17
American Institute of Aeronautics and Astronautics 1 Frequency Locking and Vortex Dynamics of an Acoustically Excited Bluff Body Stabilized Flame Benjamin Emerson 1 , Jacqueline O’Connor 1 , David Noble 2 , Tim Lieuwen 3 Georgia Institute of Technology, Atlanta, GA, 30318 This paper presents measurements of the forced response of a bluff body stabilized flame. The nonreacting, unforced flow exhibits intrinsic oscillations associated with an unstable, global wake mode. This same global mode persists in the reacting flow at low density ratios, but disappears at high flame density ratios where the flow is dominated by the convectively unstable shear layers. The flow responds quite differently to forcing in these two situations, exhibiting a roughly linear input-output character in the convectively unstable regime, but exhibiting nonlinear behavior, such as frequency locking, during global mode oscillations. In this work, a reacting bluff body wake is subjected to harmonic, longitudinal, acoustic forcing, providing a symmetric disturbance. This experiment is conducted at several density ratios as well as different spacing between the forcing frequency and global mode frequency. As the spacing between these frequencies is narrowed, the wake response is drawn away from the global mode frequency and approaches (and eventually locks-into) the forcing frequency. This observation seems to be linked to the spatial distribution of vorticity fluctuations, as well as the symmetry of the vortex shedding. For example, for large spacing between the forcing and global mode frequencies, there is significant response at the forcing frequency in the shear layers, but little response is observed along the flow centerline (which itself exhibits strong oscillations at the global mode frequency). This seems to be due to the symmetry of vortical structures that are shed at the forcing frequency. For small spacing between these frequencies, however, the vortices shed symmetrically at the forcing frequency, but quickly stagger into an asymmetric configuration as they convect downstream. For such cases, we observe a strong response of the wake at the forcing frequency. The axial distance required for such staggering to occur is a function of the spacing between the forcing frequency and natural frequency, the flame density ratio, and the intensity of the forcing. Nomenclature A = flow cross-sectional area D = bluff body diameter = mass flowrate N e = number of ensembles N s = number of samples Re = Reynold’s number, U lip D/ν St D = Strouhal number, fD/U lip T = temperature U = time-averaged axial velocity c = modal wave propagation speed f = frequency, Hz j = any integer k = wavenumber r U,L = correlation coefficient between upper and lower flame edge positions 1 Graduate Research Assistant, School of Aerospace Engineering, 270 Ferst Dr, AIAA Student Member. 2 Research Engineer, School of Aerospace Engineering, 270 Ferst Dr, AIAA Member. 3 Professor, School of Aerospace Engineering, 270 Ferst Dr, AIAA Associate Fellow.

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Page 1: Frequency Locking and Vortex Dynamics of an Acoustically

American Institute of Aeronautics and Astronautics

1

Frequency Locking and Vortex Dynamics of an Acoustically

Excited Bluff Body Stabilized Flame

Benjamin Emerson1, Jacqueline O’Connor

1, David Noble

2, Tim Lieuwen

3

Georgia Institute of Technology, Atlanta, GA, 30318

This paper presents measurements of the forced response of a bluff body stabilized

flame. The nonreacting, unforced flow exhibits intrinsic oscillations associated with an

unstable, global wake mode. This same global mode persists in the reacting flow at low

density ratios, but disappears at high flame density ratios where the flow is dominated by the

convectively unstable shear layers. The flow responds quite differently to forcing in these

two situations, exhibiting a roughly linear input-output character in the convectively

unstable regime, but exhibiting nonlinear behavior, such as frequency locking, during global

mode oscillations. In this work, a reacting bluff body wake is subjected to harmonic,

longitudinal, acoustic forcing, providing a symmetric disturbance. This experiment is

conducted at several density ratios as well as different spacing between the forcing frequency

and global mode frequency. As the spacing between these frequencies is narrowed, the wake

response is drawn away from the global mode frequency and approaches (and eventually

locks-into) the forcing frequency. This observation seems to be linked to the spatial

distribution of vorticity fluctuations, as well as the symmetry of the vortex shedding. For

example, for large spacing between the forcing and global mode frequencies, there is

significant response at the forcing frequency in the shear layers, but little response is

observed along the flow centerline (which itself exhibits strong oscillations at the global

mode frequency). This seems to be due to the symmetry of vortical structures that are shed

at the forcing frequency. For small spacing between these frequencies, however, the vortices

shed symmetrically at the forcing frequency, but quickly stagger into an asymmetric

configuration as they convect downstream. For such cases, we observe a strong response of

the wake at the forcing frequency. The axial distance required for such staggering to occur

is a function of the spacing between the forcing frequency and natural frequency, the flame

density ratio, and the intensity of the forcing.

