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International Journal of Heat and Fluid Flow journal homepage: www.elsevier.com/locate/ijhff

Effects of inﬂow pulsation on a turbulent coaxial jet Seong Jae Jang, Hyung Jin Sung * Department of Mechanical Engineering, KAIST, 335 Gwahangno, Yuseong-Gu, Daejeon 305-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 14 August 2009 Received in revised form 2 January 2010 Accepted 6 January 2010 Available online 29 January 2010 Keywords: Coaxial jet Mixing enhancement LES Pulsation frequency Turbulent pulsating ﬂow Kelvin–Helmholtz vortices

a b s t r a c t The effects of inﬂow pulsation on the ﬂow characteristics and mixing properties of turbulent conﬁned coaxial jet ﬂows have been studied. Large eddy simulations were performed at Re = 9000 and the mean velocity ratio of the central to annular jet, Ui/Uo, was 0.6. Pulsation was generated in the inﬂow jet by varying the ﬂow rates. First, inﬂow pulsation was applied at frequencies in the range 0.1 < St < 0.9 while other parameters were ﬁxed. The pulsation frequency responses were scrutinized by examining the phase- and time-averaged turbulence statistics. The pulsation frequencies St = 0.180 and 0.327 were found to produce the largest enhancement in mixing and the largest reduction in the reattachment length, respectively. The effects of the phase difference between the two inﬂow jets at these two optimal frequencies were then investigated. The optimal phase difference conditions for mixing enhancement and the reduction in the reattachment length were obtained when the strength of the outer vortices was high. Further, we found that the strength of the inner vortices was reduced by varying the phase difference, and the reattachment length was minimized, and that if the strength of the inner vortices was increased, mixing was enhanced. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Coaxial jets in which a central jet is surrounded by an annular jet are commonly utilized where mixing between the two jet streams is desired, and are often encountered in engineering applications such as gas turbine combustors, jet pumps, and chemical reactors. Two turbulent shear layers that undergo interaction and mixing are present in coaxial jet ﬂows, and the near-ﬁeld structures of such jets are very complex. The mixing between two streams is determined by the dynamics and interactions of the vortical structures in the shear layers that develop between the two jets and also between the annular jet and the ambient ﬂow. The vortical structures can be altered by varying the inﬂow conditions, such as the velocity ratio of the two jets and the inﬂow pulsation frequency. Knowledge of the dynamics of coaxial jets and suitable pulsation conditions should be useful to efforts to improve mixing efﬁciencies in combustion devices and industrial chemical systems. Many studies of the reactions of jets to changes in the velocity ratio and to periodic forcing have been performed. Dahm et al. (1992) investigated the near-ﬁeld vortex structures and dynamics of a coaxial, naturally developing jet for various velocity ratios. They referred to the vortex patterns in the inner layer as wake-like if they involved vorticity of opposing signs and shear-layer-like if they involved vorticity of only one sign. It was also found that * Corresponding author. Tel.: +82 42 350 3027; fax: +82 42 350 5027. E-mail address: [email protected] (H.J. Sung). 0142-727X/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatﬂuidﬂow.2010.01.003

the coaxial near-ﬁeld vortical structures in the inner and outer layers do not develop independently and that there is strong coupling between the two layers. The near-ﬁeld vortex structures and dynamics were found to be functions of the velocity ratio and the absolute velocities of the two streams. Wicker and Eaton (1994) studied the responses of coaxial jets to axial acoustic excitation. They found that excitation of the inner jet produces periodic structures in the inner layer, but has no signiﬁcant effects on the evolution of the outer layers. Both studies (Dahm et al., 1992; Wicker and Eaton, 1994) found that the vortical motion is dominated by the vortices emerging in the outer shear layer when the annular jet velocity is larger than the central jet velocity. Rehab et al. (1997) studied the effects of the velocity ratio on the ﬂow regimes in the near-ﬁeld. In particular, these authors have shown how the velocity ratio determines the inner jet potential core length and the conditions for a recirculating, wake-like transition, which occurs when the velocity ratio is increased above a critical value. Villermaux and Rehab (2000) focused on the role played by coherent vortices in near-ﬁeld mixing in coaxial jets. They showed that the area of the interface between the two streams increases with the instability of the outer shear layer and thus that the vorticity thickness of the outer shear layer is an important parameter. Several studies have focused on the manipulation of ﬂow vortices through active control. The control of mixing in synthetic jets was studied by Ritchie et al. (2000). They found that large scale structure modiﬁcations, especially in the outer mixing layer, have large effects on the mixing properties of coaxial jets. Angele et al.

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(2006) experimentally investigated the evolution of a threedimensional structure in a free coaxial jet controlled by micro ﬂap actuators. This study showed that non-axisymmetric forcing enables the faster development of streamwise vortices and leads to enhanced mixing. In these studies (Ritchie et al., 2000; Angele et al., 2006), the forcing was only applied to the outer shear layer because of its dominant role in jet dynamics. Both studies found that the mixing rate of a coaxial jet ﬂow can be altered by applying a suitable excitation to the outer jet. Recently, several interesting studies using direct numerical and large eddy simulations (DNS and LES) have been carried out: the DNS of a free coaxial jet by da Silva et al. (2003), and the LES of a conﬁned coannular jet at practically high Re by Akselvoll and Moin (1996). These studies revealed the detailed dynamics of vortical structures in uncontrolled jets. Jahnke et al. (2005) studied the inﬂuence of various parameters (Re, Sc, and the density ratio) by using LES. Balarac et al. (2007b) demonstrated that the turbulent mixing process is strongly affected by upstream jet conditions, which modify the transitional state. Inﬂow conditions that favor the appearance of streamwise vortices were found to strongly enhance jet mixing properties. Balarac et al. (2007a) and Mitsuishi et al. (2007) studied the effects of excitations that were applied only to the outer jet and that mimic microactuators by using DNS. This study investigated the effects of inﬂow pulsation on the ﬂow characteristics and mixing properties of turbulent conﬁned coaxial jet ﬂows. Large eddy simulations were carried out at Re = 9000, based on the bulk velocity and the outer radius of the annular jet. The mean velocity ratio of the central to annular jet was 0.6. The main control parameters were the pulsation frequency (0.1 < St < 0.9) and the phase difference (/) between the

annular and central jets. The pulsations in the annular and central jets were generated by varying their ﬂow rates by 5% and 20%, respectively. The pulsation frequency responses were scrutinized by examining the phase- and time-averaged turbulence statistics. The pulsation frequencies St = 0.180 and 0.327 were found to result in the largest enhancement in mixing and the largest reduction in the reattachment length, respectively. The effects of varying the phase difference between the two inﬂow jets at the two optimal frequencies were then investigated. The inner and outer Kelvin– Helmholtz vortices were investigated in detail to observe the effects of these coherent vortices on the mixing enhancement and the reduction in the reattachment length.

