16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012

Simultaneous high-speed PIV and OH PLIF measurements and modal analysis
for investigating flame-flow interaction in a low swirl flame
Per Petersson1,2,*, Rikard Wellander1, Jimmy Olofsson2, Henning Carlsson3,
Christian Carlsson3, Bo Beltoft Watz2, Nicolas Boetkjaer2, Mattias Richter1,
Marcus Aldén1, Laszlo Fuchs3, Xue-Song Bai3
1: Division of Combustion Physics, Lund University, Lund, Sweden
2: Dantec Dynamics A/S, Skovlunde, Denmark
3: Division of Fluid Mechanics, Lund University, Lund, Sweden* Correspondent author: per.petersson@forbrf.lth.se

Abstract Simultaneous High-Speed OH planar laser-induced fluorescence (PLIF) and particle image
velocimetry (PIV) measurements and Large Eddy Simulations (LES) were performed in turbulent low swirl
stratified premixed flames to investigate the flame/flow interaction in detail. The combined PIV and OH
PLIF images where captured with up to 4 kHz, sufficient to follow the large-scale spatial and temporal
evolution of flame and flow dynamics. Based on the PIV and OH PLIF images collected for two different
flame conditions modal analyses were performed to identify the large scale flow structures and their impactof the flame stabilization. Important characteristic features identified from the data include the strong local
dilatation of the flow field due to expansion in the flame creating regions with low axial velocity upstream of
the flame where the flame can propagate upstream and expand in the radial direction. The fluctuation part ofthe velocity field shows strong impact of the wrinkling of the flame front. The large-scale vortex structurescontinuously created in the inner shear layer influence the mixing of unburnt and burnt gases and thereby
they allow for the flame stabilization in the low-swirl flame. Oscillating Pattern Decomposition (OPD) isapplied on the time-resolved data for characterization of the flow field in different regions whereas Proper
Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are applied on 3D LES datafor visualization of coherent large scale helical structures.

1. Introduction
Flame/flow interactions are of fundamental importance in combustion science; both the interaction
between the flame and large-scale flow structures [1] and the interaction on the micro scale [2] canplay an important role in the propagation and stabilization of the flames. The structure anddynamics of turbulent premixed stratified flames are influenced by many parameters, e.g. local gas
velocity and turbulence intensity, entrainment of the ambient, large-scale structures of the flow,
curvature and strain rate of the flames. To gain an overall insight to the flame flow interaction
simultaneous measurements of the velocity field and scalars are required [3,4]. In this content
planar laser-induced fluorescence (PLIF) is frequently used to investigate turbulent premixed flame
structure and fuel distribution, and particle image velocimetry (PIV) is used to measure velocity
fields. The complex time-dependent and three-dimensional nature of the flame-flow interaction is achallenge for both numerical and experimental investigations. In order to capture the importantcharacteristics of the flames it is necessary to perform time-resolved simultaneous measurements ofthe velocity field and one or several appropriate scalars such as a flame front marker. Depending one.g. the investigated flame regimes (e.g., flamelets, thin flame, or distributed reaction zone regimes)
different flame front markers can be utilized. The OH radical are the first choice for high-speed(kHz) measurements due to the relatively strong signal compared with minor species, e.g. the CH
radicals [5]. Being an active species participating in various elementary reactions, OH is reaching
the highest levels at the flame front and thereafter it decreases in the post flame region. In flames
with a continuous flame surface the maximum gradient of the OH signal may be used to define an-1-

