12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

VORTICAL STRUCTURES IN THE INITIAL REGION OF A ROUND JET
WITH BIFURCATING FLOWS
Akinori Muramatsu
Department of Aerospace Engineering,
College of Science and Technology,
Nihon University
7-24-1 Narashinodai, Funabashi, Chiba, 274-8501,
Japan
muramatsu.akinori@nihon-u.ac.jp

Kohei Tanaka
Major of Aerospace Engineering,
Graduate School of Science and Technology,
Nihon University
7-24-1 Narashinodai, Funabashi, Chiba, 274-8501,
Japan
csko19017@g.nihon-u.ac.jp

ABSTRACTThe formation of a side jet in the initial region of a round jet
is concerned with the wavy deformation of a vortex ring and apair of streamwise vortices. These phenomena are difficult toinvestigate to be unstable, and the details are not clear. Therefore,
this study aims to clarify the formation mechanism of side jets.
To achieve this, we fixed the forming positions of the side jetsby controlling both the wavy deformation of the vortex ring andthe formation of streamwise vortices, using synthetic jets
installed in a round nozzle. The following became clear from thecontrolled experiment and its numerical simulation. The side jets
occur due to the secondary instability that develops in the braidregion. The side jet is the flow induced outwardly in the radial
direction by the circulations of a vortex ring and a pair of
streamwise vortices. The apparently continuous flow of a side
jet results from the periodic repetition of induced flow between
vortex rings and the streamwise vortices.

formation have not been well understood. The reason is that theposition of the side-jet formation changes irregularly and it isdifficult to measure the flow involved the side jet. That is, since
the starting position of the side jet rotates irregularly, it isparticularly difficult to measure the streamewise vortices in theinitial region of a jet.
We have considered that an experiment is conducted bysuppressing the azimuthal rotation of the side-jets, namely theazimuthal rotation of a vortex ring formed in the jet shear layer(Muramatsu and Tanaka, 2021). Three synthetic jets are locatedin the nozzle, and give the jet periodic disturbances, so that theazimuthal rotation of the vortex ring is stopped and the formation
positions of the side jets are fixed. In this condition, since theside jets change from an irregular phenomenon to a periodic
phenomenon, it is possible to process the measured data bysynchronizing with the phase. Vortical structures for the side-jet
formation were determined using a time-resolved PIV and 3Dimaging with Mie scattering (Kawabe et al., 2014). In the adition,
a numerical simulation of a round jet with bifurcating flows was
performed by simulating the experimental equipment used here,
using an open-source software OpenFOAM, so that details ofvortical structure were obtained in the jet with bifurcations.

INTRODUCTION
Bifurcating flows are formed at the initial region of a jet,when the mass density of jet fluid is lower than that of ambient
fluid. Monkewitz et al. (1990) observed the bifurcating flows ina round heated air jet, and Kyle and Sreenivasan (1993) foundthem in a round jet with helium gas and air mixture. The
bifurcating flows are called side jets. The side jet is easily
observed in a low density jet. However, Muramatsu, A. et al.
(2015) found that the side jet is also formed in a uniform massdensity jet. The formation of side jets in a round jet with uniform
mass density has been suggested using a numerical simulation
by Brancher et al. (1993). When the side jets are formed in around jet, velocity fluctuation in the potential core is relatively
high and periodic, so that vortex rings are periodically formed
and deformed wavy at the downstream (Monkewitz et al, 1990;
Kyle and Sreenivasan, 1993; Muramatsu et al., 2008). The
generation of the high and periodic velocity-fluctuation isconcerned with the velocity gradient of a jet flow at the nozzle
exit, and the side jets are formed in the certain range of thevelocity gradient (Kyle and Sreenivasan, 1993; Kato andMuramatsu, 2017). Moreover, it is thought that streamwise
vortex is related to the wavy deformation of a vortex ring
(Toyoda et al., 1999).
Various mechanisms for the side-jets formation have been
proposed. For the case of a low density jet, it is considered thatthe vorticity of a lateral vortex or vortex ring is enhanced by thebaroclinic torque (Chassaing et al., 2002). Brancher et al. (1993)
indicate that a side jet is a flow induced by a pair of streamwise
vortices with different rotational directions formed in the braidregion. However, the details of mechanism for the side-jets