2. Numerical method Many ﬁnite difference schemes have been constructed in physical space with cylindrical coordinates. However, the mapping of independent variables is a useful tool for constructing ﬁnite difference methods on a non-uniform mesh (Morinishi et al., 2004). In the present study, (x, r, h) coordinates in physical space are mapped into (fx, fr, fh) in computational space. The scaling factors and Jacobians are deﬁned as,

hx ¼

dx ; dfx

hr ¼

dr ; dfr

hh ¼ r

dh ; dfh

J ¼ hx hr hh :

ð1Þ

In the LES approach, ﬂow variables are decomposed into largescale and subgrid-scale components via a ﬁltering operation. The ﬁltered variables are deﬁned by,

Fig. 1. (a) Schematic diagram of the computational domain; (b) inﬂow pulsation.

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f ðxÞ ¼

Z

0

f ðx0 ÞGðx; x0 Þdx ;

ð2Þ

D

where G(x) is the ﬁltering function and D is the computational domain. The ﬁltered and transformed governing equations for the continuity, incompressible Navier-Stokes, and mixture fraction transport equations are,

1 @ J uj ¼ 0; J @fj hj @ux 1 @ J 1 @p 1 @ sjx srx u u þ þ þ ; j x ¼ J @fj hj hx @fx hj @fj @t r @ur 1 @ J uh uh 1 @p 1 @ sjr ðsrr shh Þ uj ur þ þ ; þ ¼ j J @f hj hr @fr hj @fj r @t r @uh 1 @ J ur uh 1 @p 1 @ sjh srh þ ¼ ; uj uh þ þ þ2 J @fj hj hh @fh hj @fj @t r r @f 1 @ J 1 @qj qr uj f ¼ þ ; þ @t J @fj hj hj @fj r

ð3Þ ð4Þ ð5Þ ð6Þ ð7Þ

where uj (j = x, r, h) is the velocity, p is the pressure, and f is the mixture fraction (the ﬁltering operation is omitted for simplicity). Here, the repeated indices j imply summation over j = x, r, h. The viscous shear stresses sij (i, j = x, r, h) and the scalar ﬂuxes qj (j = x, r, h) are given by,

1 1 @ux ; Re hx @fx 1 1 @ur srr ¼ 2 mt þ ; Re hr @fr 1 1 @uh ur shh ¼ 2 mt þ þ ; Re hh @fh r

1 1 Re hx 1 1 sxh ¼ mt þ Re hx 1 1 srh ¼ mt þ Re hr 1 1 qj ¼ at þ ReSc hj

sxr ¼ mt þ

@ur 1 @ux þ ; @fx hr @fr @uh 1 @ux þ ; @fx hh @fh @uh uh 1 @ur ; þ r r hh @fh @f @f ; ðj is not repeatableÞ: @fj

ð9Þ

ð10Þ ð11Þ

All variables are non-dimensionalized by the bulk velocity (Uo) and the outer radius (Ro) of the annular jet. The Reynolds number based on the bulk velocity and the outer radius of the annular jet is 9000. The mean velocity ratio of the central to annular jet, Ui/Uo, is 0.6. The working ﬂuid is air, so the Schmidt number (Sc) is 1. A dynamic subgrid-scale stress model was used to account for subgrid-scale stresses. By using the eddy-viscosity assumption, 2 j Sj, the turbulent eddy viscosity mt can be expressed as mt ¼ C D qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ with j Sj ¼ 2 Sij Sij . In this study, the model coefﬁcient C was determined by using the dynamic eddy viscosity model proposed by Germano et al. (1991), as modiﬁed and extended by Lilly (1992). In this model, C is not given a priori, but is computed from the ﬂow variables during each simulation. The model constant is averaged over the h direction. A detailed description of the method for determining the model coefﬁcient can be found in the papers of

sxx ¼ 2 mt þ

Re = 9000 0.9

4

x / Ro = 0.44 3

Ux / Ui

1

xr / xro

(a)

ð8Þ

2

0.7

x / Ro = 5.15

1

0.8

x / Ro = 8.60

0

St = 0.327

0.6

Present Akselvoll and Moin (1996)

0

-1

0.2

0.4

0.6

0.8

1

St

u x′ / U i

(b) 0.8

x / Ro = 5.15

Fig. 3. Effects of varying the pulsation frequency on the mean reattachment length.

0.6

x / Ro = 8.60

0.4

x / Ro = 0.44

1

0.2 St = 0.327

0.8

0

St = 0.180

1

0.6 St = 0.108

x / Ro = 0.44

0.6

1.3

e(x)

0.8

St = 0

x / Ro = 5.15

∫e(x) / ∫e(x)0

(c)

1.2

f

0.4

0.4

St = 0.180 St = 0.327

1.1

x / Ro = 8.60

0.2

0.2 St = 0.600

0 0

0.5

1

1.5

2

r / Ro Fig. 2. Comparison of mean velocity, turbulent intensity and mean mixture fraction.

0

0

5

1

0.1

10

0.2

0.3

St

15

0.4

0.5

20

x / Ro Fig. 4. Effects of varying the pulsation frequency on the mixing efﬁciency.

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2 j Sj, where lent eddy diffusivity at is expressed as at ¼ C f D qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ jSj ¼ 2Sij Sij and Cf is a model constant for mixture fraction trans-

is set Germano et al. (1991) and Lilly (1992). The grid ﬁlter width D equal to the grid spacing. The test ﬁlter is a box ﬁlter in real space, applied by using three-point averaging with quadratic interpolation. The box ﬁlter is applied in the streamwise and azimuthal directions. No explicit test ﬁltering is applied in the radial direction. The total viscosity 1/Re + mt is constrained to be non-negative to ensure the numerical stability of time integration, i.e., 1/ Re + mt = 0 when 1/Re + mt < 0. The total viscosity is forced to be zero at any point where the model returns a negative value. Following the dynamic subgrid-scale model for scalar transport, the subgrid-scale mixture fraction transport was modeled by assuming it is aligned with the scalar gradient vector. The turbu-

(a) St = 0

port. The test ﬁlter and the clipping procedure for mixture fraction transport are the same as those of momentum transport. Details can be found in a paper of Moin et al. (1991). The governing equations were integrated in time by using the fractional step method with the implicit velocity decoupling procedure proposed by Kim et al. (2002). In this approach, the terms are ﬁrst discretized in time using the Crank–Nicholson method, and then the coupled velocity components in the convection terms are decoupled by using the implicit velocity decoupling procedure.

xr = 9.907

r / Ro

0 1 2

-1

0

1

2

(b) St = 0.108

3

4

5

6

7

8

9

10

5

6

7

8

9

10

5

6

7

8

9

10

5

6

7

8

9

10

5

6

7

8

9

10

xr = 8.079

r / Ro

0 1 2-1

0

1

2

(c) St = 0.180

3

4

xr = 6.896

r / Ro

0 1 2 -1

0

1

2

3

4

xr = 6.473

(d) St = 0.327 r / Ro

0 1 2 -1

0

1

2

(e) St = 0.600

3

4

xr = 8.414

r / Ro

0 1 2 -1

0

1

2

3

4

x / Ro Fig. 5. Time-averaged streamlines.

