hard to distinguish the unburned gases from burned

displayed for readability: the visualized vector spacing isgases only from the CH PLIF. To overcome these defects

1.7. From these visualizations, it is fair to say that thefine scale structures are well resolved and the size of thein single radical PLIF, simultaneous PLIF for mul-measurement area is enough to have several units of suchbeen required. In addition to the PLIF measurement, par-structures in the present measurements. From the highticle image velocimetry (PIV) has been adopted tospatial resolution multi-plane PIV enables us to evaluate

measure turbulence characteristics near the flame (Don-characteristics of the fine scale eddies in turbulence inbar et al., 2001). To obtain the detailed information aboutcomparison with DNS. The QPSPIV can be applied toturbulent velocity field, 3C velocity measurement suchvarious flow fields and to the higher Reynolds number

as a stereoscopic PIV is desirable.

hand, CH PLIF has been used to investigate characteris-ti-species, such as OH and CH (Donbar et al., 2000), hasconditions.

Figure 14 shows schematic of the measurement system of simultaneous CH-OH PLIF and stereoscopic PIV

3.3 Fundamentals of PIV and Planar Laser Induced

measurement (Tanahashi et al., 2005) to clarify relation

Fluorescence (PLIF)As denoted in Sec. 2, characteristics of the turbulent

between local flame structure and turbulence characteris-premixed flames are investigated extensively in the re-ample of measured CH and OH distributions and 3C

cent 3D DNS, and local flame structures which differ

velocity map obtained in turbulent methane-air premixed

from theoretical classifications have been clarified. 3Dflames on a model swirling combustor of gas turbine.

flame structures, which are caused by strong fine scale

The area denoted by a white box in the CH and OH im-

eddies in turbulence and described as the handgrip, spireages corresponds to the measurement region of the ste-

structures and radical fingerings, appear even in the cor-reoscopic PIV. In CH and OH images, red color repre-

rugated flamelets regime of the combustion diagram, andsents high concentration. In the velocity map, the x and yenhance flame wrinkling and local heat release rate. The

components are shown by arrows and the z component isimportance of these structures should be investigated byrepresented by color. The velocity from behind to front

the experiments, while there are a few experimental

has positive value and is denoted by red. Flame fronts

techniques which have high accuracy enough to compare

have large-scale wrinkling of the order of the integral

tics in turbulent premixed flames. Figure 15 shows ex-- 37 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

(a)

(b)

Fig. 16 Typical three-dimensional flame structure obFig.14 Schematic of simultaneous OH-CH PLIF andtained by triple-plane PLIF. (a) and (b) represent

stereoscopic PIV system.

different realizations.
(Ueda et al., 2009) are useful to achieve the objectives.
Figure 16 shows examples of 3D measurements of flame
surfaces by a triple-plane PLIF (Ueda et al., 2009). The
isolines drawn by blue and white represent fluorescenceintensity distributions of CH radicals in each plane, andFig. 15 CH (left) and OH (center) fluorescence images

the yellow area represents the unburned mixture obtained

and three-component velocity map (right) (Reby fluorescence intensity distribution of OH radicals.

= 115.0,  = 1.0).

The visualized distance between two measured planes istwenty times the real scale for easy understanding. By

length scale and small-scale wrinkling less than the Tay-using the multi-plane PLIF, two principal curvatures ob-

lor micro scale. By detecting flame front from CH PLIF

tained and curvature terms in the flame stretch concept

image and estimating a unit vector normal to the flame

will be evaluated.

front from the gradients of OH at the flame front, flame

Moreover, to investigate effect of tangential strain

curvature can be obtained experimentally. Furthermore,

rate simultaneously, dual-plane CH planar laser induced

from the 3C velocity vector, strain rate acting on the

fluorescence (PLIF), single-plane OH PLIF and du-flame surfaces can be evaluated. Since behavior of local

al-plane stereoscopic particle image velocimetry (PIV)

flame elements is discussed based on the flame curvature

measurement has been developed. This simultaneousand tangential strain theoretically, this kind of combined

measurement can provide three-dimensional flame front,

laser diagnostics is very important to investigate turbu-

three velocity components and nine velocity derivatives.

lent combustion mechanism. Noted that, even in this

Details can be found in the literature (Shimura et al.,combined laser diagnostics, analyses are limited in two2011). Dual-plane stereoscopic PIV realizes simultane-

dimensions.

ous measurement of 3 velocity components and 9 velocity gradients, which gives strain rate exactly without any

3.4 Multi-dimensional/Multi-variables PLIF and PIV

assumptions. Since flame normal direction can be evalu-

in Turbulent Combustion
The above mentioned combined laser diagnostics

ated from the OH distribution, strain rate tangential to
the flame surface can be obtained. Hence, this method ishave been limited to measure two-dimensional behaviors

oneof turbulent premixed flames. To evaluate flame curva-

flame-turbulence interactions.

ofpromisingmethods

toinvestigate

ture and tangential strain rates exactly, 3D measurementsof turbulent flame are required. Crossed-plane PLIF
(Knaus and Gouldin 2000) and multi-parallel-plane PLIF
- 38 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

Fig.17 Local flame displacement speed obtained from double-pulsed CH PLIF. Red: 1st shot, blue: 2nd shot, vectors:
local flame displacement speed.
3.5 Insight into Dynamics of Turbulent Flames with

has

been

significantly

developed.