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012

approximate representation of the flame front.
Simultaneous stereo PIV and CH/OH PLIF measurements in turbulent premixed flames werereported in [6]. Boxx et. al. [7] demonstrated the potential of a combined high-speed PIV and PLIF
system in lifted-jet and swirl-stabilized flames. The out-of-plane velocity component was found to
be a significant factor in the flame dynamics and it is desirable to acquire such data to better
understand 3D effects and turbulent transport in flames.
In the present study high-speed simultaneous OH PLIF and PIV were applied to study a low-swirl
lean premixed methane/air flame. Various versions of the low-swirl burner have been used inseveral research groups [8-16] to study the fundamental structure of turbulent premixed flames.
Characteristics for the low-swirl flames are that the interaction between turbulence eddies and thechemical reactions occur in both the laminar flamelet and the distributed reaction-zone regimes of
the turbulent premixed flame regime diagram [17,18]. In a joint LES and single-shot PIV/OH PLIF
study [14,16] it was found that the large scale structures generated in the inner shear layer could be
responsible for the flame to be stabilized at a relatively low height. However, due to the lack of time
resolved experimental data this finding could not be confirmed experimentally. This has motivatedthe present investigation in which we perform time resolved (up to 4 kHz), simultaneous OH PLIF
and PIV in the leading flame fronts.
The 2D and 3D time resolved data pose a great challenge in the post-processing process.
Conventional method such as the decomposition of the instantaneous data to mean and fluctuations
can be useful to compute the statistics. Fourier spectrum analysis is often used to identify theenergetic scales and low frequency coherent flow structures. Special analysis methods such as
Proper Orthogonal Decomposition (POD) are often used to reduce large amount data and to identify
the most energetic modes in a flow field or scalar field. To analyze time-resolved experimental data
Bi-Orthogonal Decomposition (BOD) and Oscillating Pattern Decomposition (OPD) are applied forvisualization and characterization of coherent structures the flow field. Dynamic Mode
Decomposition (DMD, [20,21]) are applied on 3D LES data for visualization of coherent large scale
helical structures. In this paper we demonstrate several of the above-mentioned techniques inanalysis of the flame dynamics of the low swirl stratified premixed flame at two Reynolds numbers.

2. Experimental Set-up and Methods
To generate the low-swirl flame the same burner set-up presented in previous work performed at
Lund University was used [14-16], Fig.1a. To assure well-controlled experimental conditions the
set-up, newly calibrated mass flow controllers for methane and air (Bronckhorst Hi-Tec, EL-Flow)
were used, and the co-flow and exhaust system were adjusted. The low-swirl burner is based on thedesign by Bedat & Cheng [9] but updated with a different swirl arrangement. The low-swirl flow iscreated by an outer annular swirler (Fig. 1a) where ~60 % by volume of the mixture pass, with eightswirl-vanes, in combination with an inner perforated plate that allows for about 40% by volume of
the mixture to pass through [14]. After passing the swirler/perforated plate the premixed
methane/air mixture of equivalence ratio 0.62 discharges through a 60 mm long nozzle (diameter of
50 mm) into a co-flow of air at ~0.25 m/s. The resulting outflow from the nozzle has an inner low
velocity non-swirling region (r < 10 mm, r = radius) and an outer region with higher axial andtangential velocities [14].
The turbulent flames considered here are categorized in the laminar flamelet (leading edge of theflame) and the distributed reaction-zone regimes (typically at the trailing edge of the flame) of the
turbulent premixed flame regime diagram and have previously been investigated in several-2-

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012publications [14-16].

(a)

(b)

Figure 1. (a)The low-swirl burner placed in a co-flow region of air. The position of the annular
swirler is schematically shown. The perforated plate creating the inner non-swirling flow region is
not visible; (b) Schematic of the combined high-speed OH PLIF and PIV experimental
arrangement.
Two separate laser systems were used for the combined high-speed PIV and PLIF measurements. A
schematic diagram of the experimental setup is shown in Fig. 1b. The synchronization of the lasers
and cameras was controlled from the DynamicStudio (Dantec) software platform via a highresolution synchronizer unit.
The high repetition rate laser system, used to excite the OH Q1(6) line in the A2-X2Π (1,0) band,consists of a diode pumped Nd:YAG laser (EdgeWave HD 40IV-E) in combination with a highrepetition rate dye laser (Sirah Credo). For the repetition rates reported in this paper (up to 4 kHz)the pulse energy of the 283 nm radiation is kept at ~300 µJ. The laser pulses are formed into ~27
mm high laser sheets using ordinary UV-coated sheet forming optics.
The excited OH radicals were imaged onto an image intensified (Lambert HiCATT 25 Gen 2) highspeed camera (Photron Fastcam SA5), which at full resolution (1024×1024 pixels) has a maximum
frame rate of 7200 fps. An interference filter, designed to reflect light between 275-295 was used tosuppress scattered UV laser light. Although not required for detection, a UG 11 band pass filter isused to protect the image intensifier from any scattered PIV laser light.
For PIV double cavity diode pumped kHz Nd:YLF laser (DualPower 1000-10, Dantec) withbelonging light sheet optics was used. The laser can generate double laser shots with a repetition
rate of up to 10 kHz. For imaging a high-speed camera (SpeedSense 9060) with 1280×800 pixels
was used. The camera has at full resolution a maximum frame rate of 6242 fps. This means that
double-frame images required for PIV can be captured with up to 3.1 fps with full resolution. In thepresent investigation the active chip size was reduced to 865x600 pixels to reach a 4 kHz sampling
rate for PIV. To eliminate background light and flame emission a narrowband 532 nm (+/-10 nm)
interference filter was used.
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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012

