CFD Dominant flow features of twin jets and.pdf

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Journal of Wind Engineering

and Industrial Aerodynamics 95 (2007) 1199–1215

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Dominant flow features of twin jets andplumes in crossflow

V. Kolara,�, E. Savoryb

aInstitute of Hydrodynamics, Academy of Sciences of the Czech Republic, 166 12 Prague 6, Czech RepublicbDepartment of Mechanical and Materials Engineering, Faculty of Engineering, University of Western Ontario,

London, Canada N6A 5B9

Available online 28 March 2007

Abstract

A brief survey of the recent studies of ‘‘twin jets in crossflow’’ (TJICF) by the present authors is

carried out. A particular emphasis is put on the new velocity-field analysis based on kinematic

decomposition techniques. In addition, some new results dealing with mixing and dispersion of twin

buoyant plumes from different stack arrangements are also presented. The configuration of the

TJICF consists of a pair of identical jet nozzles and jet-flow conditions at the nozzle exits and the jets

are issuing normally into a crossflow/crosswind. The paper deals predominantly with the nozzle

arrangements placed flush with the ground plane. The flow phenomenon represents just the problem

of interaction and mixing between two adjacent non-buoyant jets or buoyant plumes governed by

large-scale vortical structures and background turbulence. Similarities and differences between the

dominant mean-flow vortical features, vorticity and circulation associated with three basic nozzle

arrangements (tandem, side-by-side and oblique arrangements) are examined, and the comparison

with the single jet case is also carried out. Some dispersion aspects reflected in the concentration

measurements of twin buoyant stack plumes are also discussed.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Multiple jets/plumes in crossflow; Nozzle arrangements; Vortical structures; Circulation; Vorticity

decomposition; Mixing and dispersion

see front matter r 2007 Elsevier Ltd. All rights reserved.

.jweia.2007.02.025

nding author. Tel.: +420 2 33109095; fax: +420 2 33324361.

dresses: [email protected] (V. Kolar), [email protected] (E. Savory).

www.elsevier.com/locate/jweia

dx.doi.org/10.1016/j.jweia.2007.02.025

mailto:[email protected]

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Nomenclature

A vortex area (m2)b velocity along vortex boundary (m s�1)C vortex boundary contour (m)C concentration (ppm)Cmin minimum concentration in any plane x ¼ Const. (ppm)Cs plume concentration at stack exit (ppm)CL centreline separation boundary of CVP (m)D jet nozzle diameter (m)Fr Froude number, where Fr2 ¼ U2

Cra=DrgD (�)g gravitational acceleration (m s�2)H stack height (m)I centreline line integral defined by (3) (m2 s�1)lb buoyancy length scale (m)Lu longitudinal (x-component) turbulent integral length scale (cm)R velocity ratio, R ¼ UJ/UC (�)ReJ Reynolds number, ReJ ¼ UJD/n, (�)t time (s)u, v, w velocity components (m s�1)UC crossflow velocity (m s�1 or mm s�1)UJ jet-outlet velocity (m s�1 or mm s�1)x, y, z coordinates (m)z0 roughness length (mm)a nozzle arrangement angle, see Fig. 2 (1)a ¼ 01 tandem arrangementa ¼ 901 side-by-side arrangementa ¼ 451 oblique arrangementn kinematic viscosity of fluid (m2 s�1)ra density of the crossflow fluid (kgm�3)rp density of the plume fluid (kgm�3)Dr density difference, Dr ¼ ra– rp (kgm�3)o vorticity aligned with the crossflow (s�1)G vortex circulation (m2 s�1)h. . .i long-time-averaged value

V. Kolar, E. Savory / J. Wind Eng. Ind. Aerodyn. 95 (2007) 1199–12151200

1. Introduction

Many engineering and environmental problems deal with the basic jet-flow configura-tion of a single circular jet issuing normally into a crossflow, frequently called ‘‘jet incrossflow’’ (JICF), which has been investigated by researchers for more than 60 years (e.g.,the extensive review of Margason, 1993). Engineering applications include typicalenvironmental flows such as vehicle exhaust, chimney jet/plume dispersion and buildingventilation. These applications are closely associated with a crosswind effect upon the

ARTICLE IN PRESSV. Kolar, E. Savory / J. Wind Eng. Ind. Aerodyn. 95 (2007) 1199–1215 1201

issuing jet/plume. The crosswind effect is responsible for the jet/plume deflection andformation of a specific vortical structure and, naturally, in the case of external wind flows,for the resulting atmospheric pollution dispersion.