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Qannular = Qo{1 + aosin(2pfpt)}, Qcentral = Qi{1 + aisin(2pfpt + /)}, where / is the phase difference between the annular and central jets and fp is the pulsation frequency. The pulsation amplitudes of the annular and central jets were 5% (ao) and 20% (ai), respectively. The pulsation frequency was varied in the range 0.1 < St < 0.9 where the deﬁnition of St is St = fpRo/Uo, while other parameters were ﬁxed. The convective boundary condition ou/ot + Ucou/ox = 0 was used at the exit, where Uc is the mean exit velocity. Periodic conditions were applied in the azimuthal direction for the velocity components, and no-slip boundary conditions were imposed at the solid walls. For the mixture fraction, periodic condition was used in the azimuthal direction. The Neumann condition was applied along all solid walls. The inﬂow boundary conditions were determined by using Dirichlet conditions: f = 1 at the central jet, and f = 0 at the annular jet. The convective boundary condition was the same as for the momentum equations. To ascertain the reliability and accuracy of the present numerical simulation, we simulated the conﬁned coaxial jet ﬂow with the same conditions of Akselvoll and Moin (1996), i.e., Re = 48,100 and Uo/Ui = 3.132. As shown in Fig. 2, the mean velocity, turbulent intensity and mean mixture fraction are in good agreement with those of Akselvoll and Moin (1996).

The decoupled velocity components are then solved without iteration. Since the implicit decoupling procedure relieves the Courant–Friedrichs–Lewy restriction, the computation time is reduced signiﬁcantly. The overall accuracy in time is second-order. All the terms in Eq. (3)–(6) and the diffusion terms in Eq. (7) are resolved using a second-order central difference scheme in space with a staggered mesh. The bounded QUICK scheme proposed by Herrmann and Blanquart (2006) is used to resolve the convection terms in Eq. (7). Note that the central difference scheme applied to the scalar equation with inﬂow-outﬂow boundary conditions might lead to numerical instability (Akselvoll and Moin, 1995). Further details of the numerical algorithm can be found in a paper of Kim et al. (2002). By using the homogeneity in the azimuthal direction, the three-dimensional Poisson equation was reduced to a set of decoupled two-dimensional Helmholtz equations through Fourier decomposition. Each Fourier mode was solved with a multigrid algorithm to accelerate the convergence of the iterative procedure. A schematic diagram of the computational domain and the scheme for inﬂow pulsation are shown in Fig. 1. The numbers of grid points in the x, r, and h directions were 305 105 129, respectively. 80 of the 305 axial grid points were used to cover the inlet section (upstream of the expansion). The axial grid points were compressed around the expansion point and stretched on either side. The radial grid points were clustered along all solid þ walls. The grid resolutions were Dxþ min ¼ 2:10; Dxmax ¼ 28:0; Drþmin ¼ 0:50; Drþmax ¼ 5:35; and 2Ro Dhþ ¼ 12:7 based on us for the chamber. The time step was 0.0058Ro/Uo and the total averaging time was 611Ro/Uo. At each time step, stored two-dimensional instantaneous velocity data obtained from a simulation of periodic turbulent pipe and annular ﬂow with pulsation were provided at the inlet of the main simulation. Pulsation was generated in the inﬂow jets by varying the ﬂow rates according to the equations:

(a)

0.08

St = 0

St increases

Mi

0.02

First, we examined the effects of varying the inﬂow pulsation frequency on the mean reattachment length without phase difference. The variation of the mean reattachment length with pulsation frequency is shown in Fig. 3. The pulsation frequency was varied in the range 0.1 < St < 0.9 while other parameters were ﬁxed. The reattachment length was normalized by that found in

St 0 0.108 0.144 0.180 0.216 0.327

0.06

0.04

3. Effects of varying the pulsation frequency

St 0.327 0.450 0.600 0.720 0.900

St = 0.900 St increases

St = 0.327

St = 0.327

0

(b)

St = 0.180

St = 0.327

Mo

0.1

St = 0.900

St = 0 St increases

0.05

St increases

St = 0.327 0

0

2

4

6

8

10

0

2

4

x / Ro Fig. 6. Momentum ﬂux: (a) inner shear layer; (b) outer shear layer.

6

x / Ro

8

10

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S.J. Jang, H.J. Sung / International Journal of Heat and Fluid Flow 31 (2010) 351–367

the case of no pulsation, xr0/Ro = 9.907 at St = 0. We found that the reattachment length is optimally reduced at St = 0.327 by 34.7% with respect to that of the case of no pulsation. When St is less (more) than 0.327, the reattachment length increases as St decreases (increases). The reduction in the reattachment length is more than 30% in the range 0.180 < St < 0.360; this range is the same as the quarter- to half-harmonic St range predicted by the

(a)

0

linear stability theory (Ho and Huerre, 1984), for which Sto = fpho/ U = 0.032 (St = 0.720). The variation with pulsation frequency in the mixing efﬁciency along the streamwise direction is shown in Fig. 4. The mixing efﬁciency is deﬁned as,

R jf ðx; rÞ f1 jdA eðxÞ ¼ 1 R ; jf0 ðx; rÞ f1 jdA

(b)

0.5

1 1.5 2

0

r / Ro

r / Ro

0.5

ð12Þ

x / Ro = -1

0

St = 0.108 St = 0.180 St = 0.327 St = 0.600 0 1

0

0

1 1.5

2

0

3

4

0

5

0

6

0

0

2

0.2

x / Ro = -1

0

ux ′ / Uo

0

1

2

3

4

5

6

0

0

0

0

0

0

0

0.2

u r ′ / Uo

(c)

0

r / Ro

0.5 1 1.5 2

x / Ro = -1

0

0

1

2

3

4

5

6

0

0

0

0

0

0

0

0.2

uθ ′ / Uo Fig. 7. Resolved turbulence intensities.

(a)

(i)

(ii)

(i)

(ii)

St = 0.108 St = 0.180 St = 0.327 St = 0.600

ux ′max / Uo

0.3

0.2

0.1

0

(b)

ux ′max / Uo

0.3

0.2

0.1

0

0

2

4

6

x / Ro

8

10

0

2

4

6

8

10

x / Ro

Fig. 8. Maximum turbulence intensities: (i) random ﬂuctuation; (ii) the oscillating component of the phase-averaged velocity. (a) Streamwise direction; (b) radial direction.