Nomenclature

A = flow cross-sectional area

D = bluff body diameter

ṁ = mass flowrate

Ne = number of ensembles

Ns = number of samples

Re = Reynold’s number, UlipD/ν

StD = Strouhal number, fD/Ulip

T = temperature

U = time-averaged axial velocity

c = modal wave propagation speed

f = frequency, Hz

j = any integer

k = wavenumber

rU,L = correlation coefficient between upper and lower flame edge positions

1 Graduate Research Assistant, School of Aerospace Engineering, 270 Ferst Dr, AIAA Student Member.

2 Research Engineer, School of Aerospace Engineering, 270 Ferst Dr, AIAA Member.

3 Professor, School of Aerospace Engineering, 270 Ferst Dr, AIAA Associate Fellow.

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t = time

v = instantaneous transverse velocity

Greek Characters

β = backflow ratio, -U(y=0)/U(y=∞)

Ω = unsteady vorticity

ζL = lower flame edge position

ζU = upper flame edge position

ν = kinematic viscosity

ρ = gas density

φ = equivalence ratio

ω = frequency, rad/s

Subscripts

0 = zero group velocity

b = burned

i = imaginary component

in = inlet

lip = bluff body trailing edge lip

max = maximum

ref = reference velocity

u = unburned

Superscripts

’ = unsteady component

f = forcing

n = natural

r = response

I. Background

The unsteady flow fields in reacting bluff body flows are often dominated by large scale coherent structures,

embedded upon a background of acoustic waves and broadband fine scale turbulence. These large scale structures

play important roles in such processes as combustion instabilities1-3

, mixing and entrainment, flashback, and

blowoff4. These structures arise because of underlying hydrodynamic instabilities of the flow field

5. This paper

extends recent work which characterized the effects of the flame upon the absolute and convective instability

characteristics of the flow6-9

. Here we analyze the effects of harmonic forcing upon this flow. The response of the

flame and flow to harmonic forcing is motivated by the combustion instability problem10

, where heat release

oscillations couple to natural acoustic modes of the combustion chamber and lead to large amplitude, narrowband

tones.

As shown in Figure 1, there are two key flow features downstream of the bluff body in high Reynolds number

flows; these include the separating free shear layers11

and the wake, both of which strongly influence the flame.

These two flow features are discussed next.

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a)

b)

Figure 1. a) Schematic of non-reacting flow past a bluff body at ReD = 10,000.

Reproduced from Prasad and Williamson12

b) Instantaneous streamlines from

current experiment, for non-reacting wake behind v-gutter at Ulip = 50 m/s,

produced from PIV measurement

The separated shear layer is convectively unstable due to the Kelvin-Helmholtz mechanism for ~1200DRe 12

,

leading to shear layer rollup into tightly concentrated vorticity. In most practical configurations, the flame lies

nearly parallel to the flow and, thus, almost directly in the bluff body shear layer for high velocity flows. The

separating vorticity sheet on both sides of the bluff body rolls up, which induces a flow field that wraps the flame

around these regions of concentrated vorticity.

In non-reacting flows, the bluff body wake is absolutely unstable, and characterized by large scale, asymmetric

rollup of the separating shear layer into staggered vortical structures13

, often referred to as the Von Karman vortex

street. This instability has a characteristic frequency of12

lip

BVK D

Uf St

D (1)

where DSt is the Strouhal number. For circular cylinders, DSt is independent of Reynolds number ( 0.21DSt ) in

the turbulent shear layer, laminar boundary layer regime, ~ 200,000D~ 1000 < Re 14

. Above ~ 200,000DRe ,

the boundary layer starts to transition to turbulence and there are some indications that this Strouhal number value

changes15-16

. Bluff body shape also influences the Strouhal number for the Von Karman vortex street17

. In

particular, DSt is lower for “bluffer” bodies, i.e., those with higher drag and wider wakes18

. For example,

~ 0.18DSt for a 90 degree “v-gutter” and drops to 0.13 for a sharp edge, vertical flat plate19

.