EXPERIMENTAL APPARATUS AND METHODS
Experimental Apparatus
Figure 1 shows a schematics diagram of a round contraction
nozzle with three holes for acoustic excitation. The outlet
diameter of the round nozzle is D0 = 12 mm and the area
contraction ratio is 30.3. A cylindrical actuator equipped withthree holes with a diameter d0 of 2 mm is attached to the top of
the round nozzle, as shown in Fig. 1. The height of the actuatoris 12 mm. The periodic disturbance added to the jet is generatedusing loudspeakers. The loudspeaker is sinusoidally vibrated in
the speaker box, and the vibration propagates through a vinyl
tube to the excitation holes and forms synthetic jets at theexcitation holes.
A downstream view of the nozzle is shown in Fig. 2. The
nozzle has three holes at azimuthally equal intervals. Three
synthetic jets are used to excite the jet at equal phases from theoutside in the radial direction toward the center.
The coordinate system has the origin at the center of thenozzle outlet, with x in the mainstream direction, y in the lateral
direction, and z in the depth direction, as shown in the left
diagram of Fig. 2.

Experimental Conditions
Experimental conditions are shown in Table 1. Air is ejected1

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
Table 1. Experimental conditions.

Figure 1. Round nozzle with small holes to add disturbances tothe jet in the nozzle.

Jet gasAir

Jet Reynolds number, Re

2,000

Nozzle diameter, D0 [mm]12Issuing velocity, Uc0 [m/s]2.55

Frequency for generation of vortex
rings, fv [Hz]120

Non-dimensional frequency, St

0.565

Input signal to loudspeaker

Sine wave

Frequency of input signal, fs [Hz]120

Turbulence intensity, u’/Uc0 [-]0.10

Figure 2. Excitation directions and visualized section.

vertically upward from the round nozzle into the ambient air.
The Reynolds number Re of the jet is set to be 2,000. The issuing
velocity Uc0 under this condition is 2.55 m/s at the nozzle exiton the centerline, as the Re is defined by the Uc0 and D0. The
formation frequency of vortex rings of the round jet in the naturaltransition at Re = 2,000 is 120 Hz. This value was determined byflow visualization and velocity measurement using a hot wire
anemometer. Therefore, the input signal to the loudspeaker is asine wave with a frequency of fs = 120 Hz. The nondimensionalfrequency St = fs D0/Uc0 is 0.565. The velocity in the flow
direction of the synthetic jet changes periodically from -0.4 to
0.5 m/s, and the turbulence intensity is approximately 10%

Figure 3. Computational domain.

SETUP FOR NUMERICAL SIMULATION
Computational Domain and Elements
Figure 3 shows a computational domain. The computational
domain consists of the cylindrical part of the nozzle with theexcitation holes and the main computational domain (spaceejected from the nozzle). The sizes of the main computational
domain are 5D0 in the radial direction, 2π in the azimuthal
direction, and 12 D0 in the mainstream direction. The
computational elements are made smaller from the center of thenozzle toward the shear layer and larger from the shear layer
toward the lateral boundary, and they are made smaller near thenozzle exit in the mainstream direction. The computational
elements are made in a cylindrical coordinate system outside theradial direction from the Cartesian coordinate system near thecenter of the nozzle.
The nozzle at the bottom of the main computational domain
is cylindrical with a diameter of 12 mm (D0) and a height of 12mm, and the experimental actuator is reproduced. The excitation
holes are a cylinder with a diameter of 2 mm (d0) in theexperiment, but in the calculation, they are rectangular with aside of 2 mm, and three holes are provided at equal intervals in
the azimuthal direction.

Experimental Methods
The jet cross-section was visualized using a laser sheet witha thickness of 1 mm. Fine particles with a nominal diameter of 1
μm generated by a smoke generator were mixed with the jet fluid
and then the air jet issued into the still atmosphere from thenozzle. A high-speed camera was used to capture cross-sectionsin the streamwise direction (position is shown in Fig. 2) and thehorizontal sections perpendicular to the mainstream.
For a PIV measurement, it is necessary to mix particles inthe surrounding air. Therefore, a duct was installed around thenozzle, and the inside of the duct was filled with particles.
Images for time-resolved PIV were recorded using a high-speed
camera at 12,500 fps with a resolution of 1,024 × 1,024 pixels.
Velocity fields were obtained from the images using a PIV
analysis software with an overlap of 50% and an nterrogation
window of 32 × 32 pixels.
In the addition, an oscillating mirror method was adopted fora time-resolved 3D imaging. The laser sheet was scanned at highspeed by the oscillating the mirror. Several cross-sectional
images were taken with a high-speed camera almost
simultaneously, and three-dimensional images were constructedby stacking these images.