S.J. Jang, H.J. Sung / International Journal of Heat and Fluid Flow 31 (2010) 351–367

1; 0 6 r=Ro 6 12 and f1 is the mixture fraction 0; otherwise associated with a completely mixed state, f1 = 0.1667. The mixing efﬁciency monotonically increases from the expansion point along the streamwise direction and gradually approaches 1, the completely mixed state, at the end of the simulation domain in all cases. Near the expansion point (x/Ro < 3), the mixing efﬁciency increases as St decreases. For lower St (St = 0.108, 0.180), the mixing efﬁciency undergoes rapid growth in the range 3 < x/Ro < 6. To investigate the overall effects on mixing enhancement of varying the pulsation frequency, we deﬁned the global mixing efﬁciency as the integral of the mixing efﬁciency along the streamwise direction from the where f0 ðx; rÞ ¼

357

expansion point to the end of domain. The global mixing efﬁciency was normalized with respect to that of the case of no pulsation. The global mixing efﬁciency of all inﬂow pulsation cases was found to be higher than that of the case of no pulsation. In the inset in Fig. 4, it can be seen that the mixing is most enhanced at St = 0.180. When St is greater than 0.180, the global mixing efﬁciency decreases as St increases. The optimal pulsation frequency, St = 0.180, for the mixing enhancement is the same as the frequency of the general wake instability. Time-averaged streamlines for various St values are shown in Fig. 5. As shown in Fig. 3, the recirculation zone is smallest for St = 0.327. The average mass-ﬂow in the recirculation zone is

Fig. 9. Streamwise distributions of ur. The white and black lines represent the positive and negative distributions of radial velocity.

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0.2

(a)

(b)

1

4 0.8 0.15 2

a 2 (t)

0.05

St = 0.327 λi Σλi

0.6

Σλ i

St = 0.180 λi Σλi

λi

0.1

0

0.4 -2 0.2 -4

St = 0.327

St = 0.180 0

0

10

20

30

40

0

-6

-4

-2

0

2

4

6

a 1 (t)

i

Fig. 10. Relationship of eigenmodes for St = 0.180, 0.327: (a) level of eigenvalues; (b) phase diagram of time-varying coefﬁcients a1(t) and a2(t).

approximately 30% of the total mass-ﬂow for low St (0.108, 0.180) and 20% for high St (0.327, 0.600). The entrainment of ﬂuid into the recirculation zone occurs from the center of the recirculation zone (indicated by ‘+’) to the reattachment point (indicated by the vertical dashed line). As St decreases, the center of the recirculation zone shifts downstream and the distance between the center of the recirculation zone and the reattachment point decreases. Therefore, a large positive radial velocity occurs in this region for low St. The variation of the momentum ﬂux along the streamwise direction is shown in Fig. 6. We deﬁne the quantities Mi for the inner shear layer and Mo for the outer shear layer as follows:

M i ðxÞ ¼ M o ðxÞ ¼

1 A 1 A

Z Z

3R 4 o

0

Z Z

ðu2x þ u2r þ u2h Þr dr dh;

2Ro

3R 4 o

ðu2x þ u2r þ u2h Þr dr dh;

ð13Þ ð14Þ

where A ¼ pð2Ro Þ2 is the cross-sectional area of the chamber. The momentum ﬂux in the inner shear layer is smallest at St = 0.327, when the reattachment length is smallest, due to the better homogeneity of the axial velocity component. The momentum ﬂux in the outer shear layer is largest at St = 0.180, when the mixing is most enhanced. The x-location for the maximum Mo shifts downstream as St decreases for St < 0.45. The global trends in Mi are similar to the variations in the reattachment length shown in Fig. 3, and the variation in the maximum values of Mo is similar to that of the mixing enhancement. Thus, when the pulsation frequency is the only control parameter, Mi and Mo seem to track the variations of the reattachment length and the mixing enhancement, respectively. Fig. 7 shows the time-averaged streamwise, radial and azimuthal turbulence intensities. The global features of u0x , u0r , and u0h are similar. The turbulence intensities have a dominant peak in each shear layer near the expansion point. As the outer shear layer grows, two peaks merge to one peak in the outer shear layer. For high St (0.327, 0.600), the turbulence intensities inside the recirculation zone increase signiﬁcantly near the expansion point (x/Ro < 2). For St = 0.327, which is the optimal St for the minimum reattachment length, all turbulence intensities inside the recirculation zone for the range x/Ro < 3 are maximum. The local maximum values of the streamwise and radial turbulence intensities are shown in Fig. 8(i). The local maximum turbulence intensities are largest at St = 0.327, and smallest beyond x/ Ro = 6. The x-locations for the largest values shift downstream with decreasing St. Fig. 8(ii) shows the local maximum velocity proﬁles

of the r.m.s. of the oscillating component, which is deﬁned as the local maximum of:

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ x Þ2 ; ~ x Þ2 ¼ ðhux i u ðu qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ r Þ2 : ~ 0r ¼ ðu ~ r Þ2 ¼ ðhur i u u

~ 0x ¼ u

ð15Þ ð16Þ

This quantity has a clear dependence on St. The rates of increase ~ 0r;max increase with increasing St. However, the rates of ~ 0x;max and u of u decrease are similar downstream. Thus, the size of the effects on ~ 0r;max of the Kelvin–Helmholtz vortex increases as St de~ 0x;max and u u creases. The x-locations for the largest values shift downstream ~ 0r;max shift further with decreasing St, as in Fig. 8(i). The locations of u ~ 0x;max . downstream (0.5Ro) than those of u To investigate the characteristics of vortical structures in the inner and outer shear layers, the streamwise distributions of ur as a function of time at the ﬁxed radial location (r/Ro = 0.5 and 1.0 for the inner and outer shear layers) are shown in Fig. 9. The largescale azimuthal vortical structures, such as the Kelvin–Helmholtz vortices due to the inﬂow pulsation, convect with broad positive and negative ur at the front and back of the vortical structures in the outer shear layer and at the back and front of the vortical structures in the inner shear layer, respectively. In the inner shear layer (r/Ro = 0.5), the inner vortices move faster than the outer vortices close to the expansion point with the same velocity (AI in Fig 9) for three cases (St = 0, 0.180 and 0.327). However, as ﬂow goes downstream x/Ro > 6, 4 and 2 for St = 0, 0.180 and 0.327, respectively, the inner vortices slow down with the same velocity as that of the outer vortices. The outer vortices move with the same velocity Ao in Fig. 9a and c for St = 0 and 0.327, respectively. However, for St = 0.180, the velocity of the outer vortices (Bo in Fig. 9b) is slow in comparison with the velocity for St = 0 and 0.327, i.e., the slope Bo is larger than the slope Ao and the velocity is a half of the velocity of the inner vortices near the expansion point (AI). Thus, the vortical structures for St = 0.180 and 0.327 are closely related to the characteristics of the inner and outer shear layers of St = 0, respectively. The snapshot-based POD (Proper Orthogonal Decomposition) analysis (Lee et al., 2008) is carried out by using 480 instantaneous snapshots of ur ﬂuctuations to elucidate quantitative characteristics of large-scale vortical structures. The contribution of each mode is displayed in Fig. 10a, which is represented by the level of eigenvalues with circle symbols normalized by the total sum of all eigenvalues. The accumulation of eigenvalues at each mode is represented by the lines, which are also normalized by the total