It is well known from the fluid-dynamical viewpoint that the near field of the JICF ischaracterized by a dominant contrarotating vortex pair (CVP) studied intensively fordecades. However, for operational reasons (e.g., increased efficiency), some practicalengineering applications may require substitution of the single JICF by multiple jets incrossflow. Then, the flow phenomenon represents an interaction of two or more singleJICFs and the gross qualitative features of the vortex formation process of the resulting(mean-flow) vortical structure have to be taken into consideration.

The present contribution aims at summarizing recent experimental studies of ‘‘twin jetsin crossflow’’ (TJICF) by the present authors (Kolar et al., 2003, 2006) where the mean-flow velocity fields of the TJICF have been determined using the standard crossed hot-wireanemometry (HWA) technique. Some results of the kinematic analysis of the TJICFvelocity fields are also presented (Kolar, 2005). In addition, the survey includes some newresults on mixing and pollutant dispersion (Duan and Savory, 2005) using planar laser-induced fluorescence (PLIF) to obtain the buoyant plume concentration fields for multiple(including twin) stack arrangements.

The survey concentrates predominantly upon the vorticity and circulation aspectsassociated with the dominant vortical structure of the TJICF and some closely relatedresults of the new velocity-gradient decomposition technique applied to vorticity fields aredescribed. In addition, the present paper is concerned with jets/plumes that mix freely withthe external crossflow, such that the effects of any nearby buildings or structures are notincluded.

It should be mentioned that hundreds of papers have studied the single JICF problem indetail, while the literature on the TJICF or multiple JICF problem is relatively scarce, e.g.,Holdemann and Walker (1977), Makihata and Miyai (1979), Gregoric et al. (1982), Isaacand Jakubowski (1985), Karagozian et al. (1986), Savory and Toy (1991), Barata et al.(1991). These papers bring useful information predominantly on global characteristicssuch as jet path or vortex trajectory, on multiple or confined multiple jets in crossflow byvarying the number of jets in a row, or by considering specific geometrical configurationsof nozzles (including their overlapping).

The configuration of the TJICF consists of a pair of identical jet nozzles and jet-flowconditions at the nozzle exits and the non-buoyant jets/plumes are issuing normally into acrossflow. Two geometrically symmetric nozzle arrangements, namely tandem arrange-ment (see Fig. 2, a ¼ 01) and side-by-side case (a ¼ 901), and an oblique nozzlearrangement (for a ¼ 451) are treated in detail. The different arrangements essentiallycorrespond to different wind directions. The TJICF flow phenomenon is characterized bythe interaction and mixing between two adjacent non-buoyant jets or buoyant plumesgoverned by large-scale vortical structures and background turbulence.

The dominant TJICF vortical mean-flow structure remains rather similar to that of thewell-known secondary-flow CVP of the single JICF (though asymmetric for an obliqueTJICF nozzle arrangement). The CVP of a single JICF (schematically depicted in Fig. 1)occurs as a result of the impulse of the jet on the crossflow, forming itself in the near fieldand becoming dominant in the far field. For a description of the fundamental vorticalstructure associated with the JICF flow phenomenon, see Fric and Roshko (1994), Kelsoet al. (1996), Morton and Ibbetson (1996).

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UC

z

y

x

Fig. 1. Sketch of the CVP and coordinate system.


The JICF flow phenomenon is closely associated with buoyant plumes where the densitydistribution, due to the initial jet-to-crossflow differences and/or crossflow stratification,plays a crucial role (e.g., Contini and Robins, 2001, 2004; Macdonald et al., 2002; Jirka,2004; and the references therein). It should be emphasized that the dominant vorticalmean-flow feature of buoyant plumes is, again, the above-mentioned secondary-flow CVP.