Boundary and Initial Conditions
As the velocity inflow boundary condition, a velocity profilein the mainstream direction flowing into the computational
domain from the bottom of the cylindrical nozzle (inlet in Fig.
3) gives a Top-Hat type velocity profile as expressed by thefollowing equation (1).

212th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

(1)
Here, 𝑟 = 𝑦 + 𝑧 , and two constants a and r0 are a = 30 andr0 = 0.45 on the based the measured velocity profile using a hot
wire anemometer. To set the Re of the issuing air to 2,000, theUcinlet is 2.5 m/s. For the velocity Ux in the mainstream direction,a spatially and temporally random disturbance Unoise with ± 0.5%of the Ucinlet is added as a disturbance simulating white noise.
The disturbances caused by the periodic vibration of theloudspeaker are reproduced under the pressure boundary
condition at the excitation hole, and the boundary condition is
given by the following equation (2).
𝑝/𝜌 = 𝐴 sin(2𝜋𝑓 𝑡)(2)

The amplitude A is adjusted so that the turbulence intencity isclose to the experimental value, and the A is chosen to be 2.0
m2/s2. The frequency fs is set to 120 Hz and the same as thesinusoidal input signal to the loudspeakers.
The boundary conditions of the side and outlet in Fig. 3 areconstant total pressure and take into account the inflow andoutflow. The area corresponding to the wall surface in Fig. 3 (thebottom surface excluding the cylindrical nozzle in the main
computational domain and the inner wall of the cylindricalnozzle) adopts a non-slip condition.
Initial conditions are set to 0 for velocity U [m/s] andpressure divided by density p/ρ [m2/s2] in all regions except theboundaries expressing in Eqs. (1) and (2).

Figure 4. Visualized streamwise cross-sections of a excited air
jet.

Computational Methods
The governing equations are the continuity equation and theNavier-Stokes equation for an incompressible fluid. A numericalsimulation was performed using an open source software
OpenFOAM under the same conditions as in the above
experiment. We used the pimpleFoam solver for uncompressible
and unsteady flows. The solver discretizes the governing
equations using the finite volume method and calculates velocity
and pressure using the PISO (Pressure Implicit with Splitting ofOperators) method. The turbulence models are not used, that is
an implicit LES. The calculation time interval is adjusted foreach calculating step so that the maximum Courant number does
not exceed 0.5. The time integration is an implicit Euler method.

Figure 5. Visualized streamwise cross-sections on the centerlineof a round helium gas jet at Re = 1,000.

RESULTS AND DISCUSSION
Flow Visualizations by a Planar Laser Mie Scattering
Figure 4 shows visualized images of an x-y cross-section of
the excited jet in the streamwise direction. Figure 4(a) shows an
instantaneous image, and Fig. 4(b) is a phase-averaged image at
the same phase as Fig. 4(a) based on the phase of the exciting
frequency. A white arrow at the bottom in Fig. 4 indicates theexciting direction. The periodic excitation from the radial
direction brings the formation position of a vortex ring closer tothe nozzle exit than that of the jet of natural transition. In
addition, the collapse position of the vortex ring is also on the
upstream side, and a series of phenomena from the formation to
the collapse of the vortex ring is enhanced (Muramatsu andTanaka, 2021). A side jet branching radially outward near x/D0
= 2.0 are formed on the unexcited side. As shown by red lines inFig. 4, the vortex ring precedes on the side where the side jet isformed. This excited side jet is considered to be a bifurcating

Figure 6. Visualized horizontal cross-sections of a excited air jet
at x/D0 = 2.0.

flow by the same mechanism as side jets in a helium gas jet
shown in Fig. 5.
Figure 6 shows the visualized images of the horizontal crosssection (y-z cross section) at x/D0 = 2.0. Figures 6 (a) and (b) arerespectively an instantaneous image and phase average image.
Figure 6 has the same phase as that of Fig 4. As seen in Fig. 4,
the images of the horizontal cross section at x/D0 = 2.0 are thephase of a vortex ring when the jet passes the cross section. The
excitation changes the shape of the y-z cross section of the jet toa triangle. The vortical structures inside and outside the vortex
ring shown in red circles in Fig. 6(a) are pairs of streamwise3