S.J. Jang, H.J. Sung / International Journal of Heat and Fluid Flow 31 (2010) 351–367

sum. As shown in Fig. 10a, the contribution of the ﬁrst two modes is similar, which is approximately 17% of total ur ﬂuctuations for St = 0.180 and 10% for St = 0.327. Close values of k1 and k2 support that the two modes are originated from identical motions. The relationship of the two modes is clearly shown in the phase diagram of Fig. 10b, where the horizontal and vertical axes represent the time-varying coefﬁcients a1(t) and a2(t), respectively. The timevarying coefﬁcient am(t) of the mth mode is calculated from the following formulation:

am ðtÞ ¼

Z

ur ðx; tÞ wm ðxÞdx

ð17Þ

X

where wm(x) is the mth eigenmode. It is interesting to note that a torus is generated by the phase diagram of a1(t) and a2(t). Distortion of the circular shape represents the variation of phase difference between the two eigenmodes, whereas the dispersion of circulation is caused by the amplitude change of oscillations. The present torus shape means that small ﬂuctuations of oscillating amplitudes are expected in the ﬁrst two modes while the phase difference between the two eigenmodes is nearly constant.

359

The ﬁrst and second eigenmodes of ur ﬂuctuations are shown in Fig. 11. As mentioned above, the ﬁrst and second modes are originated from identical motions and the phase difference between the two modes is estimated to be a quarter of one period. These two eigenmodes clearly show the most energetic motions of vortical structures due to the inﬂow pulsation. For St = 0.180, the energetic motions appear in the range of 2 < x/Ro < 7 and 1 < x/Ro < 5 for ~ 0r;max has a large value in this range as shown in St = 0.327 and u Fig. 8. In the calculation of the kernel matrix of POD using the instantaneous snapshots, we have tested three sub-domains. One is to use the entire radial domain with 0.5 < x/Ro < 6. Others are to use the restricted radial domains in the inner and outer shear layers, 0.48 < r/Ro < 0.52 and 0.97 < r/Ro < 1.03, in the range 0.5 < x/Ro < 6. The ﬁrst two modes for three sub-domains are almost the same as shown in Fig. 11b–d. Thus, the most energetic motions of the vortical structures in the inner and outer shear layers are almost identical. The ﬁgures on the left and right of Fig. 12 show the time- and phase-averaged mixture fractions, respectively. The snapshots of the phase-averaged mixture fractions were obtained at t = 0/4T,

Fig. 11. Eigenmodes of ur ﬂuctuations. The gray and black contours represent the positive and negative distributions of eigenmodes.

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where T is the pulsation period. It can be seen that the stream issuing from the central jet moves further towards the corner of the expansion point as St decreases. Thus, the mixing efﬁciency becomes higher near the expansion point as St decreases, as shown in Fig. 4. The vortex identiﬁcation method of Zhou et al. (1999) was adopted to visualize the vortical structures by using a contour of swirling strength kci, where kci is the imaginary part of the complex eigenvalue of the velocity gradient tensor. The swirling strength is a quantity that can be used to detect vortex cores and to distinguish vortical structures from shear regions. In the ﬁgures on the right of Fig. 12, the two-dimensional swirling strength kci (indicated by solid lines) clearly shows the azimuthal vortical structures due to the inﬂow pulsation; these vortices convect downstream over time. The rotation direction of the azimuthal

vortices is indicated in Fig. 12c. The inner vortex rotates in the opposite direction to the outer vortex. The inner vortex is trapped in the free space between two consecutive outer vortices in Fig. 12b–d; this is the locking phenomenon reported by many previous studies. At St = 0.600, the inner vortex is divided into two parts due to the strong outer vortex: one is trapped between outer vortices as in the other cases, and the other vortex is located beneath the outer vortex, as shown in Fig. 12e. The ﬁrst outer azimuthal vortex forms faster and the distance between the two outer vortices decreases as St increases. However, the radius of the outer vortical structure increases as St decreases. The dashed line is the mixture fraction of the completely mixed state, f1 = 0.1667. It can be seen that the outer vortices pinch the inner shear layer, as indicated by the dashed line. This pinching effect

Fig. 12. Averaged mixture fractions and spanwise vortical structures (kci). Left: time-averaged; right: phase-averaged. Dashed line: f = 0.1667; Solid line: kci.

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(a)

6

(b)

3

St =0.108 St =0.180 St =0.327 St =0.600

5 4

2

3

St increases

St increases

2

1

1 0

0

2

4

6

0

8

0

2

4

x / Ro

6

8

x / Ro

Fig. 13. Swirling strength of the vortex center: (a) outer shear layer; (b) inner shear layer.

means that the central jet expands in the radial direction at the front of the outer vortex and the annular jet invades at the back of the outer vortex. Thus, the mushroom-shaped structures are formed by the dashed line (f1 = 0.1667). When the mushroomshaped structures are generated, the mixture fraction in the central region of the structures diffuses quickly in the radial direction. The area of the interface between the two inﬂow jets increases because the stream issuing from the central jet invades the annular jet region and the annular jet penetrates deeply in the central jet area. This structure enhances the large scale mixing that arises due to the Kelvin–Helmholtz vortices. The variation of the swirling strength of the vortex center due to inﬂow pulsation is shown in Fig. 13. The x-location of the max-

Fig. 14. Fluctuations in the r.m.s. of the mixture fraction.

imum swirling strength of the outer vortex shifts downstream with decreasing St. This maximum location is the same as that ~ 0x shown in Fig. 8a. The maximum value and the increase rate of u of the swirling strength of the outer vortex increase with increasing St. The swirling strength of the outer vortex is twice as large as that of the inner vortex. Fig. 14 shows the ﬂuctuations in the r.m.s. mixture fraction. The peak in f0 in the inner shear layer diffuses to the center line and chamber wall as the streamwise distance increases. Downstream the peak moves from the inner shear layer to the center line of the chamber. For the r.m.s. velocity ﬂuctuations shown in Fig. 7, the r-locations of the maximum value are in the outer shear layer due to the large shear and the azimuthal vortical structures in the outer shear layer. Even though the turbulence intensities increase inside the recirculation zone due to the inﬂow pulsation, f0 is very small due to the lack of mixture fraction in the outer shear layer near the expansion point. The local maximum in the ﬂuctuations of the r.m.s. mixture 0 describes small scale mixing fraction is shown in Fig. 15a. fmax ~ r;max near and seems to be less sensitive to the pulsation St than u the expansion point, as shown in Fig. 8(ii). Fig. 15b shows the local maximum mixture fraction proﬁles of the r.m.s. of the oscillating component. ~f 0max describes large scale mixing and has a clear 0 . The locations of largest ~f 0max dependence on St, in contrast to fmax are the same as the maximum swirling strength locations of the outer vortex, as shown in Fig. 13a. The largest value of ~f 0max increases with increasing St.