2. Basic experimental parameters

The HWA measurements were carried out in a wind tunnel in the School of Engineeringat the University of Surrey (UK), using the standard crossed HWA technique (for detailssee Kolar et al., 2003, 2006). The tunnel working cross-section was 0.62m width� 0.75mheight and the twin jet nozzles of diameter D ¼ 13.5mm were placed flush with the groundplane. All experimental data were obtained for a nozzle centre-to-centre separation ofapproximately 5D (66.3mm, i.e., 4.91D), see Fig. 2.The jet outlet velocity was UJ ¼ 25.0m/ s�1, resulting in ReJE2.24� 104. Only one jet-

velocity/crossflow velocity ratio, R ¼ UJ/UC, was studied, namely the ratio R ¼ 8. Hence,the crossflow velocity was set to be UC ¼ 3.125m/ s�1. The velocity fields, that is theprojections of the mean velocity vectors ðhui; hvi; hwiÞ in the planes x ¼ Const., weredetermined in five (or four) rectangular cross-sections located at x/D ¼ 10, 12.5 (notmeasured for an oblique nozzle arrangement), 15, 17.5 and 20. Flow symmetry is assumedfor tandem and side-by-side arrangements in order to reduce the range of measurements toa half-plane. All the quantities obtained from this study and presented in the figures arenormalized by D and UC.The water flume measurements of buoyant plume dispersion from multiple stacks, of

height H ¼ 100mm and inside diameter D ¼ 11.5mm, were carried out at the Universityof Waterloo, Canada. For details see Duan and Savory (2005). A nominally suburbanboundary layer was modelled in the flume at a scale of 1:200 (model roughness lengthz0 ¼ 1.8mm, power law exponent ¼ 0.29, integral length scale Lu ¼ 40 cm at z ¼ H) usingbarrier and ground roughness methods. The crossflow velocity was maintained atUC ¼ 69mm/ s�1 (measured at stack height), whilst the plume velocity at the stack exit wasUJ ¼ 138mm/ s�1, corresponding to a velocity ratio of R ¼ 2. The water for the stacksupply was heated to give turbulent buoyant plumes having a Froude number of Fr ¼ 4.1.A number of stack arrangements were studied, namely single stack (denoted by 1S in thelegend on the graphs presented here in Figs. 9 and 10), two tandem (2SIL) and then twoside-by-side (2SBS) stacks (centre-spacing of 4D) and three tandem (3SIL) and then three

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UC z

y

x

≈ 5D

UJ

0

�

Fig. 2. TJICF nozzle arrangements, a ¼ 01/451/901.

V. Kolar, E. Savory / J. Wind Eng. Ind. Aerodyn. 95 (2007) 1199–1215 1203

side-by-side (3SBS) stacks (centre-spacing of 2D). In every experiment all the stacks wereoperational, with the plumes issuing at the same velocity and Froude number.

The PLIF method, which involves a fluorescent dye (Rhodamine WT) tracer absorbingAr-ion laser excitation light over a range of wavelengths and then re-emitting light at alonger wavelength, was used to measure the plume cross-section concentration fields.Under appropriate experimental conditions, the intensity of the fluoresced light isproportional to the local dye concentration and laser light intensity (Crimaldi and Koseff,2001). The PLIF system was composed of a laser, a scanning mirror, a calibrated CCDcamera with cut-off filter and a video image acquisition and processing system. The plumecross-section concentration fields were obtained from the video images by applyingcalibration charts of pixel intensity against concentration. The concentrations fields arepresented here in the form (C�Cmin)/Cs, where C is the time-averaged concentration, Cmin

is the minimum concentration at each downstream location and Cs is the concentration atthe stack exit.

For each stack configuration, measurements were taken in planes located at x/D ¼ 8.7,17.4, 34.8 and 69.6. The experiments were repeated with the dye first issuing from all thestacks and then from each stack in turn. By examining the different data sets, it waspossible to observe the spread of each plume individually and assess its contribution to theoverall plume at each downstream location.

3. TJICF vortex formation of the CVP

The formation and decay of the dominant vortical structure, the CVP, is closelyassociated with the centreline turbulent vorticity transport across the plane of symmetry(Kolar et al., 2000, 2003). This transport is supposed to be the main cause of thecancellation of the vortex strength of the CVP in terms of circulation. Considering TJICFproblems, the vortical structure is naturally more complex than for a single JICF, althoughit has been found that for geometrically symmetric TJICF arrangements (tandem and side-by-side arrangements) the mean flow becomes rapidly similar to that of the single JICF.For an oblique ground arrangement, the resulting CVP exhibits a natural asymmetry.However, it resembles the structure of the single-jet CVP (Fig. 1) as well. The term‘‘resulting’’ should indicate that the TJICF flow phenomenon represents just an interactionof two single JICFs and that the dominant vortical structure formed downstream (here inthe long-time-averaged sense) is a result of a merging process of ‘‘more simple’’ vorticalformations associated with the single JICF. The early vortex formation close to the nozzle