The laser beams generated by the two systems were arranged in a counter-propagating manner andformed into overlapping sheets to illuminate a planar region of the flame, crossing the verticalcenterline of the burner, Fig. 1b. In the probe volume the PIV light sheets had a height of ~50 mm
and a thickness of ~1 mm. For PLIF the sheet height was ~27 mm. The simultaneous acquisition ofvelocity and scalars requires a precise superposition of the measurement volumes in the postprocessing. In order to perform this, a transparent calibration target designed to fit the burner nozzle
was imaged on both cameras. In the post-processing, the PLIF images were mirrored to fit the PIV
coordinate system prior to superposition.
The 283 nm laser pulse was fired in between the two PIV pulses to eliminate PIV particle lightscattering and to freeze the flow/flame behavior. Typically a pulse separation of ~30 µs, wasrequired for PIV to resolve the high axial and tangential (out-of-plane) velocities in the outer part of
the flame. As flow tracer ~1 µm-sized ZrSiO4 particles were used.
For the evaluation of the velocity field and the following BOD and OPD analysis the
DynamicStudio software (v. 3.30) was used.

3. Results and Discussion
Two methane/air flames are investigated both with an equivalence ratio of 0.62. The Reynolds
numbers (Re) based on the bulk flow velocity (6.2 and 9.3 m/s) and diameter at the burner exit areabout Re=20 000, and Re=30 000. The laminar flame speed calculated using Peters’ mechanism is
12 cm/s for the inlet gas temperature of 300 K and pressure of 1 atm [19]. An overview of the flow
field [14] and mean flame position [14] is given in Fig. 2. The mean flame brush thickness is ~15mm for Re=20 000, and ~17 mm for Re=30 000 [14].
Figure 2. The time-averaged velocity field (Re=30 000)from stereoscopic PIV and the schematic mean flame
position. Background color corresponds to the tangential
velocity component (swirl). The two regions used for themodal analysis of high-speed PIV data are indicated.
The simultaneous PIV and OH PLIF sequences are used tovisualize characteristic flame-flow interaction in theinvestigated flames. Of particular interest is the vortex
motion in the inner shear-layer and how these structuresinteract with the flame [16]. At the nozzle exit the inner shear-layer is located between the inner low
velocity non-swirling region (r < 10 mm) and the outer region with higher axial and tangential
velocities [14] (see Fig. 2). In this layer, originated from inside the nozzle, vortices are continuously
formed due to the shear-layer instability (Kelvin-Helmholtz instability). A Strouhal number (St)based on the characteristic length and velocity scales of high-speed region axial component gives
St=0.2-0.3 for frequencies in the range of 100-200 Hz.
Several set simultaneous PIV and PLIF images (each with 2000 samples) were collected for both
flame conditions. With a sampling rate of 4 kHz the sample time is 0.5s. An example of a combined
PIV and PLIF image is shown in Fig. 3. The color (gray scale) corresponds to the OH PLIF signal
indicating the flame front and the burnt region. The OH radicals show the highest level at the flame
front and thereafter decreasing to its equilibrium value in the post-flame zone. In the present
unconfined flames OH can also decrease in the post-flame zone due to the entrainment of ambient-4-

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012

air which cools down the hot gas. The steepest OH gradient in the most upstream region or leadingedge of the flame was here used to define the flame front location. Note that the OH contour
resolves only the relative large scale wrinkling of the flame.
a) t=0b) t=2 msc) t=4 ms