(a)

(b)

0

1 1.5 x / Ro = -1

0

0

2

0

3

0

4

0

0

5

6

0

0

0.2

f′

St = 0.108 St = 0.180 St = 0.327 St = 0.600

0.4

0.3 max

2

St = 0.108 St = 0.180 St = 0.327 St = 0.600 0 1

f′

r / Ro

0.5

0.2

0.1

0

0

2

4

x / Ro

6

8

10

0

2

4

6

8

10

x / Ro

Fig. 15. Maximum r.m.s. of the mixture fraction: (a) random ﬂuctuation; (b) oscillating component of the phase-averaged mixture fraction.

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4. Effects of the varying the phase difference between the two inﬂow jets

1

0.68

xr / xro

xr / xro

0.9 0.67 0.66 0.65

0.8

0.64

-30

0

30

φ (°)

60

90

0.7

St = 0.327

0.6 0

30

60

90

120 150 180 210 240 270 300 330

φ (°) Fig. 16. Effects of varying the phase difference on the mean reattachment length for St = 0.327.

1 St = 0.180 & φ = 330°

St = 0

0.8

St = 0.180 & φ = 210°

0.6

∫e(x) / ∫e(x)0

e(x)

1.3

1.2

0.4

1.1

0.2

St = 0.180

St = 0.180 & φ = 90° 1

0

0

5

0

30 60 90 120 150 180 210 240 270 300 330

10

φ (°)

15

20

x / Ro Fig. 17. Effects of varying the phase difference on the mixing efﬁciency for St = 0.180.

To investigate the effects of varying the phase difference between the two inﬂow jets, we carried out large eddy simulations with the same pulsation inﬂow and 30° increments of the phase difference for the two optimal pulsation frequencies: St = 0.327 and 0.180. The variation of the reattachment length with the phase difference for St = 0.327 is shown in Fig. 16. We found that the reattachment length is most reduced at / = 30° by 34.9%. A phase difference of / = 30° reduces the reattachment length by an amount that is 0.026Ro (0.4%) more than the reduction at / = 0°. It can be seen that the reattachment length increases signiﬁcantly in the range 150° < / < 240°. For / = 210°, the reattachment length is almost the same as that of the case with no pulsation. The variation seems to be symmetric about / = 30° (or 210°). The variation of the mixing efﬁciency with the phase difference for St = 0.180 is shown in Fig. 17. The most effective phase difference for mixing enhancement is / = 330°. / = 330° increases the mixing efﬁciency by an amount that is 0.26% more than that at / = 0° in the range 0 < x/Ro < 8. For / = 210°, the trend in the mixing efﬁciency is similar to that in the case with no pulsation. However, the mixing efﬁciency is higher than that found in the no pulsation case by 5%. The global mixing efﬁciency gradually decreases as / increases from 330° (30°) to 210°, and abruptly increases as / increases from 210° to 330°. The vortical structures and the iso-surfaces of the mean mixture fraction (f = 0.1667) are shown in Fig. 18. To visualize the vortical structures, we used an iso-surface of swirling strength kci = 3.0. Four snapshots taken at intervals of T/4 are shown, where T is the pulsation period. For St = 0.327 and / = 30° in Fig. 18b, both of the streamwise and radial vortical structures are more activated near the expansion point and rapidly weaken beyond x/Ro = 5. The large scale spanwise vortical structures consist of small scale vortices crowded in the outer shear layer between the annular jet and the expansion chamber, and are observed in the range 1 < x/Ro < 4. Due to the streamwise and spanwise vortical structures near the expansion point, the center of the recirculation region is biased towards the expansion point and the maximum reduction in reat-

Fig. 18. Vortical structures and iso-surfaces of the mean mixture fraction (kci = 3.0).

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tachment length occurs. In Fig. 18c, the large scale spanwise vortical structures are observed in the range 2 < x/Ro < 6. Near x/Ro = 6, the streamwise vortical structures are observed on the center line. The iso-surfaces of the mixture fraction contain a mushroomshaped structure in the range 3 < x/Ro < 6. The mushroom-shaped structure is formed in the free space between two consecutive outer spanwise vortical structures and indicates the ejection of mixture fraction seeded in the central jet towards the lateral wall of the expansion chamber. This event corresponds to large scale mixing due to the Kelvin–Helmholtz coherent vortices. There are small-scale streamwise and radial vortices crowded into the center and shoulder regions of the mushroom-shaped structure, respectively. These small scale vortices promote turbulent scale mixing. The same POD analysis is examined by using 480 instantaneous snapshots of ur ﬂuctuations at the optimal phase difference for mixing enhancement at St = 0.180 and 0.327, respectively. In

(a)

0.2

Fig. 19, the contribution of each mode and the phase diagram between the ﬁrst two eigenmodes are similar to those of no phase difference at the same St as shown in Fig. 10. The ﬁrst and second modes are originated from identical motions and the phase difference between the two modes is estimated to be a quarter of one period. In Fig. 20, the ﬁrst two eigenmodes at the optimal phase difference are expanded to the center line in comparison with those of no phase difference as shown in Fig 11. This means that the large-scale vortical structures near the center line, i.e., inner vortices in the inner shear layer, are strengthened by the phase difference. Moreover, in the ﬁrst two eigenmodes for St = 0.180 with / = 330°, the vortical structures move through the center line near x/Ro = 6. The mixing is enhanced by these motions which cut the central jet with the annular jet. Fig. 21a and b show the time- and phase-averaged mixture fractions at the optimal phase differences for mixing enhancement at

(b)

1

4

0.8 2

0.1

St = 0.327 & φ = 270° λi Σλi

0.05

0

0

10

20

i

30

40

0.6

a2 (t)

St = 0.180 & φ = 330° λi Σλi

Σλ i

λi

0.15

0

0.4

-2 0.2

0

-4

St = 0.327 & φ = 270°

St = 0.180 & φ = 330° -4

-2

0

2

4

a1 (t)

Fig. 19. Relationship of eigenmodes for St = 0.180 and / = 330°, St = 0.327 and / = 270: (a) level of eigenvalues; (b) phase diagram of time-varying coefﬁcients a1(t) and a2(t).