These baseline flow stability characteristics are altered by heating the fluid in the wake20-24

, by addition of

splitter plates11

, and through base bleeding/blowing, to name several examples. Of most interest to this study are

wake density ratio effects (due to heating, for example). It has been shown that a sufficiently hot wake relative to

the free stream eliminates the absolute instability of the wake, so that the flow’s dynamics are then controlled by the

convectively unstable shear layers. For example, Yu and Monkewitz22

performed parallel stability analyses of

variable density wakes, characterized by an outer flow with a density, u , and time-averaged velocity, uU , and

wake region with density b and velocity bU (see Figure 2). For an inviscid flow with a step jump in properties

between the two fluids, they show that the absolute stability boundary depends upon density ratio between the outer

flow and the wake, as well as the ratio of reverse flow velocity in the wake to outer flow velocity, , defined

below:

ub

uu (2)

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Figure 2. Sketch of the heated wake problem profile.

The convective/absolute stability boundary predicted by their analysis is plotted in Figure 3. For the purpose of

this study, only the sinuous (asymmetric) mode stability boundary is plotted, as it is more easily destabilized in

wakes than the varicose (symmetric) mode. This plot shows that absolute instability is promoted with lower density

ratios, u b , and higher wake reverse flow velocities. For the reacting mixtures considered in this study, the

density ratios plotted in the figure are also essentially identical to the flame temperature ratio, b uT T . Similar

stability calculations have also been performed using more realistic velocity and density ratio profiles, such as

hyperbolic tangent profiles and incorporating viscous effects25-26

. A particularly significant effect is differences in

location of the velocity and density jump in the wake, which can cause the stability boundary in Figure 3 to move

significantly.

Figure 3. Dependence of convective/absolute stability limit upon backflow ratio

and density ratio, as predicted by 2D parallel flow stability analysis22

These points have important implications on the effects of a flame on the bluff body flow field, as the density

and velocity profiles in Figure 2 are a good first order approximation of a reacting flow profile in a high velocity

flow. Indeed, a variety of prior studies have noted fundamental differences in the dynamic character of the flame

and/or flow field at different velocity and fuel/air ratio conditions27-28

, particularly under near blowoff conditions29-35

or in flames utilizing highly preheated reactants23, 29, 36

. Erickson et al.23

and Emerson et al. 37-38

presented calculated

results of flames with various density ratios, showing that a large sinuous flow feature gradually grows in

prominence as density ratio across the flame is decreased below values of approximately 2-3. A significant

additional observation from the latter study was that the transition to absolute instability did not appear abruptly as

the density ratio crossed some threshold value. Rather, the flow intermittently flipped into and out of the sinuous

mode in a transitional range of density ratios. The fraction of time that the flow spent in the sinuous flow state and

oscillating in a narrowband fashion increased monotonically with decreasing density ratio.

These observations are quite significant, as they show that the dominant fluid mechanics in a burner with non

preheated reactants, such as lab burners or atmospheric pressure boilers, which have a “high” density ratio, can be

very different from those of facilities with highly preheated reactants. For example, stoichiometric methane air

flames with reactant temperatures, Tu = 300 and 1000 K have density ratios of 7.5 and 2.6, respectively. Examples

of low temperature ratio practical applications include gas turbines, where Tu ~ 700K-900K39

, recuperated cycles

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where Tin ~1000K-1100K40-41

, or applications using vitiated combustion products, such as duct burners used in

waste heat boilers downstream of ground power gas turbines42

.

The implications of this absolute/convective instability distinction are particularly significant in thermoacoustic

instability problems, where vortical structures excited by acoustic waves play important roles in the feedback

mechanism43

. A “convectively unstable” system is quite sensitive to acoustic excitation6. In contrast, an absolutely

unstable system is an oscillator – it exhibits intrinsic, self-excited oscillations and does not require external

disturbances to persist. In such a self-excited system, the limit cycle behavior may remain independent of the

external forcing, may shift in frequency with increasing amplitude of excitation, and/or may monotonically decrease

in amplitude with increased forcing amplitude 17, 44-45

. In one case, low amplitude acoustic excitation induces a

proportional response while in the other it may not. In turn, this has important implications on which type of flow

instabilities can be involved in linear combustion instability mechanisms. For example, it suggests that simulating

combustion instabilities in simplified lab combustors with high density ratios will lead to completely different

acoustic-hydrodynamic stability coupling processes than what may actually be occurring in the low flame density

ratio application of interest.

The objective of this study is to characterize the unsteady flow evolution of longitudinally forced flames as a

function of flame density ratio. The rest of paper is organized as follows. First, we describe the experimental

facility in which flame density ratio is monotonically varied across this bifurcation in flow structure at constant Ulip.