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
vortices with streamwise vorticity, as will be shown later in the
results by PIV measurements. It can be observed that the twocounter-rotating streamwise vortices are paired, and a side jet isformed between a pair of streamwise vortices.
Because the phase-averaged images in Fig. 4(b) and 6(b)clearly show the structures of the potential core, vortex rings,
streamwise vortices, and side jet until the vortex ring collapses,
it is indicated that these structures are a flow locked-in at theexciting frequency. Therefore, these structures can be treated as
the aimed periodic phenomenon.
Focusing on the deformation of the vortex ring, it isconsidered that the vortex ring is three-dimensionally wavy
because it is deformed in the streamwise direction in Fig. 6 andin the azimuthal direction in Fig. 7.

Figure 7. The streamwise vorticity field in the horizontal crosssections.

Visualizations of Vortical Structures in the Horizontal
Plane by PIV Measurement
To visualize the vortical structures in the horizontal crosssection (y-z cross section) more clearly, the streamwise vorticity
ωx was obtained from the measured results by a PIV and isshown as a contour diagram in Fig. 7. The contour diagrams are
dimensionless by the maximum vorticity in each cross-section,and the positive values are counterclockwise. Figure 7(a) and (b)respectively show the cross sections at x/D0 = 1.5 and x/D0 = 2.0,
and they are in phase. The flow structures indicated by the redcircles in Fig. 6 are streamwise vortices, because they havevorticities ωx as indicated by the circles in Fig. 7(b).
The vorticity of the streamwise vortex can be seen at six
locations in Fig. 7(a). Since the jet is excited from three locations,
two streamwise vortices are formed for each excited hole. On theother hand, in Fig. 7(b), streamwise vortices can be seen at 12
locations, which is twice as many as in Fig. 7(a). It can also beseen that the outgoing flows are generated between thestreamwise vortex pairs.
Next, we focus on the y-z cross section in the vortex ring at
x/D0 = 2.0 shown in Fig. 8. In this y-z cross section, there are six
regions where the absolute value of the streamwise vorticity ωxis larger than that of the streamwise vortices, as indicated by thecircles in Fig. 8. Comparison with the visualized image in Fig. 6
shows that the vorticities are expressed in the cross-section of
the vortex ring. If the vortex ring is not deformed in thestreamwise direction, the vorticity ωx does not appear in thehorizontal cross-section (y-z cross-section). However, if thevortex ring is deformed in three dimensions as described
previously, the vorticities are measured as shown in theschematic diagram in Fig. 9. When the vortex ring is threedimensionally deformed as shown in a yellow ring in Fig. 9 andflows downstream while rotating, the vorticities rotating in the
red and blue directions are measured in the plane as the vortex
ring passes through the measurement plane. The numbers in Fig.
9 correspond to the numbers assigned to the vorticity in Fig. 8,
and the red and blue arrows represent the vorticity that appears
when the vortex ring rotating in the poloidal direction passes
through the plane. It can also be seen that the outflows are
generated at the upper part of the mountainous part of the wavy
deformed vortex ring from Figs. 8 and 9.

Figure 8. The streamwise vorticity field in the horizontal crosssection at x/D0 = 2.0.

Figure 9. Wavy deformation of a vortex ring.

in a wavy shape, and a pair of streamwise vortices can be seenaround the vortex ring. The streamwise vortices extend from theoutside of the upstream vortex ring to wrap around the inside ofthe downstream vortex ring.
Figure 11 is a schematic diagram of the vortical structuresinferred from the three-dimensional imaging of Fig. 10. The side
jets are formed between the upper side of mountains of thepoloidally rotating vortex rings and the streamwise vortex pairsthat develop in the braid region.

Visualizations of Vortical Structures by Numerical
Simulation
Figure 12 shows the round air jet with bifurcating flows,
namely side jets, by a numerical simulation. Based on thecalculated velocity field, the jet is visualized with particles
ejected from the nozzle. This visualization allows us to confirm
the vortex rings and side jets. Since the stripe pattern between
the side jets and the mainstream that appears in the crosssectional image in Fig. 4 also is observed in Fig. 12, it is judgedthat the reproducibility of the flow field by calculation is high.