Fig. 20. Eigenmodes of ur ﬂuctuations. The gray and black contours represent the positive and negative distributions of eigenmodes.

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St = 0.180 and 0.327, respectively. The global mixing efﬁciencies are 1.25 for St = 0.180 with / = 330° and 1.20 for St = 0.327 with / = 270°. For St = 0.180 with / = 330°, which are the conditions resulting in the most enhanced mixing, the mushroom-shaped structure is detached from the main central jet stream near x/ Ro = 5. When this structure is detached, the outer jet penetrates to the center line. For St = 0.327 with / = 270°, the stream issuing

from the central jet is closer to the corner of the expansion point than in Fig. 12d. A mushroom-shaped structure is also observed. The variation of the swirling strength of the vortex center with phase difference is shown in Fig. 22. When the reattachment length is minimized (indicated by the dashed-dotted line; St = 0.180 and / = 90°, St = 0.327 and / = 30°), it can be seen that the outer swirling strength is large and the inner swirling strength

Fig. 21. Averaged mixture fractions and spanwise vortical structures (kci). Left: time-averaged; right: phase-averaged. Dashed line: f = 0.1667; Solid line: kci.

(a)

2

(i)

(ii)

St = 0.180 φ = 90° φ = 210° φ = 330°

(i)

(ii)

St = 0.327 φ = 30° φ = 210° φ = 270°

1.5

1

0.5

0

(b)

4

3

2

1

0

0

2

4

x / Ro

6

8

0

2

4

6

x / Ro

Fig. 22. Swirling strength of the vortex center: (a) St = 0.180; (b) St = 0.327. (i) Outer shear layer; (ii) inner shear layer.

8

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is small. When the mixing is enhanced (indicated by the solid line; St = 0.180 and / = 330°, St = 0.327 and / = 270°), both the outer and inner swirling strengths are somewhat large. In this case, the large positive and negative ur, which is located at the front and back of the outer vortex, is expanded near the center line by the strong inner vortex. Thus, the central jet is expanded towards the lateral wall of the chamber and the annular jet penetrates to the center line. As a result, mushroom-shaped structures form. At / = 210° for both St = 0.180 and 0.327, the outer swirling strength is the lowest and the effects of the inﬂow pulsation are the weakest. However, the inner swirling strength for St = 0.327 is large. Thus, the effects of the phase difference are greater for the outer vortex. The phase-averaged axial velocities are shown in Fig. 23; the two axial velocities are those of the annular and central jets. Ux,max and Ux,centerline denote the maximum axial velocity and the axial velocity of the center line along the streamwise direction,

respectively. The dashed lines are the time-averaged results. In Fig. 23a, the effects of inﬂow pulsation vanish beyond x/Ro = 7. The spatial peaks of the two velocities are not matched up to approximately x/Ro = 5. In the region 2 < x/Ro < 5, the number of local peaks of Ux,centerline is reduced to one, which is different to that of Ux,max. This result suggests that the strong outer vortex does not strongly inﬂuence the central jet, because the local maximum positions of Ux,max are linked to the center positions of the outer vortex. In Fig. 23b, the effect of inﬂow pulsation disappears beyond x/Ro = 10 and the spatial peaks of the two velocities are well matched along the streamwise direction. This phenomenon is closely related to the mushroom-shaped structures. In the neck and head regions of the mushroom-shaped structures, the two velocities become the local maximum and minimum, respectively. This behavior is also observed at St = 0.327 and / = 270°.

(a) St = 0.327 & φ= 30°

(b) St = 0.180 & φ= 330°

1.4

t=0/4T

1.2

t=0/4T U x,max

U x,max

1 0.8 0.6

U x,centerline

0.4

U x,centerline

1.4 1.2

t=1/4T

t=1/4T

t=2/4T

t=2/4T

t=3/4T

t=3/4T

1 0.8 0.6 0.4 1.4 1.2 1 0.8 0.6 0.4 1.4 1.2 1 0.8 0.6 0.4

0

2

4

6

8

10

0

2

4

6

x / Ro

x / Ro Fig. 23. Phase-averaged streamwise velocity proﬁles.

8

10

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The time-averaged mixture fractions at r/Ro = 0, 0.75, and 1.5 are shown in Fig. 24. The mixture fractions of the cases with enhanced mixing (St = 0.180 and / = 330°, St = 0.327 and /

1

(a)

St = 0.180 φ = 90° φ = 210° φ = 330°

f

0.8

0.6

= 270°) diffuse most rapidly and approach f1. The region with f = 1 and 0 for St = 0.327 is shorter than that for St = 0.180 at the center of both the central and annular jets (r/Ro = 0 and 0.75). This

(b)

St = 0.327 φ = 30° φ = 210° φ = 270°

r / Ro = 0

r / Ro = 0

r / Ro = 0.75

r / Ro = 0.75

r / Ro = 1.5

r / Ro = 1.5

0.4

0.2 0.2 0.1 0 0.2 0.1 0

0

2

4

6

8

0

10

2

4

6

8

10

x / R0

x / R0 Fig. 24. Time-averaged mixture fractions: (a) St = 0.180; (b) St = 0.327.

(a)

0.5

(i)

(ii)

St = 0.180 φ = 90° φ = 210° φ = 330°

(i)

(ii)

St = 0.327 φ = 30° φ = 210° φ = 270°

0.4

f ′max

0.3

0.2

0.1

0

(b)

0.5

0.4

f ′max

0.3

0.2

0.1

0

0

2

4

6

x / Ro

8

10

0

2

4

6

8

x / Ro

Fig. 25. R.m.s. of the mixture fraction. (i) Random ﬂuctuation; (ii) oscillating component of the phase-averaged mixture fraction.