To do this, it is necessary to highly preheat the reactants to obtain flames that are stable (i.e., well removed from

blowoff), yet of low density ratio. This was done by developing a facility capable of vitiating the flow, with

independent control of the fuel/air ratio of the two burners. Such a facility is able to achieve broad ranges of flame

density ratio. In order for these density ratio sweeps to be conducted at a fixed bluff body lip velocity requires a

second, independently controlled air injection source. The ability to control two independent air flows and two

independent fuel/air ratios leads to substantial flexibility in operating conditions. Furthermore, great care was taken

to stay well away from blowoff boundaries, where additional flame dynamics can occur4, 46-47

.

Next, we experimentally characterize the unsteady flow evolution as the density ratio is varied at constant Ulip.

We observe that a gradual shift in flow structure exists when the flame is operated near the absolute-convective

stability limit, and the flame density ratio is swept across this limit. Finally, we investigate the effects of

longitudinal acoustic forcing on these low density ratio wakes. We show that the shear layers primarily exhibit

oscillations at the forcing frequency and the wake primarily exhibits oscillations at its natural frequency. However,

when the flow is either forced sufficiently strongly or sufficiently near the natural wake frequency, the wake

oscillations may be drawn away from the wake’s natural frequency, and may even lock-into the forcing frequency.

The symmetry of the coherent vortex shedding at the forcing frequency turns out to be related to this lock-in

phenomenon.

II. Experimental Facility, Diagnostics, and Testing Procedures

The experimental rig, shown in Figure 4, consists of two premixed, methane-air combustors in series. This

facility, the bluff body geometries, and the diagnostics are detailed in previous work37-38

and so are only briefly

described here. The main components are a vitiator, secondary air and fuel inlets, a flow settling section, acoustic

drivers, and the test section. The test section is rectangular with a bluff body spanning the width of the combustor,

creating a nominally 2D flow. The aspect ratio of bluff body height to chamber width is 0.15. This combustor has

quartz windows for optical access from all four sides. Two different bluff bodies were used in the test section: a 2D

ballistic shape and a V-gutter. From here on, the 2D ballistic shape will be referred to as the ballistic bluff body.

Figure 4. Schematic and photograph of the atmospheric pressure, vitiated, bluff

body rig

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The primary and secondary air and fuel each have their flowrates adjusted individually. This design allows the

flame density ratio and bluff body lip velocity to be varied independently in the test section, by adjusting the air

flowrate and stoichiometry in the vitiator and test section separately. The test matrix was developed such that

primary and secondary air flowrates and primary and secondary equivalence ratios could be adjusted to achieve a

variety of density ratios at a fixed velocity.

The measured primary air and fuel flowrates as well as the measured temperature just before the test section

were recorded. The measured absolute temperature entering the test section was ~25-30% lower than the calculated

adiabatic temperature. Bluff body lip velocity was calculated as Ulip=ṁlip/(ρlipAlip). For this lip velocity calculation,

the total measured mass flowrate and the flow cross sectional area at the bluff body trailing edge were used. The

density for the lip velocity calculation was determined from the temperature measured just before the test section

and the gas composition of the secondary air and fuel adiabatically mixed with the equilibrium vitiated gas. An

equilibrium gas solver was then used to calculate the adiabatic density ratio across the test section flame.

Experimental diagnostics consisted of high speed, line of sight imaging of flame chemiluminescence and particle

image velocimetry (PIV). High speed video was captured for the ballistic bluff body only. High speed imaging was

performed with a Photron Fastcam SA3 camera to capture chemiluminescence at a 3000 Hz frame rate at a

resolution of 512 x 256 pixels. This CMOS camera operates nearly continuously at 3000 Hz. The flame was

imaged through a BG-28 filter for CH* chemiluminescence imaging. This filter transmission exceeds 10% for

wavelengths between 340 and 630 nm, and peaks at 82% transmission at 450 nm. The flame was imaged from the

bluff body trailing edge to approximately nine bluff body diameters downstream. A total of 8029 images were

stored in each run. The resulting images were used to quantify flame motion via the transverse position of the top

and bottom flame branch edges, ζU(x,t) and ζL(x,t), as a function of axial position and time (see Figure 5). This

resulted in time series for edge positions along both flame branches, at each axial position. These time series are

Fourier transformed to determine their temporal spectra, given by ζU(x,f) and ζL(x,f).

Figure 5. Coordinate system and instantaneous flame edge position definition

from edge tracking, Ulip = 50 m/s, ρu/ρb = 1.7.