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UC

zy

x

plane of symmetry

//

//

//

cross-section plane

UC

z

x

y

//

//

//

a

b

Fig. 3. Vortex formation of the TJICF: (a) tandem and (b) side-by-side arrangement.


exits represents an interaction between the originally separated single-jet CVPs, asindicated schematically for tandem and side-by-side arrangements in Fig. 3.The tandem arrangement CVP is a result of the merging process of the stronger (initial)

CVP of an upstream jet (Fig. 3a) with the slightly weaker CVP of a downstream ‘‘shielded’’jet, the latter possessing higher transverse penetration ability and having a lifting effect onthe resulting CVP. As shown in the next section, the resulting tandem arrangement CVPappears—in terms of vorticity—much stronger than that of the single JICF.The CVP of the side-by-side arrangement is expected to form during the strong

interaction of inner vortices (Fig. 3b) rapidly cancelling each other out. The transversenozzle separation distance as projected on to the planes x ¼ Const. is associated with theentrainment blockage. The velocity data of Toy et al. (1993) for the side-by-side case showthat the deflected jets are dominated by one single resulting CVP such that the innervortices of each original single-jet CVP are not evident. Schluter and Schonfeld (2000)employed these data as a test case for their Large Eddy Simulation (LES) computations.They suggested a plausible explanation of the early stage interaction phenomenon duringformation of the resulting CVP in terms of the Coanda effect due to the mutual blockageof entrainment in between the original single JICFs. The resulting CVP is slightly weakerfor the side-by-side case than for the single JICF.

4. Vorticity and circulation

The secondary-flow vortical structure, the CVP, is described using the streamwisevorticity component aligned with the crossflow direction, which can be directly inferred


from the projections of the velocity vectors on the planes x ¼ Const. as hoi ¼ hwiy � hviz.Velocity gradients are computed using the three-point centred scheme.

Fig. 4 provides a summary of all the calculated downstream vorticity and circulationcharacteristics that are discussed below or in the following section. The typical examples of(negative) vorticity distributions for tandem, side-by-side and oblique arrangementsincluding single JICF are shown in Figs. 5–7 (the residual vorticity depicted in these figuresfor comparison purposes is explained in the following section).

The z-locations of vorticity peaks for three basic TJICF arrangements, and a single JICFfor comparison purposes, are depicted here. Fig. 4 shows that the averaged z-locations ofvorticity peaks for an oblique nozzle arrangement at x ¼ Const. are a little higher thanthose of the side-by-side case and clearly lower than those of the tandem arrangement, thelatter exhibiting the highest transverse penetration ability into the crossflow. Note that thedata for an oblique nozzle arrangement are averaged due to a natural flow asymmetry ofthe resulting CVP (cf. the typical vorticity distribution for an oblique case in Fig. 6).

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x/Dx/D

Fig. 4. Summary of downstream vorticity and circulation characteristics of the TJICF and single JICF.

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SIDE BY SIDE (x/D = 12.5)

Fig. 5. Vorticity (left) and residual vorticity (right) for symmetric TJICF arrangements (negative contours).


The vorticity distributions depicted in Figs. 5 and 6 exhibit, for all TJICF arrangements,the features of a single CVP (with the exception of a slightly disordered asymmetricstructure for an oblique case, Fig. 6), rather similar to that of a single JICF (cf. Fig. 7).

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OBLIQUE for y > 0 (x/D = 10)

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Fig. 6. Vorticity (left) and residual vorticity (right) for an oblique TJICF arrangement (negative contours).


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2 4 6 8 0 2 4 6 8

Fig. 7. Vorticity (left) and residual vorticity (right) for a single JICF (negative contours).


The peak vorticity values for an oblique arrangement are on average slightly lower thanthose of the tandem and much higher than those of the side-by-side arrangement.The integral vortex strength is usually described in terms of the circulation G associated

with the resulting CVP (as given by mean-flow definition). The CVP evolves downstreamin the crossflow direction x with its vortex strength G and the rate of decay given by dG/dt.At an arbitrarily chosen instant (this is due to the mean-flow definition of G), G can beassociated with the material surface of the single vortex of the CVP in the cross-sectionplane x ¼ Const. for which it is calculated, due to Green’s theorem, as a surfacequadrature of /oS.