Figure 3. The instantaneous flow field (absolute velocities) and the lifted flame. Images
corresponds to Fig.4 a),c)and e) below.
In Fig. 3 the instantaneous flow field, presented with absolute velocities, is shown together with thelifted flame (corresponding to Fig. 4b). Large scale wrinkling of the flame front can be seen in
Fig.3, and the wrinkling structure develops with time. The flow appears to diverge around the
propagating lower flame segment in the center partly of the images in Fig. 3, creating the low speed
regions in both the axial and radial direction (out-of-plane motion is not captured). Carefulinspecting the velocity vector in front of the flame front one can see that the flame front tends topropagate to low speed regions. This typical flame-flow interaction can partly be due to a “bluff
body effect” of the expanding flame, and partly due to the swirling inflow, which is not strong
enough to generate vortex breakdown but strong enough to induce low speed flow region where theflame can reside.
In Fig. 4 (a- f) a short sequence (compare Fig. 3) is shown where the mean value for each velocity
vector has been subtracted to reveal the fluctuating part of the flow. The OH PLIF signal showing
the flame front and the burnt region are also displayed. As will be shown below the flame flowinteraction in the central region can better be understood by observing the flame dynamic inperspective to the fluctuating part of the flow. The following observation can be summarized:a) Vortex structures are found in the central region upstream of the flame and further out towards
the inner shear layer.
b) two counter rotating vortex structures convected downstream by the mean flow start to interact
with the flame.
c-d) the region between the two counter rotating vortex structures (here shown as a reverse axial
flow with the mean subtracted) allows for the flame to locally propagate upstream.
e-f) the flame region between the two counter rotating vortex structures expands in the radial
direction.
Fig. 3 shows that the absolute flow field corresponding to Fig. 4a),c) and e) is not closely correlatedwith the local flame shape (dynamics) in the inner region. The mean velocity, which is stationary
and determines the mean flame position, is not responsible for the dynamics and evolution of flame
wrinkling. The fluctuating velocity, on the other hand, is shown to satisfactorily correlate with thewrinkling and the temporal evolution of the flame front.

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Lisbon, Portugal, 09-12 July, 2012

Note also that the flame dynamics in the upper outer regions (e.g. upper left region in Fig. 4 a-e) is
faster than in the inner region, which is likely due to strong shear in the outer region where KelvinHelmholtz instability can induce large scale vortices. Since the flow is swirling, a strong 3D flow
exists. The 2D images are limited to explain the 3D nature of the flow and flame in this region.
High-speed film sequences indicate a strong erosion of the flame towards the trailing edge of theflame.
a) t=0b) t=1 msc) t=2 msd) t=3 mse) t=4 msf) t=5 ms

Figure 4. a -f) The fluctuating part of the flow field and the lifted flame. Flow structures created
upstream the flame are convected downstream and interacts with the flame. Time between images is
1ms (Re=30 000 ).
In Fig. 5 a sequence of six simultaneous PIV and OH PLIF images shows the typical roll-up of theouter part of the flame base [16]. Note that the outer counter-clockwise rotating vortex structure
creates a reverse flow that can mix burnt and unburnt. This can be compared with the typical flowin the inner region (see Fig. 3) where a reduction of the axial velocity component is created
upstream the flame but no flow reversal occurs.
The presented study indicates that vortices in the outer flame region (in shear layers) are frequently
created, resulting in large-scale reverse flow structures. The premixed flame front propagates in thesame direction as the reverse flow upstream and radially towards the unburned mixture. Thisobservation is consistent with the previous LES and single-shot imaging of the flame [16]. In the
inner region the vortices are mainly noticeable in the fluctuating part of the flow (convection speed
dominates, cf. Fig. 3 and 4). These vortices may not enhance the macro mixing but can create
significant local flame wrinkling. Flame propagation in low speed regions can increase the flame
surface that in turn would increase the consumption speed. Flow reversal can randomly occur also
in the center region and seems to be related to out-of plane motion of the flame/flow.

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Lisbon, Portugal, 09-12 July, 2012a) t=0b) t=0.25 msc) t=0.50 msd) t=0.75 mse) t=1.0 msf) t=1.25 ms

Figure 5. Characteristic flow flame interaction in the outer region (lower flame brush region,
Re=30 000).
Further analysis indicates that certain vortex structures may emanate from the nozzle flow although
the shear layers downstream of the nozzle are responsible for the generation of turbulence as well as
coherent structures. As can be seen in Fig. 4 vortex structures in the inner region are seemingly
convected downstream with the (in mean) rather weak swirling flow in this region (see Fig. 2). The
vortex structures are captured in a time resolved manner with the planar imaging technique (PIV)
due to the low out-of plane motion in this region. In the outer region the swirl is creatingcomplicated flow structures and shear layer(s), Fig. 5. However, the mean and fluctuating part ofthe velocity vectors are not sufficient to identify the origin of the flow structures. Other analysis
methods have to be employed as discussed below.
Modal analysis - Oscillating Pattern Decomposition (OPD)
For multi-dimensional systems a first order linear differential equation has the form:
X t   A  X t 
and the solution can in general be expressed as
X t    cn K n exp n t 

where λn and K n are eigen-values and eigen-vectors of A . K n are normalized unit vectors andcn scale the contributions from the various terms in the summation above. Eigen-values λn of A
can be complex, which makes exp(λnt) describe both growth/decay and oscillations:n   1  n  in 

exp n t   exp  t  n  exp in t   exp  t  n cosn t   i sinn t 
It is evident that the imaginary part ωn of each eigen-value determines the frequency (rad/sec),
while the real part determines stability in terms of the so-called e-fold time τn (sec):τn ≥ 0 : stable mode (amplitude constant or decaying with time)-7-