PIV was conducted for both the ballistic bluff body and the V-gutter at conditions identical to those for high

speed video. The PIV system is a LaVision Flowmaster Planar Time Resolved system that allows for two-

dimensional, high-speed velocity measurements. The laser is a Litron Lasers Ltd. LDY303He Nd:YLF laser with a

wavelength of 527 nm and a 5 mJ/pulse pulse energy at 10 kHz repetition rate. The laser is capable of repetition

rates up to 10 kHz in each of the two laser heads. The Photron HighSpeed Star 6 camera has a 640x448 pixel

resolution with 20x20 micron pixels on the sensor at a frame rate of 10 kHz. In the current study, 2000 PIV image

pairs were taken at a frame rate of either 10 kHz or 5 kHz, with 12 s between images for a given image pair. The

DaVis 7.2 software from LaVision was used to process the PIV data, performing background subtraction and then

calculating the velocity fields.

Longitudinal acoustic waves were excited with a pair of 100 Watt speakers. Because the combustor has a

limited number of discrete, resonant acoustic frequencies, the flow was driven consistently at a fixed frequency of

515 Hz. Rather than performing a sweep of the forcing frequency, the natural wake frequency of the bluff body was

swept by altering the lip velocity. For each condition, three tests were performed: one with no forcing, one with

“medium” forcing, and one with “high” forcing. The “medium” and "high" forcing had longitudinal acoustic

velocity amplitudes of approximately 4% and 6% of the lip velocity, respectively. In order to ensure the same

combustor characteristics for the unforced and forced conditions, these data were taken during the same test runs by

allowing the combustor temperature to equilibrate and then to take the data without shutting off the combustor or

adjusting any valves. This allowed for flow conditions (e.g., flowrates and equivalence ratios) to be maintained very

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consistently between the three data sets. This procedure was repeated for all of the density ratios and flow velocities

of interest.

It is important to note that, for the acoustically forced cases, sampling was performed such that spectral leakage

from the forcing frequency would be negligible. This ensures that measurements of dynamics at the global mode

frequency are not contaminated by energy leaked from the forcing frequency, even when the two frequencies are

close. This was accomplished by setting the forcing frequency, sampling frequency, number of samples and number

of ensembles such that Eq. 3 was satisfied49

. In the equation, Ne represents the number of ensembles for spectral

averaging; this is not necessarily the same as the total number of ensembles, which may be increased further by

spatial averaging. In essence, this equation ensures that an integer number of cycles of the forced response are

sampled, which places a frequency bin exactly at the forcing frequency.

/f e sample sf jN f N (3)

III. Results - Flame and Flow Dynamics

A. Overview of Unforced Flow and Flame Dynamics

This section summarizes key results shown in previous work37-38

on the unforced flame dynamics by presenting

characteristics of the flame position, ζ (see Figure 5), and velocity as a function of flame density ratio. Velocity data

shown in this section were taken from the transverse centerline velocity ( , 0, )v x y t , whose Fourier transform is

given by ˆ( , 0, )v x y f .

Figure 6a shows spectra from the upper flame branch at x/D=3.5 at three different density ratios as a function of

Strouhal number, StD=fD/Ulip. At the highest density ratio shown, ρu/ρb=2.4, the spectrum is fairly evenly distributed

across all frequencies, with a small bump at StD ~0.24. As the density ratio is decreased to 2.0, a clear feature

appears centered near StD = 0.24. As the density ratio is further decreased to 1.7, the response at StD = 0.24 becomes

narrowband and quite prominent. Figure 6b shows spectra of the unsteady transverse velocity for the same flow

conditions, and demonstrates that the flow similarly exhibits a growing narrowband spectral feature at StD=0.24.

a)

b)

Figure 6. Spectra for ballistic bluff body at 50 m/s and x/D = 3.5 for a) upper

flame edge and b) centerline transverse unsteady velocity

In order to focus on the characteristics near the frequency of peak response, the integrated power under the

spectral peak between Strouhal numbers of 0.20 and 0.28 as a function of density ratio is computed for the flame

response spectra in Figure 6a, and for the unsteady transverse velocity spectra in Figure 6b. Integrated spectral

energy was converted to a root mean squared (rms) by use of Parseval’s theorem. These rms values are presented in

Figure 7 for both the flame and the flow. The flame data in Figure 7a show that the normalized fluctuation in flame

position has a value of roughly 4% over the 2.4<ρu/ρb<3.4 range. Below a value of ρu/ρb=2.4, the response gradually

increases to 18% at ρu/ρb=1.7. Comparing the flame to the flow data again demonstrates similarities. This plot also

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indicates that the transition in flow and flame characteristics is not an abrupt bifurcation with change in density ratio,

but a more gradual increase in narrowband response as the density ratio decreases.

The correlation coefficient between the two flame branches, rU,L (defined below), provides important information

on the scale and/or correlation between the underlying flow structures perturbing them.