G ¼I

C

hbidC ¼

ZA

hoidA. (1)

Fig. 4 shows G for TJICF arrangements and a single JICF for comparison. Thedownstream development of circulation is clearly dependent on the nozzle arrangement.The CVP is naturally responsible for convective entrainment of the JICF (Morton and

Ibbetson, 1996) or TJICF, which is expected to persist far downstream depending on theCVP decay by the (usually turbulent) vorticity transport between its vortex cores acrossthe plane of symmetry. The centreline vorticity exchange between the single vortices of theCVP is a basic vorticity transport feature of the CVP configuration. Turbulent vorticitytransport can be characterized and visualized in a straightforward manner by turbulentvorticity fluxes as shown for a single JICF and the symmetric TJICF arrangements inKolar et al. (2000, 2003). The centreline vorticity exchange is the main cause of the


cancellation of the vortex strength. In the present paper, we are summarizing the grossvortical features without sophisticated estimates based on local turbulence statistics.

Another interesting feature of the centreline region was found by Savory and Toy(2000). They established that, for a single JICF, the relative centreline portion of G, givenby the centreline line integral I, is constant

GI¼ 1:2014, (2)

I ¼

ZCL

hwidC, (3)

with correlation coefficient 0.996 for a number of different experimental data sets of asingle JICF (for various conditions including different velocity ratios R). The centrelineportion CL of the vortex boundary separates the single vortices within the CVP. Relations(2) and (3) indicate that the mean-flow CVP evolves downstream with a certain flowsimilarity. In the present case of a—much more complex—TJICF flow phenomenon theratio G/I was also examined, though no ideal ‘‘constant behaviour’’ of G/I was found forall TJICF arrangements in comparison with the single JICF (Fig. 4).

5. Advanced velocity-field analysis

Although vorticity appears as the primary quantity for the description of vorticalstructures, it is well known that vorticity does not provide a true vortex geometry becauseof not distinguishing pure shearing motions from the ‘‘actual swirling motion of a vortex.’’

In the following text, an outcome of the application of a novel approach to vortex-coreidentification (Kolar, 2005) based on the triple decomposition of the local relative motionnear a point is presented. Unlike the conventional double decomposition of motion—intoa pure irrotational straining motion and a rigid-body rotation—the triple decompositionaims, moreover, at the extraction of an effective pure shearing motion. Consequently,vorticity is decomposed into two parts, shear vorticity and residual vorticity. The residual

vorticity is associated with the local residual rigid-body rotation near a point obtained afterthe extraction of an effective pure shearing motion.

Though the CVP mean flow is generally of 3D nature, the downstream bending of the jetto the crossflow direction results in a relatively small slope of the jet in the far field (by thepresent data approx. for x/DX10). Consequently, velocity gradients in the crossflowdirection are relatively small and the triple-decomposition algorithm specialized to 2Dvelocity fields (for details see Kolar, 2004) can be employed for the analysis of vorticalsecondary flow of the TJICF and the single JICF in the cross-sections x/D ¼ Const.

The vorticity decomposition result regarding the TJICF vortex geometry clearlyindicates (Figs. 5 and 6) that while the CVP vorticity distribution is characterized by theprolonged contours subjected to the nozzle arrangement, the vortex-core geometrydescribed by the residual vorticity (after extracting an effective pure shearing motion) is onaverage nearly circular within the measured range downstream. The similar results areshown in Fig. 7 for the single JICF. The residual vorticity—representing a specific portionof the total vorticity—appears as a proper physical quantity for the kinematicidentification of true vortex cores. Note that the geometry of an isolated ideal viscousvortex is always axisymmetric.

ARTICLE IN PRESSV. Kolar, E. Savory / J. Wind Eng. Ind. Aerodyn. 95 (2007) 1199–12151210

Note that an arbitrary additive vorticity decomposition implies the possibility of acorresponding surface-quadrature decomposition. By integration of residual vorticitydistributions, the circulation (strictly said, a portion of total circulation G) based onresidual vorticity can be obtained. The results in Fig. 4 indicate almost universal constantbehaviour of this quantity downstream within the measured downstream range (no datasmoothing has been applied) for all three TJICF arrangements and the single JICF.Some basic issues of the advanced velocity-field analysis are discussed in Section 7.

