1. Chapter 2. Physical processes responsible for evolution and downstream breakdown of a subsonic round jet
Multimedia files Nos. 2.1 – 2.8
The results of researches presented in presentation are published in the following
main articles:
1. V.V. Kozlov, G.R. Grek, L. Löfdahl, V.G. Chernorai, M.V. Litvinenko Role of Localized Streamwise Structures in the Process
of Transition to Turbulence in Boundary Layers and Jets (Review) // J. Appl. Mech. Tech. Phys Vol. 43, No. 2, pp
2. M.V. Litvinenko, V.V. Kozlov, G.V. Kozlov, G.R. Grek, Effect of streamwise streaky structures on turbulisation of a circular
Jet // J. Appl. Mech. Tech. Phys., 2004, Vol. 45, No. 3, pp
Scheme and experimental technique
Experimental conditions (top – hat mean velocity profile)
Multimedia files Nos. 2.1 – 2.7
Smoke visualization of the round jet and its cross sections at different
distances from the nozzle exit.
Smoke visualization of the round jet at region of the azimuthal
vortex structures
Multimedia file No.2.8
Acoustic effect on the ring vortices scale
8. Scenarios of the 3D distortion of 2D ring vortex and instability wave

2. 1. Scheme and experimental technique
Scheme of experiment with a round jet is shown. Methods of research: hot - wire anemometer measurements and jet flow smoke visualization with stroboscopic analysis of the observed phenomena.
1 − settling chamber, 2 − grids, 3 − nozzle, 4 − potential jet core, 5 − Kelvin-Helmholtz ring vortices ,
6 − streaky structures, 7 − laser sheet position,8 − dynamic loudspeaker, 9 – video camera, 10 – hot – wire
anemometer probe,11 – hot – wire anemometer, 12 – analog to digital converter, 13 - computer.

3. 2. Experimental conditions (top – hat mean velocity profile)
Mean velocity profile at the short nozzle exit (at the left) is top - hat mean velocity profile. Large velocity gradient is observed in a narrow shear layer. In this shear layer the maximum level of root-mean-square oscillations of the streamwise velocity component is observed.
Distribution of a mean (U) and fluctuation (u’) streamwise velocity components at various distances from a nozzle exit (1-8 mm; 2–12 mm; 3–22 mm; 4–32 mm),
U0 = 4 m/s.

4. Scheme of the experiment
3. Smoke visualization of a round jet (cross section) Natural streaky structures
Scheme of the experiment
Double click
here
Video file No. 2.1
Development of the natural streaky structures (the acoustic field is imposed) is shown. Feature of the natural streaky structures is that their appearance is chaotically. On the other hand, the forced streaky structure generated by roughness elements are stable and clear.

5. 3. Smoke visualization of a round jet (cross section). Natural case
Video file No. 2.2
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here
Development of a round jet with natural disturbances is shown at the imposed acoustic field. In development of a jet there is an interaction of the ring vortices with the natural longitudinal streaky structures. There is the disturbances intensity growth downstream.

6. 3. Smoke visualization of a round jet (cross-section)
3. Smoke visualization of a round jet (cross-section). Generation of the streaky structures by roughness elements
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here
Video file No. 2.3
Development of a round jet under the action of acoustic field. Longitudinal streaky structures are generated by roughness elements pasted on inner surface at the nozzle exit. This allows to stabilize of azimuthal positions of the L- or W - like vortices and to study them. Velocity oscillations observable in the shear region are connected to passage of the Kelvin-Helmholtz ring vortices.

7. Scheme of the experiment
3. Control by a round jet mixing with use of local blowing/suction (f = 480 Hz)
Scheme of the experiment
Video file No. 2.4
Secondary instability of a round jet excited by means of local blowing/suction is realized. A source position of high frequency disturbances is placed in region of the roughness position. Distinguishing feature is, that the disturbed area from a dotted source increases and covers all jet far downstream.
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here

8. 3. Stroboscopic visualization of a round jet at various frequency of acoustic influence
Video file No. 2.5
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here
Round jet evolution under action of acoustic field with different frequencies is shown in this video at stroboscopic effect. It is possible to observe of a round jet contours as a whole and some ring vortices (Kelvin-Helmholtz instability)

9. 3. Smoke visualization with use of a laser sheet
3. Smoke visualization with use of a laser sheet. Scanning of a round jet at constant jet velocity but various distance from a nozzle exit
Double click
here
Video file No. 2.6
As a light source the laser sheet is used. Jet flow direction is from top to down at constant jet velocity. At the laser sheet downstream movement it is possible to observe various stages of the round jet evolution. So for example, ring vortices and longitudinal streaky structures which at interaction locally destroy a ring vortex result in azimuthal structures origin (L- or W - like vortices).

10. 3. Smoke visualization with use of a laser sheet
3. Smoke visualization with use of a laser sheet. Jet longitudinal section in region of the streaky structures evolution.
Video file No. 2.7
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here
Scanning of the longitudinal section of a round jet with help of laser sheet travelling across jet is demonstrated. Flow direction from top to down.

11. 4. Smoke visualization of the round jet and its cross sections at different distances from the nozzle exit. Interaction between ring vortices and streaky structures result in azimuthal vortices origin. Process is synchronised by acoustic effect.
2D ring vortices
3D azimuthal vortex structures

12. 5. Smoke visualization of the round jet at region of the azimuthal vortex structures (Λ or  - like vortices ) “heads” (R1) and its “legs” (R2).
1- ring vortices
2- streaky structures

13. 6. Smoke visualization with use of a laser sheet
6. Smoke visualization with use of a laser sheet. Scanning of a round jet at constant distance from a nozzle exit but various jet velocity
Video file No. 2.8
Double click
here
On given video it is shown as the structure of a jet in the same section with velocity growth changes. As appeared, the jet velocity growth result in to earlier development of disturbances but the scenario of their development does not vary.

14. 7. Acoustic effect on the ring vortices scale
Smoke visualization patterns of the round jet at acoustic effect with frequency
f = 110 Hz (а) and f = 250 Hz (b), U0 = 5 m/s (Re = U0  d / ν  6700).

15. 8. Scenarios of the 3D distortion of 2D ring vortex and instability wave
Scenarios of the disturbances downstream evolution are shown at interaction of longitudinal streaky structure with Tollmien - Schlichting wave (right) or a ring vortex (left).
Scenario of the 3D distortion of 2D disturbances at interaction of the ring vortex with streaky
structures (a) and scenario of the 3D distortion of the Tollmien – Schlichting wave (b): 1 − Λ or
– like vortices; 2 − streaky structures; 3 − ring vortex; 4 − 3D distortion of the 2D wave;
5 − roughness elements.

16. KEY POINTS :
Summarizing the above experimental results, we focus on the following aspects of the laminar jet breakdown:
 The longitudinal structures of velocity perturbations contributing to the laminar flow breakdown can be generated close to the jet origin, i.e., the nozzle exit.
 Interaction of the ring vortices with the longitudinal disturbances is similar to the deformation of two-dimensional instability waves in a boundary layer by local flow non-uniformities.
 A result of the interaction is the generation of “beams” in the form of Λ- or Ω – like structures spaced over the ring vortex.
 An intensive mixing of the jet with the surrounding air occurs in the region of the heads of Λ- or Ω – like structures enhancing the jet spreading and its transition to the turbulent state.
 Under the external acoustic forcing, the passage frequency and the scales of the ring vortices are modified as well as mixing of the jet with ambient air becomes more profound.