,2 2

( , ) ( , )( )

( ( , )) ( ( , ))

U L

U L

U L

x t x tr x

x t x t

(4)

Note that negative and positive correlation coefficients indicate an asymmetric and symmetric flame, respectively.

Furthermore, a nearly zero correlation coefficient implies that the flame is disturbed by uncorrelated structures much

smaller than their transverse separation distance, roughly the bluff body diameter. The dependence of correlation

coefficient upon ρu/ρb is plotted in Figure 7b, showing that the correlation coefficients are near zero, but possibly

slightly positive, for ρu/ρb> ~2.5. The correlation coefficient monotonically decreases towards values of rU,L ~ -0.6,

indicative of growing correlation and antisymmetric motion with decreasing flame density ratio.

a)

b)

Figure 7. a) Spectral energy about StD = 0.24 for ballistic bluff body at Ulip = 50

m/s and x/D = 3.5, expressed as rms flame edge fluctuation averaged over both

flame branches, and rms centerline transverse unsteady velocity b) Correlation

coefficient between top and bottom flame edge position for ballistic bluff body at

Ulip = 50 m/s.

B. Forced Response Characteristics

As mentioned in the Background section, absolutely unstable flows exhibit nonlinear disturbance response

characteristics. In the vicinity of low amplitude disturbances, the global mode may persist relatively independently

of the external forcing. However, if the disturbance amplitude is large enough, or the frequency of the disturbance is

close enough to the natural frequency of the oscillator, the amplitude and/or frequency of the global mode will be

altered. For example, “lock-in17, 44

” is a function of the difference between the disturbance frequency and natural

instability frequency, as well as the amplitude of the disturbance.

Streamlines of the time-averaged flow are presented in Figure 8. Notice that the forced flows tend to have a

shorter recirculation zone than their unforced counterpart; such behavior persisted at all conditions and was the

primary influence of the forcing on the base flow. This observation is consistent with that of Cardell11

, who found

that the presence of vortex shedding shortens the recirculation zone. Presumably, the more intense vortex shedding

present in the forced cases would be associated with a shorter recirculation zone.

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a) fn/ff = 0.78

b) fn/ff = 0.96

Figure 8. Time-averaged streamlines for the ballistic bluff body at ρu/ρb = 2.0 and

two different fn/ff values. Top half of each image shows unforced case, bottom half

shows high forcing case.

We next focus on the flow dynamics. A typical centerline vorticity spectrum is presented in Figure 9, showing

the presence of two peaks, corresponding to the global mode frequency, fn, and forcing frequency, ff.

Figure 9. Typical centerline unsteady vorticity spectrum, in this case for ρu/ρb =

1.7, fn/ff = 0.78, x/D = 4, y/D = 0, showing responses at both the natural and forcing

frequency.

The characteristics of these two peaks are a strong function of their frequency spacing, as shown by the vorticity

spectra in Figure 10. When ff is greater than approximately 1.3 times the value of fn (ie fn/ff < 0.8), little change was

observed in the global mode oscillations. This may be seen by comparing Figure 10a to Figure 10d, which show

unforced and forced spectra, respectively, for such a case. Next, as the two frequencies approach each other, the

global mode frequency begins to move from fn and drift toward (but not necessarily exactly to) ff. This drift in fn is

evident when comparing the unforced spectrum in Figure 10b to its forced counterpart in Figure 10e. Finally, when

forced even closer to the natural frequency, the wake response locks into the driving frequency. This is clear by

comparing the unforced spectrum in Figure 10c and forced spectrum in Figure 10f, which show strong responses at

fn and ff, respectively.

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No Forcing High Forcing

a) fn/ff = 0.78

d) fn/ff = 0.78

b) fn/ff = 0.82

e) fn/ff = 0.82

c) fn/ff = 0.93

f) fn/ff = 0.93

Figure 10. Vorticity spectra located at x/D = 4, y/D = 0 for ρu/ρb = 1.7. Frames a-c

show results from unforced flows, and frames d-f show results from the

corresponding flows forced with an acoustic velocity magnitude of 6% of the lip

velocity. The natural wake frequency is indicated on all plots.