6. Dispersion of buoyant plumes from different stack arrangements

It is not the purpose of the present paper to discuss the detailed aspects of plumedispersion and the applicability of the experimental data to prediction models. Rather, theaim is to illustrate how the different configurations result in a greater or lesser initialmixing between the plumes. The turbulent structures associated with a single plume areillustrated by the flow visualization image in Fig. 8, which also shows the planes in whichmean concentrations were assessed. The large-scale motion, associated with interconnectedvortex rings, is clearly well developed at x/D ¼ 17.4 but is obscured, due to turbulentdiffusion, by x/D ¼ 69.6. It is these structures, deformed by the shearing action of thecrossflow, that give rise to the CVP observed in the time-averaged flow. The ability of theCVP to entrain fluid from the boundary layer is much reduced, when compared to jetsissuing from ground plane nozzles, due to the elevation of the plume by the stack. Fig. 9compares the centreline (plane y ¼ 0) trajectories of the plumes between the different cases,expressed as a plume rise (the height of the geometrical centre of the plume above H), withthe geometrical locations normalized using the conventional buoyancy length scale, lb.Note that x is measured from the geometrical centre of each stack arrangement. It may beseen that the trajectories are higher for the tandem (2SIL and 3SIL) cases than for eitherthe single stack (1S; some 1S points are hidden behind others on the plot in Fig. 8), whichfollows almost exactly the expected (2/3)rd power law associated with full-scale plumes(Briggs, 1984), and the side-by-side (2SBS and 3SBS) cases. Whilst direct quantitativecomparison with the jet studies reported here is not possible because of different boundaryconditions, notably the closer source spacing for the stacks, these results demonstrate a

Fig. 8. Visualization of structures associated with the plume from a single stack showing the planes in which

concentration measurements were analysed.

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10

100

1000

10 100 1000

z/l

b

2/3 Law

1S Experiments

2SIL Experiments

3SIL Experiments

2SBS Experiments

3SBS Experiments

x/lb

Fig. 9. Comparison of plume rise between the different stack cases.


more rapid combining of the tandem plumes (as in the jet case), leading to increasedpenetration into the crossflow.

As an illustration, the profiles of concentration across two side-by-side plumes at threeconsecutive downstream planes, shown in Fig. 10, clearly show that the plumes have stillnot mixed together very well as far as x/D ¼ 17.4 downstream. Because entrainment offluid only happens where the two plumes adjoin and the contrarotating vortices haveopposite rotations when the plumes are side-by-side, mixing and re-organization are notaccelerated but delayed. By the time the two plumes have re-organized into a single plume(around x/DE34.8), the plume has passed the initial phase and has become bent over. Inaddition, an accumulation of plume material at the underside of the combined plumelowers the centre of mass and causes a downwash effect (Contini and Robins 2004) so thatlittle enhancement occurs.

7. Discussion

The gross vortical mean-flow features of the TJICF can be briefly summarized asfollows. By comparing the symmetric TJICF cases in terms of vorticity and circulation, thetandem arrangement exhibits much higher transverse penetration ability (inferred from thetrajectory of peak values of vorticity) and much stronger vortex strength (circulation) thanthe side-by-side case, although both nozzle arrangements rapidly form the resulting CVPthat is rather similar to that of a single JICF.

The oblique nozzle arrangement represents—as an intermediate geometrical case—anatural blend of vortical features of the tandem and side-by-side arrangements. However,due to a flow asymmetry, nonlinearity and complexity, these features are far from a mereaverage of the tandem and side-by-side arrangements. The penetration ability is closer tothat of the tandem arrangement, whilst the vortex strength is closer to the side-by-sidecase, and the resulting CVP is, quite naturally, of a slightly disordered asymmetric

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0.04

-3 -2 -1 0 1 2 3

(C-C

min

)/C

s

2SBS17.4A

2SBS17.4L

2SBS17.4R

-0.01

0

0.01

0.02

0.03

-3 -2 -1 0 1 2 3

(C-C

min

)/C

s

2SBS34.8A

2SBS34.8L

2SBS34.8R

Normalized lateral distance (y/D)



Fig. 10. Comparison of concentrations laterally across the plume geometrical centre at x/D ¼ 8.7, 17.4 and 34.8

for two side-by-side stacks (legend: A ¼ all stacks traced by dye, L ¼ left-hand stack traced by dye, R ¼ right-

hand stack traced by dye).



structure. It should be recalled that the oblique case in the present study employs only onenozzle arrangement angle a (Fig. 2), namely a ¼ 451.