Consequently

High Temporal Resolution PIV and PLIF
Dynamics of turbulence and flame affects turbulent

high-repetition-rate PLIF and PIV measurements haveflame structures, and high repetition rate PLIF and PIV

bulent flames. The high-repetition-rate dual-plane OH

measurements are significant to understand turbulent

PLIF and stereoscopic PIV has been developed and used

combustion mechanism. In the concept of the flamefor investigation on local turbulent displacement speed

stretch, increasing rate of the flame area is expressed by(Trunk et al., 2013, Peterson et al., 2015). In our previ-

flame displacement speed, flame curvature and strain

ous work, a high-repetition-rate simultaneous measure-rate at the flame front. To confirm the flame stretch con-ment of CH–OH PLIF and stereoscopic PIV has been

cept experimentally, measurements of the flame dis-developed (Johchi et al., 2015). The high-repetition-rate

placement speed are necessary. For this purpose,

simultaneous measurement system has an acquisition

time-resolved OH PLIF has been developed (Kaminski

rate of 10 kHz and its measurement duration time ex-

et al., 1999). CH double-pulsed PLIF, which was origi-

ceeds 1.0 s.

got to have potential to investigate the dynamics of tur-nally developed for study on stabilization mechanism of

The developed system was applied to the me-turbulent non-premixed jet flame (Schefer et al., 1994;

thane–air turbulent jet premixed flame in order to inves-

Watson et al., 2002), has been utilized to evaluate flame

tigate the flame and flow dynamics of the turbulent

displacement speed in turbulent premixed flames

combustion field. Examples of the representative PLIF

(Tanahashi et al., 2008b). Figure 17 shows vector map ofimages and distributions of fluctuating velocity acquired

flame displacement speed with successive CH fluores-by this system are shown in Fig. 18. The illustrated area

cence images. In these figures, red and blue color repre-

is approximately 13 mm × 13 mm. The quantized lightsents 1st and 2nd CH fluorescence image, respectively.

intensity in the OH and CH images was mapped from 0

White vectors represent local flame displacement speed.

(minimum) to 1 (maximum), and the color scale in Fig.

Directions of flame displacement vectors coincide very

18 is adjusted to better represent the instantaneous dis-well with observations from successive CH fluorescence

tributions. The arrows and background indicate theimages. Detailed analyses of this experimental result

in-plane (x and y) and out-of-plane (z) component of thesuggest that theoretical conventional model might bevelocity fluctuation, respectively. The mean velocity

modified. If the CH double-pulsed PLIF is conducted

distribution obtained from 10,000 samples was subtract-with a stereoscopic PIV simultaneously (Tanahashi et al.,ed from the instantaneous velocity. The continuous flame

2008c), direct measurement of local burning velocity isshape at U0 = 10 m/s (result not shown) turns to exhibit

possible, and direct validation of the concept of the flame

complicated geometry with several flame wrinkles at U0

stretch will be realized.

= 15 m/s (Fig. 18). Due to the entrainment of burned gas

Recently technologies for high speed visualizationin the unburned region, flame front sometimes appears to- 39 -C 2017 The Heat Transfer Society of Japan○

Thermal Science & Engineering Vol.25 No.3 (2017)

Fig. 18 PLIF images of OH (upper row) and CH and fluctuating velocity fields (lower row) for U0 = 15 m/s, x/Din = 7
and Re = 173.1.
be disconnected. Between the disconnected unburned

compare with DNS. The direct comparisons between

mixtures, the velocity field exhibits strong flow motion.

experiment and DNS lead to deep insight into turbulence

At U0 = 20 m/s (not shown here; see Johchi et al., 2015),

and turbulent combustion, which will contribute to thethe unburned mixture in the main jet frequently divides

development of high efficient and low emission com-in two in the streamwise direction. The large scale tur-bustors.

bulent structures in the downstream region are caused by
Kelvin–Helmholtz instability of the shear layer. Consequently, large scale islands of unburned mixture are cre-

ACKNOWLEDGMENT
The authors acknowledge the financial support ofated and ejected into the downstream. Around the flame

Grant-in-Aid for Scientific Research (A) (No.15206023),

tip under the condition of high Reynolds number, the(S) (No.18106004), (B) (No.19360097), and (S)formation of fine scale unburned mixture islands was(No.23226005) of Japan Society for the Promotion ofobserved and its contribution to enhance the turbulent

Science, by Grant-in-Aid for Young Scientists (S)burning velocity was also investigated by statistically(No.20676004) of Japan Society for the Promotion ofanalyzing the local consumption rate (Johchi et al., 2015;

Science, by Grant-in-Aid for Young Scientists (A)Shimura et al., 2016). These high-repetition-rate and(No.16686011) of the Ministry of Education, Culture,long-duration simultaneous measurement of PLIF andSports, Science and Technology, Japan and by the NEXTPIV will give more detailed information on dynamics ofprogram (No. GR038) initiated by the Council for Sci-

turbulent flames in the near future.

ence and Technology Policy (CSTP), Japan.

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