In order to track the variation in frequency of the global mode, we define the response frequency, fr as the

frequency at which the strongest narrowband oscillation is observed. This may correspond to the natural frequency,

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fn, the forcing frequency, ff, or something in between. Lock-in maps are presented in Figure 11 which plot the

variation of fr with fn. These show how, as the natural frequency becomes very close to the forcing frequency, the

absolutely unstable wake ceases to operate most strongly at its natural frequency and adjusts its dynamics to operate

primarily at the forcing frequency. Figure 11a presents a lock-in map for the lowest density ratio case. Notice that

lock-in was observed only for the stronger forcing amplitude, and for natural frequencies only very close to the

forcing frequency. Also notice for the high forcing cases that, as ff approaches fn, the response frequency is drawn

towards ff but neither equals fn nor ff. This behavior has been observed previously in non-reacting wakes forced by

longitudinal bluff body motion48

. Figure 11b shows lock-in maps for a slightly higher density ratio, where, in

comparison to the lower density ratio, lock-in occurs slightly more easily for the high forcing cases, and much more

easily for the medium forcing cases. Again, this is not surprising since in this case the wake presumably has a lower

growthrate of the global mode. Here, there is some noise in the measurement of the natural frequency due to the

somewhat less narrowband response at this condition. These spectra, shown at the flow centerline at x/D = 3, are

very representative of behavior downstream of this location. Upstream of x/D = 3, the flow has a greater tendency

to respond at the forcing frequency as the natural response becomes weak.

a)

b)

Figure 11. Response frequency vs natural frequency, demonstrating lock-in

phenomenon when the natural frequency is in the vicinity of the forcing

frequency, at x/D = 3, y/D = 0, for a) ρu/ρb = 1.7 and b) ρu/ρb = 2.0. Envelopes for

the high forcing cases have been drawn. The o’s have been offset from x’s by fn/ff

= +0.015 for visibility purposes.

Having discussed the basic flow characteristics at a single point on the flow centerline, we next discuss the

spatial character of the flow response in more detail. As mentioned in the Background section, the flow has two

major regions: the convectively unstable shear layers, and the absolutely unstable wake. In the presence of

narrowband fluctuations, these regions exhibit fundamentally different dynamics. Figure 12 shows velocity vectors

ensemble averaged at the natural frequency and the forcing frequency for ρu/ρb = 1.7, fn/ff = 0.78. For the ensemble

averaged velocity at the global mode frequency, ensembles were sampled conditionally. Conditional sampling was

based on sine fits between short duration, time-local chunks of the unsteady centerline vorticity signal at x/D = 3 and

a sine wave at the natural frequency; free parameters for the sine fits were amplitude and phase. Such a procedure is

outlined in more detail in previous work, where short duration sine fits were used to characterize the behavior of

flow intermittency37-38

. Sampling for ensemble averaging was phase-locked to the phase of these sine fits.

Furthermore, ensembles were discarded if, for that instant, the sine fit was poorly correlated to its corresponding

chunk of the vorticity time signal. Ensemble averaging at the forcing frequency was performed using the phase

locked reference of the forcing frequency. The flowfields in this figure clearly show the varicose nature of the

forced response, and sinuous nature of the natural response.

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a)

b)

Figure 12. Flow vectors for ρu/ρb = 1.7, fn/ff = 0.78, showing a) flow ensemble

averaged at the forcing frequency b) flow ensemble averaged at the natural

frequency.

Some insight into reasons for this behavior is obtained from Figure 13, which presents contours of normalized,

filtered vorticity magnitude in frequency regions centered on ff and fn. The response centered on ff is derived from

the magnitude of the vorticity measured at ff, since a frequency bin is located exactly here by design. This value will

be denoted f . The response centered on fn was determined by integrating the power spectrum over a 20 Hz band

centered at the natural frequency. These two responses were then normalized at each axial station by their mean

value at that axial station, and will be denoted Ωf and Ωn for the response at the forcing frequency and natural

frequency, respectively:

, ,,

, ,

f

f

f

x y fx y

x D y D f

(4)

10

10

10

10

, ,

,

, ,

n

n

n

n

f

f

nf

f

x y f df

x y

x D y D f df

(5)

Consider the left column, where ρu/ρb = 1.7. Note how the dominant oscillations at ff and fn occur in the shear

layers and in the wake, respectively, for the fn/ff=0.78 case. In contrast, when ff is very near fn, as in the fn/ff=0.93

case, not only the shear layers but also the entire wake responds strongly at ff.

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ρu/ρb = 1.7 ρu/ρb = 2.0

a) fn/ff = 0.78

c) fn/ff = 0.73

b) fn/ff = 0.93

d) fn/ff = 0.89

Figure 13. Contours of normalized vorticity magnitude centered around the forcing

frequency and the wake natural frequency. The forced response is represented by

lightly shaded regions bounded by thick lines, and the natural wake response is

represented by darkly shaded regions bounded by thin lines. Results are shown for

various ρu/ρb and fn/ff values, and contour lines sit at a level of 0.8.