3D Image of a Jet with Bifurcating FlowsThe spatial distribution of the wavy deformed vortex ringshown in Fig. 9 and the streamwise vortices shown in Fig. 7 isrevealed from the three-dimensional imaging of the jet shown in
Fig. 10. As shown in Fig. 9, the vortex ring is greatly deformed

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12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022

Figure 13. Comparison of extracted vortical structures bythreshold values of Δ.

Figure 10. 3D image of the jet with side jets.

Figure 11. A schematic diagram of the vortical structures.
Figure 14. Streamwise vorticity fields (upper) and radial velocity
fields (lower) in horizontal sections.

Figure 12. Visualization of flow field obtained by numericalsimulation with tracer particles.

coding in Fig. 13 corresponds to the positive (red) to andnegative (blue) values of ωx as similar in Figs. 7 and 8. Figure13(a) shows the case where the threshold value is set such thatthe roll-up of the shear layer seen in Fig. 4 is identified as avortex, and Fig. 13(b) shows the case where the threshold value
is set such that the roll-up is not identified as a vortex. Figure 13(a) also captures the deformation of the shear layer due toexcitations in the nozzle. The deformation of the shear layer islocated below the first vortex ring, and the vortex ring isaccompanied the deformation of the shear layer caused by
disturbances caused in the nozzle. However, Fig. 13(a) isdifficult to capture the relationship between the vortex ring andthe streamwise vortices, so in the subsequent discussion, thethreshold is set as shown in Fig. 13(b).

Although the results are not shown here, the values of the mean
velocity and turbulence intensity are also close to the measured
values by our experiment.
From the numerical simulation result, vortical structures are
extracted using the Δ value (Chong et al., 1990) to investigate
coherent structures such as vortex rings and streamwise vortices,and to consider the structure of the flow field. The vorticalstructures identified by the isosurface of the Δ value strongly
depends on the threshold value. Figure 13 shows the differencein vortical structures depending on the threshold value. The color

Secondary Instability
Figure 14 shows the streamwise vorticity ωx and radial
velocity Vr in three horizontal cross sections (y-z cross sections)
to consider the relationship between the growth of streamwise
vortices and the formation of side jets. The locations of the threecross sections are the braid regions (x/D0 = 1.64 and 2.58) andthe vortex ring region (x/D0 = 2.11), as shown in Fig. 13(b).
Disturbances due to excitation appear as strains in the shear layer,
and the strains are induced streamwise vorticities in the braidregion (Fig. 13(a)), causing in in-plane flows (Fig. 13(d)). The

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12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
streamwise vorticities are sufficiently developed by elongationin the deformed vortex ring region (Fig. 13(b)). Three pairs of
streamwise vortices are formed in this process, and it can be
confirmed that the streamwise vortices extend from the inside ofthe downstream vortex ring to the outside of this vortex ringfrom the isosurface of Δ in Fig. 12 (b).
It can be seen that the in-plane circulations that occur as aresult of the development of the streamwise vorticities in the
braid region induces radial velocities between streamwise vortex
pairs from Figs. 13(c) and 13(f). These radial outward flows
occur uninterruptedly from the center of the jet to the peripheryin the braid region (Fig. 13(f)). On the other hand, the outflows
are divided by the flow toward the center in the vortex ringregion (Fig. 13(e)). Therefore, the side jets are considered to be
the phenomenon by the secondary instability that develops in the
braid region.

Figure 15. Periodic changes in vortical structures.

