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. 2002 Vol. 43, No. 2, pp. 224-236.
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. 349-357.
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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
Scenarios of the 3D distortion of 2D ring vortex and instability wave
1. Scheme and experimental technique
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.
2. Experimental conditions (top – hat mean velocity profile)
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.
3. Smoke visualization of a round jet (cross section)
Natural streaky structures
Scheme of the
experiment
Video file No. 2.1
Double click
here
3. Smoke visualization of a round jet (cross section).
Natural case
Video file No. 2.2
Double click
here
3. Smoke visualization of a round jet (cross-section).
Generation of the streaky structures by roughness elements
Video file No. 2.3
Double click
here
3. Control by a round jet mixing with use of local
blowing/suction (f = 480 Hz)
Scheme of the
experiment
Double click
here
Video file No. 2.4
3. Stroboscopic visualization of a round jet at various
frequency of acoustic influence
Video file No. 2.5
Double click
here
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
3. Smoke visualization with use of a laser sheet.
Jet longitudinal section in region of the streaky structures
evolution.
Video file No. 2.7
Double click
here
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
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
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
Double click
here
Video file No. 2.8
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 /   6700).
8. Scenarios of the 3D distortion of 2D ring vortex and
instability wave
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.
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 - 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.