The dominant flow features for different TJICF arrangements appear significantly moreuniversal in terms of the residual vorticity as shown in Section 5. The advanced velocity-field analysis provides a more detailed kinematic insight and, consequently, it leads to amore adequate geometry of vortical structures. In Kolar (2004) this approach has beenapplied to the experimental data of the nominally plane turbulent wake of two side-by-sidesquare cylinders (Kolar et al., 1997).

The CVP vortex cores described by the residual vorticity are nearly circular for all threeTJICF arrangements including the asymmetric oblique case and the single JICF as well.Moreover, the CVP vortex strength obtained by the integration of the residual vorticity is,on average, almost constant and roughly the same for all cases. This fact indicates that theresidual vorticity is the reason why the CVP persists far downstream. Apparently, abackground shearing strongly affects the (total) vorticity distributions, especially in thecentreline region. This shearing effect is the most pronounced in the tandem case (wherethe single vortices of the CVP are relatively close) but much weaker for the side-by-sidearrangement. The portion of circulation associated with this shearing effect decays muchfaster than that based on the residual vorticity which remains almost constant. In view ofthe present results, the turbulent vorticity transport across the centreline and thecorresponding circulation decay (cf. Kolar et al., 2000, 2003) deal predominantly with theshear vorticity rather than the residual vorticity, at least within the measured downstreamrange.

While the z-locations of residual vorticity peaks correspond well to those of (total)vorticity, the transverse separation of residual vorticity peaks is slightly higher (afterextracting the shearing effect near the centreline).

8. Conclusions

The present study is a brief survey of the dominant flow features of TJICF. It is based onthe recent TJICF studies by the present authors for two geometrically symmetricarrangements, namely the tandem and side-by-side cases, and for an oblique nozzlearrangement for fixed angle 451. The work predominantly concentrates upon the vorticitydistribution, circulation and gross vortical features associated with the resultingsecondary-flow CVP which, undoubtedly, dominates the mean-flow behaviour. Mixingand dispersion of twin buoyant plumes from different stack arrangements are brieflydiscussed, on the basis of measured concentration fields, and these confirm the results fromthe jet studies in that tandem configurations show more rapid mixing and greaterpenetration into the crossflow than side-by-side configurations.

Hence, it is shown here how the transverse penetration ability (inferred from thetrajectory of peak values of vorticity) and vortex strength in terms of circulation(calculated as a surface integral of vorticity) are subject to the nozzle arrangement.However, all the TJICF cases lead qualitatively to the same dominant vortical structure—CVP—which is responsible for convective entrainment and, naturally, for the resultingatmospheric dispersion of pollutants.

Finally, the advanced velocity-field analysis—providing a novel approach to vortex-coreidentification based on vorticity decomposition—reveals that the dominant flow featuresfor different TJICF arrangements exhibit a relatively universal behaviour in terms of the

ARTICLE IN PRESSV. Kolar, E. Savory / J. Wind Eng. Ind. Aerodyn. 95 (2007) 1199–12151214

residual vorticity obtained after extracting the effect of a background shearing. The nearlycircular vortex-core geometry, the circulation based on residual vorticity (as an alternativevortex strength parameter) and its downstream change are quite similar for all the casesexamined, that is independent of the nozzle arrangement. Consequently, it can be inferredthat turbulent vorticity transport across the CVP centreline—the main cause of thecirculation decay downstream—deals predominantly with the cancellation of the shearingmotion along the CVP centreline (at least within the measured downstream range).

Acknowledgements

This work was financially supported by the Grant Agency of the Academy of Sciences ofthe Czech Republic through grant IAA2060302, and by the Academy of Sciences of theCzech Republic through Inst. Res. Plan AV0Z20600510. Thanks are due to the late RobertMacdonald (1961–2004) of the University of Waterloo, who collaborated with one of thepresent authors on the plume studies summarized here, and Zhiyong Duan who carried outthose measurements.

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