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012τn < 0 : unstable mode (amplitude grows with time)
If the underlying, linearized, governing equations are known (and simple) the matrix A cannormally be derived fairly easy, but in many situations they are either unknown or too complex forpractical purposes. OPD (and also Dynamic Mode Decomposition (DMD) [20, 21]) strives toestimate the temporal evolution matrix A from a time series of known state vectors (each statevector describing the state of the dynamic system at a certain point in time). OPD is essentially thesame as Principal Oscillation Pattern (POP) analysis [23]. OPD and DMD differ slightly in the way
they estimate this matrix from the experimental data, but once A has been estimated, eigen-values
and -vectors can be estimated easily.
To reduce the computational time the OPD analysis is applied on the outcome of a Bi-Orthogonal
Decomposition (BOD, related to POD) analysis [24]. BOD (and thus OPD) requires time-resolved
data as input. The applied sampling rate of 4 kHz resolved the investigated vortex structures well intime as seen in Fig. 4 and Fig 5. The investigated sequences of 2000 images are sampled with 4 kHz(sample time of 0.5 s). The short sampling time restricts the detection of low frequencies to ~12 Hz
as at least ~6 cycles are required for a reliable result. Following the Nyquist theory frequencies up
to half the sampling rate can be resolved; however, with a recommended 6 samples/cycle and asampling rate of 4 kHz frequencies up to ~600 Hz should be possible.
By analyzing several high-speed sequences of the flow and its fluctuations different flow paths andregions where vortex structures are convected (and sometime created) were identified. Based onthese findings we present several strong modes (with long decay time) from an OPD analysis tohighlight flow structures in different regions of the investigated flow. An overview of the modes isfound in Table 1. The full cycles for mode A and B and show below (Fig. 6 & 7). Both investigatedflame cases show similar modal behavior.
Mode f (Hz) Flow regionA
42.0
Upstream/in flame brushB
35.3
Inner region/following leading edge of flame brushC
137.9
Inner shear layer – swirl region
D
68.9
Inner flow path – no swirl
Table 1. Modes from the OPD analysis with frequencies and flow region of importance.
In Fig. 6. a full cycle of a low frequency (f = 42 Hz) mode (mode A) active in the flame region isshown. The background color show the mean OH signal (flame brush) for the same data set. The
large coherent structures are created just upstream of the mean flame area (Phase angle 90 and 270)
and are convected downstream through the flame. The energy behind these large structures may
come from the increased divergence of the mean flow around the lifted expanding (non-enclosed)
flame. To confirm this comparison must be made with OPD applied to data from a non-reacting(cold flow) case when available.
Mode B shows flow structures emanating from the nozzle and being convected downstream. The
structure follows a path along the leading edge of the flame brush (Fig. 7).These structures are notactive in the burnt region (compare mode A).

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Lisbon, Portugal, 09-12 July, 2012

Mode A: Phase angle 0°

180°

45°

90°

135°

225°

270°

315°

Figure 6. A full cycle for mode A (f = 42 Hz). Large coherent structures are created just
upstream of the mean flame area (Phase angle 90 and 270) and are convected downstream
through the flame. The background shows the mean OH signal (flame brush) for the samedata set. (Re=20000, 4kHz).
Mode B: Phase angle 0°

180°

45°

90°

135°

225°

270°

315°

Figure 7. A full cycle for mode B (f = 35.3 Hz). Flow structures emanating from the nozzle
are convected downstream. The structure follows a path along the leading edge of the flame
brush. (Re=20000, 4kHz).
3D analysis: POD and DMD
The planar data presented above needs to be put into the perspective of the complex 3D nature of
the flow field in the flame region. Large eddy simulations (LES) were therefore carried out for theRe=20000 flame. A flamelet model based on mixture fraction and level-set G-equation validated forboth flamelet and thin reaction zone regimes was utilized. Details of the sub-grid scale flame front-9-