Similar observations can be made from Figure 14, which plots the axial dependence of the vorticity fluctuations

at ff. Note how the oscillations are strongest in the shear layers in the fn/ff=0.78 case, while in the fn/ff=0.93 case they

have comparable values between the centerline and the shear layers.

a)

b)

Figure 14. Vorticity magnitude for ρu/ρb = 1.7 and a) fn/ff = 0.78 and b) fn/ff = 0.93.

Plots show response at the forcing frequency in both the flow centerline and the

shear layer.

Figure 14 shows how the centerline response at the forcing frequency approaches (and may perhaps exceed) the

response of the shear layers at this frequency as fn/ff approaches unity. Figure 15 plots the ratio of the vortical

fluctuations in the wake to the shear layer at the forcing frequency, as a function of fn/ff. Consider first Figure 15a,

which plots this result for ρu/ρb = 1.7. Recall that at this condition, the flow exhibits intrinsic oscillations, suggesting

self-excitation of a global mode. The figure shows that the wake begins to respond strongly at the forcing frequency

only when fn is very close to ff (within the lock-in range). In contrast, Figure 15b shows that the wake response at

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the forcing frequency increases almost linearly with fn/ff for the higher density ratio, ρu/ρb = 2.0. Under this

condition, the wake’s sinuous mode has a relatively weaker absolute growthrate.

a)

b)

Figure 15. Ratio of centerline vorticity response to shear layer vorticity response,

both at the forcing frequency, for a) ρu/ρb = 1.7 and b) ρu/ρb = 2.0

As mentioned above, vortical oscillations in the shear layers always respond strongly at the forcing frequency,

but substantially lesser so near the center of the flow, unless ff is close to fn (and perhaps an integer multiple of fn).

Figure 16 helps explain this observation and illustrates ρu/ρb = 1.7 and 2.0 data in the left and right columns,

respectively. The lack of centerline response when ff is far from fn may be explained by the symmetry of the vortex

shedding at the forcing frequency, as shown in Figure 16a and Figure 16c. The acoustic forcing introduces

disturbances which are symmetric about the flow centerline; vorticity associated with transverse gradients must

necessarily be zero along the centerline of a symmetric flow. When ff is close to fn, however, these symmetrically

shed vortices quickly stagger as they progress downstream, and become asymmetric, as can be seen in Figure 16b

and Figure 16d. This loss of symmetry associated with the staggered vortices means that strong transverse gradients

in flow velocity are present along the centerline and, hence, so are strong fluctuations in vorticity. Loss of

symmetry seems to be more likely to occur at lower density ratios, for higher amplitudes of forcing, and when ff is

closer to fn. Similar observations regarding the forcing amplitude and frequency spacing dependencies have been

made previously for non-reacting wakes forced by longitudinal velocity fluctuations50-52

.

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ρu/ρb = 1.7 ρu/ρb = 2.0

a) fn/ff = 0.78

c) fn/ff = 0.73

b) fn/ff = 0.93

d) fn/ff = 0.89

Figure 16. Vorticity contours selected to mark vortex center locations, shown for

a-b) ρu/ρb = 1.7 and c-d) ρu/ρb = 2.0. Dashed lines pass through pairs of inferred

vortex centers to demonstrate symmetry or lack thereof. Images in a given

sequence, read top to bottom, are separated by 400 μs.

IV. Concluding Remarks

These results clearly show that the bluff body wake structure is a strong function of density ratio, but that no

sharp bifurcation occurs with variations in ρu/ρb. Furthermore, when the flow becomes absolutely unstable at low

density ratios, disturbance amplification becomes a highly nonlinear process and is a strong function of both

disturbance amplitude and frequency. Longitudinal acoustic waves provide a symmetric, narrowband disturbance

which primarily excites the shear layers. However, when the frequency of this disturbance is close to the natural

frequency of the wake, the sinuous wake may exhibit fluctuations away from its natural frequency, and may even

lock into the disturbance frequency; hence the lock-on phenomenon is observed. For this type of forcing,

oscillations at the forcing frequency spawn in a symmetric mode just aft of the bluff body, and may gradually

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transition to an asymmetric mode farther downstream. Whether or not this transition occurs is a function of forcing

amplitude, fn/ff, and ρu/ρb, and helps explain the lock-in phenomenon in the wake. Oscillations at the natural

frequency are always asymmetric. Future work on this subject will look at these phenomena from a wake stability

standpoint, as well as to analyze the presence of intermittency in this forced flow, previously noted for the unforced

problem. Further understanding of the different types of flow behavior at realistic density ratios and conditions is a

critical step in predicting bluff body flame response characteristics.

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