10

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result arises because of the strong inner and outer vortices, which are formed earlier than those for St = 0.180. Beyond the core region of f, the high rate of variation of f for St = 0.180 lasts longer than that for St = 0.327. However, f for St = 0.180 approaches f1 faster than that for St = 0.327 beyond x/Ro = 6. At r/Ro = 1.5 up to approximately x/Ro = 4 (inside the recirculation zone), f in the cases of enhanced mixing is higher than in other cases by 30% due to the penetration of f issued from the central jet because of the inﬂuence of the mushroom-shaped structure. Thus, the mixing efﬁciency in the case of enhanced mixing at the expansion point is higher than in other cases, even though f does not change at the center of either of the central or annular jets. The local maximum of the r.m.s. of the ﬂuctuation of the mix0 for the cases of enhanced mixture fraction is shown in Fig. 25. fmax ing is smaller than that for the cases of minimum reattachment length (St = 0.180 and / = 90°, St = 0.327 and / = 30°) because of a reduction in the turbulent scale mixing due to the better homogenization of f. ~f 0max for the cases of enhanced mixing is larger than that for the cases of minimum reattachment length due to their strong inner vortices and mushroom-shaped structures. The effects of the outer vortices on ~f 0max are less than those of the inner vortices due to a lack of f in the outer shear layer. Thus, the effects of the turbulent scale mixing on the mixing of f are more dominant than on the large scale mixing in the cases of minimum reattachment length, and both the turbulent and the large scale mixing are dominant in the cases of enhanced mixing. 5. Summary and conclusions Large eddy simulations of turbulent ﬂows through a coaxial jet were performed at Re = 9000 to investigate the effects of inﬂow pulsation. The mean velocity ratio of the central to annular jet was 0.6. Pulsation was generated in the inﬂow jets by varying their ﬂow rates. The pulsation amplitudes of the annular and central jets were 5% and 20%, respectively. To investigate the effects of varying the pulsation frequency, the pulsation frequency was varied in the range 0.1 < St < 0.9 while other parameters were ﬁxed. The reduction in the reattachment length was more than 30% in the range between the quarter- (St = 0.180) and half-harmonic (St = 0.360) frequencies described by linear stability theory. The maximum reduction in the reattachment length, 34.7%, was obtained at St = 0.327. At this frequency, the momentum ﬂux in the inner shear layer was smallest and all the turbulence intensities inside the recirculation zone were maximized in the range x/Ro < 3. The mixing was most enhanced at St = 0.180, which is the same as the frequency of the general wake instability. At this frequency, the momentum ﬂux in the outer shear layer was largest and the pinching effect, which results in mushroom-shaped structures and enhances large scale mixing, was intensiﬁed. In order to determine the effects of varying the phase difference between the two inﬂow jets, we performed large eddy simulations by using the same pulsation inﬂow with 30° increments of the phase difference for the two pulsation frequencies: St = 0.327, which optimizes the minimum reattachment length, and St = 0.180, which optimizes the mixing enhancement. We found that the reattachment length was most reduced for St = 0.327 with / = 30° by 34.9%. When the reattachment length is minimized, the outer swirling strength is large but the inner swirling strength is small. The most effective phase difference for mixing enhancement was obtained at St = 0.180 with / = 330°. In this case, the large positive and nega-

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tive ur, which is located at the front and back of the outer vortex, was expanded near the center line by the strong inner and outer vortices, and mushroom-shaped structures formed. From the r.m.s of the mixture fraction, both turbulent and large scale mixing are dominant in the cases of enhanced mixing. The optimal phase difference conditions for mixing enhancement and the reduction in the reattachment length were obtained when the outer vortices were strong. Then, if the inner vortices are weakened by the phase difference, the reattachment length is minimized, and if the inner vortices are strong, the mixing is enhanced. Acknowledgements This work was supported by the Creative Research Initiatives (Center for OptoFluidFlexible Body Interaction) of MEST/NRF and partially supported by KISTI under the Grand Challenge Supercomputing Program (KSC-2008-G2-0001). References Akselvoll, K., Moin, P., 1995. Report No. TF-63, Thermosciences Division, Department of Mechanical Engineering, Stanford University. Akselvoll, K., Moin, P., 1996. Large-eddy simulation of turbulent conﬁned coannular jets. J. Fluid Mech. 315, 387–411. Angele, K., Kurimoto, N., Suzuki, Y., Kasagi, N., 2006. Evolution of the streamwise vortices in coaxial jet controlled with micro ﬂap actuators. J. Turbulence 7, 73. Balarac, G., Metais, O., Lesieur, M., 2007a. Mixing enhancement in coaxial jets through inﬂow forcing: a numerical study. Phys. Fluids 19, 075102. Balarac, G., Si-ameur, M., Lesieur, M., Metais, O., 2007b. Direct numerical simulations of high velocity ratio coaxial jets: mixing properties and inﬂuence of upsteam conditions. J. Turbulence 8, 22. Da Silva, C.B., Balarac, G., Metais, O., 2003. Transition in high velocity ratio coaxial jets analysed from direct numerical simulations. J. Turbulence 4, 24. Dahm, W.J.A., Frieler, C.E., Tryggvason, G., 1992. Vortex structure and dynamics in the near ﬁeld of a coaxial jet. J. Fluid Mech. 241, 371–402. Germano, M., Piomelli, U., Moin, P., Cabot, W.H., 1991. A dynamic subgrid-scale eddy viscosity model. Phys. Fluids A3, 1760–1765. Herrmann, M., Blanquart, G., 2006. Flux corrected ﬁnite volume scheme for preserving scalar boundedness in reacting large-eddy simulations. AIAA 44 (12), 2879–2886. Ho, C.-M., Huerre, P., 1984. Perturbed free shear layers. Ann. Rev. Fluid Mech. 16, 365–424. Jahnke, S., Kornev, N., Tkatchenko, I., Hassel, E., Leder, A., 2005. Numerical study of inﬂuence of different parameters on mixing in a coaxial jet mixer using LES. Heat Mass Transfer 41, 471–481. Kim, K., Baek, S.J., Sung, H.J., 2002. An implicit velocity decoupling procedure for the incompressible Navier-Stokes equations. Int. J. Numer. Meth. Fluids 38, 125– 138. Lee, S.B., Kang, W., Sung, H.J., 2008. Organized self-sustained oscillations of turbulent ﬂows over an open cavity. AIAA. Lilly, D.K., 1992. A proposed modiﬁcation of the Germano subgridscale closure method. Phys. Fluids A4, 633–635. Mitsuishi, A., Fukagata, K., Kasagi, N., 2007. Near-ﬁeld development of large-scale vortical structures in a controlled conﬁned coaxial jet. J. Turbulence 8, 23. Moin, P., Squires, K., Cabot, W., Lee, S., 1991. A dynamic subgrid-scale model for compressible turbulence and scalar transport. Phys. Fluids 3, 2746– 2757. Morinishi, Y., Vasilyev, O.V., Ogi, T., 2004. Fully conservative ﬁnite difference scheme in cylindrical coordinates for incompressible ﬂow simulations. J. Comput. Phys. 197, 686–710. Rehab, H., Villermaux, E., Hopﬁnger, E.J., 1997. Flow regimes of large-velocity-ratio coaxial jets. J. Fluid Mech. 345, 357–381. Ritchie, B.D., Mujumdar, D.R., Seitzman, J.M., 2000. Mixing in coaxial jets using synthetic jet actuators. AIAA. Villermaux, E., Rehab, H., 2000. Mixing in coaxial jets. J. Fluid Mech. 425, 161– 185. Wicker, R.B., Eaton, J.K., 1994. Near ﬁeld of a coaxial jet with and without axial excitation. AIAA 32, 542–546. Zhou, J., Adrian, R.J., Balachandar, S., Kendall, T.M., 1999. Mechanisms for generating coherent packets of hairpin vortices in channel ﬂow. J. Fluid Mech. 387, 353– 396.