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012

propagation velocity and modification of the level-set approach accounting for local extinction has
been described in [3, 15, 16]. The numerical simulations were performed with an in-house code,
where the transport equations are discretized and solved on a uniform staggered Cartesian gridusing a high order finite difference scheme [22]. The dimensions of the considered domain was
8D×8D×8D, where D is the diameter of the nozzle exit (D=5.0 cm), where two layers of local
refinements of the grid were used in the flame zone region 2.4D×2.4D×4.4D resulting in a meshspacing of 1.4 mm in all directions. LES was performed to simulate the flow field inside the burner
and the swirler to obtain the inflow boundary condition set at the nozzle exit plane. For details
about the inflow and other boundary conditions see [3, 15, 16].
The entire 3D data was sampled at 2000 Hz during 0.1 s. 3D POD on this data was carried out,
decomposing the flow field into orthogonal flow structures ranked by energy content. Figure 8
shows POD modes 0 and 1 visualized through isosurfaces of tangential vorticity. Mode 0,
corresponding to POD average, shows a close to rotationally symmetric inner and outer shear layer
where the outer shear layer is bending outwards and the inner shear layer is bending inwards. Mode
1 shows a lot of scattered structures with small coherent structures formed close to the nozzle exit.
On the POD obtained time coefficients, Discrete Fourier Transform (DFT) was taken. For the first
15 POD modes, representing more than 87% of the total kinetic energy, the DFTs showed strong
influence from two flow frequencies in the vicinity of 200 Hz (St = 0.3) as well as some rather low
frequency patterns (<40 Hz). As previously presented [16], the obtained flow frequencies in thevicinity of 200 Hz are believed to be highly important in enhancing shear layer instabilities andthereby contributing to the stabilization mechanism.

Figure 8. 3D POD modes 0 (a) and 1 (b) visualized through isosurfaces of tangential vorticity withorange and white corresponding to positive and negative vorticity respectively.
For spatial characterization of these frequencies, DMD [20, 21] was performed isolating frequency
specific flow motion. Figure 9 (a) and (b) shows the real part of the DMD modes corresponding to
the two frequencies obtained in the POD analysis, 175 Hz and 211 Hz respectively visualized
through isosurfaces of tangential vorticity. The imaginary part of the modes (not shown here)
implies a rotational motion of the modes along and against the swirling direction of the flow for
Fig. 9 (a) and (b) respectively. Both modes start from the nozzle exit, where mode (a) shows an
origin closer to the nozzle rim while mode (b) has its starting point closer to the nozzle center,corresponding to wall shear layer and inner shear layer induced motion respectively. The modes are- 10 -

16th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 09-12 July, 2012

following a helical motion downstream before breaking up at the flame height. Notable is that mode(a) is classified as an m=2 mode whereas mode (b) is classified as an m=3 mode, corresponding to
two and three “arms” of positive/negative vorticity. The reason for these numbers of arms andorigin of the structures lies in the flow within the nozzle and deserves additional study.
Distinguishing these frequency specific flow patterns with DMD offers a better understanding of
the helical structures induced by the swirler and its resulting shear layers than the decomposition
obtained with POD.

Figure 9. 3D DMD modes corresponding to frequencies 175 Hz (a) and 211 Hz (b) visualized
through isosurfaces of tangential vorticity with orange and white corresponding to positive andnegative vorticity respectively.

4. Conclusions
High-speed sequences of simultaneous flow field (PIV) and flame front location (OH PLIF) havebeen obtained. The sequences give insight into the flow dynamics and the flame-flow interaction.
Vortex structures emanating from the nozzle flow or created in the shear layers just downstream of
the nozzle are convected along different paths. The coherent structures and the paths were also
identified in the OPD modes. The results presented above indicated that vortices in the outer flame
region create a local flow reversal (recirculation) and thus contribute to the large-scale mixing ofburnt and unburnt. In the inner region the vortices are only noticeable in the fluctuating part of theflow (convection speed dominates). These vortices do not enhance the large-scale mixing but cancreate regions for the flame to propagate (in any direction) and locally increase the flame surface.
The 3D nature of the flow was shown via 3D POD and DMD analysis of LES data and visualized
through tangential vorticity. Frequency specific helical structures are found close to the nozzle exit
and in the shear layers and surround the inner flow region (and flame region).

Acknowledgements
This work was supported by the Swedish Energy Agency and the Swedish Foundation for Strategic
Research through CECOST (Centre for Combustion Science and Technology), and the Swedish
Research Council.

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Lisbon, Portugal, 09-12 July, 2012

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