REFERENCES
Brancher, P., Chomaz, J. M., and Huerre, P., 1993, "Direct
numerical simulations of round jets: Vortex induction and side
jets," Physics of Fluids, Vol. 6, pp. 1768-1774.
Chassaing, P., Antonia, R. A., Anselmet, F., Joly, L., andSarkar, S., 2002, "Variable density fluid turbulence," Vol. 69 of
Fluid mechanics and its applications. Kluwer Academic
Publishers, Chap. 8.
Chong, M. S., Perry, A. E. and Cantwell, B. J., 1990, "A
general classification of three-dimensional flow fields," Physics
of Fluids, Vol. 2, No. 5, pp. 765-777.
Kato, Y. and Muramatsu, A., 2017, "Effect of initial velocity
gradient on side-jets formation in a round jet with constantdensity,” Extend Abstract of the Ninth JSME-KSME Thermaland Fluid Engineering, TFEC9-1311.
Kawabe, K., Harada, H., and Muramatsu, A., 2014, "Threedimensional imaging of a helium gas jet flow using planer laser
Mie scattering,” Proceedings of the 16th International
Symposium on Flow Visualization.
Kyle, D. M. and Sreenivasan, K. R., 1993, "The instability
and breakdown of a round variable-density jet,” Journal of Fluid
Mechanics Vol.249, pp. 619-664.
Monkewitz, P. A., Bechert, D.W., Barsikow, B, andLehmann, B., 1990, “Self-excited oscillation and mixing in aheated round jet,” Journal of Fluid Mechanics, Vol. 213, pp.
611-639.
Monkewitz, P. A. and Sohn, K. D., 1988, “Absoluteinstability in hot jets,” AIAA. Journal, Vol. 26, No.8, pp. 911916, 1988.
Muramatsu, A., Gamba, M., and Clemens, T. N., 2008, “Sidejets generated in a round helium gas jet,” Proceedings of 2ndInternational Conference on Jets, Wakes, and Separated Flows.
Muramatsu, A., Saitoh, M. Kawabe, K and Kikuchi, T., 2015,“Side-Jet Formation in a Round Jet with Uniform Density,”
Preprint of JSME Fluid Engineering Conference, No. 0209, (inJapanese).
Muramatsu, A. and Hara, I., 2016, “Side-jets formation in around jet with an external counter flow,” Preprint of JSME Fluid
Engineering Conference, No. 0210, (in Japanese).
Muramatsu, A. and Tanaka, K., 2021, “Flow structures in theinitial region of a Round Jet with azimuthally deformed vortex
rings utilizing a sound wave,” Edited by Braza, M. et al., FluidStructure-Sound Interaction and Control, Springer, pp. 267-273.
Toyoda, K. Muramatsu, Y. and Hiramoto, R., 1999,“Visualization of the Vortical Structure of a Circular Jet Excitedby Axial and Azimuthal Perturbations,” Journal of Visualization,
Vol. 2, No. 1, pp. 17-24.

Interference of Vortex Ring and Streamwise Vortices
and Formation of Side Jet
Figure 15 shows vortical structures by the isosurface of the
Δ value in time series. The contours of the radial velocity aresuperimposed on the right side in Fig. 15. The data wereextracted within one cycle T = 1/ fs based on the exciting
frequency fs. It can be seen that the streamwise vortices growupstream from the inside of the second vortex ring in Fig. 15 (a)to the outside of the second vortex ring in Fig. 15 (c). That is, thestreamwise vortices extending from the inside of thedownstream vortex ring are connected to the vortex ring so as towrap around the outside of the upstream vortex ring. In this way,
the development of the streamwise vortices is accompanied by
the wavy deformation of the upstream vortex ring, and theinterference between the vortex ring and the streamwise vortices
is periodically repeated. However, this result on the interference
between the vortex rings and streamwise vortices seems to differ
from the previous result (Toyoda et al., 1999) that streamwise
vortices form in a hairpin shape. It can be seen that the
disturbances are propagated from the downstream to the
upstream by the streamwise vortices. Therefore, it is consideredthat the streamwise vortices cause local absolute instability(Monkewitz and Sohn, 1988; Muramatsu and Hara, 2016), sothat large velocity fluctuations due to a self-excited oscillation
produce in the potential core.

CONCLUSIONS
The formation mechanism of side jets in the initial region of around jet was investigated using a round air jet with an artificiallyformed bifurcating flows and the following findings were obtained.
Side jets are formed as a result of secondary instability thatdevelop in the braid region. A side jet is formed between the upper
part of the mountain part of a three-dimensionally wavy vortex
ring and a pair of the streamwise vortices in the blade region. The
side jet is the flow induced radially outward in the region between
the poloidal circulation of the vortex ring and the circulation of thestreamwise vortex pair. The streamwise vortex pair extends fromthe downstream vortex ring to the upstream vortex ring, and it isthought that disturbances are transmitted to the downstream by thestreamwise vortex pair and they cause absolute instability. The
formation of side jets is a periodic phenomenon linked to the rollup of shear layer, namely the formation of a vortex ring. The
apparently continuous flows of the side jets are the result of
periodically repeated interference between the wavy deformationof the vortex ring and the streamwise vortices.

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