Transcript Chapter 7.ppt

Chapter 7. Parabolic round jet in a shear cross flow
Multimedia files Nos. 7.1 – 7.5
The results of researches presented in presentation are published in the following
main articles:
1.
Grek G.R., Kozlov V.V., Kozlov G.V., Litvinenko Yu.A. Instability modeling of a laminar round jet with parabolic mean
velocity profile at the nozzle exit // Vestn. NSU. Seria: Physics. 2009. Vol. 4. Vip. 1, pp. 14-24, in Russian
2.
Kozlov V.V., Grek G.R., Kozlov G.V., Litvinenko Yu.A. Physical aspects of subsonic jet flows evolution // The collection of
proceedings «Successes of mechanics of continuum» to the 70-anniversary of academician V.A. Levin, 2009. Dalnauka,
Vladivostok, pp. 331-351, in Russian
Round jet with parabolic mean velocity profile at the nozzle exit.
Jet instability to the weak cross flow.
Mean velocity profile at the nozzle exit
1 - Mean velocity profile at the nozzle exit
without overlay
2 - Mean velocity profile at the nozzle exit
with overlay
Jet velocity (U0) – 4 m/s,
Cross flow velocity (Ujet) – 0.5 m/s.
Smoke visualization patterns of the jet cross sections
Video file No. 7.1
Double click
here
Scheme of the jet folding into the counter rotating stationary
vortex pair under action of a cross flow (Scheme is taken from the
work by Lim et al., 2001 )
High – frequency secondary instability
of the round jet tangential ejections caused by the cross flow
Video file No. 7.2
Double click
here
CONCLUSIONS
 It is found, that round jet instability to cross flow result in deformation of a jet as
tangential ejections of gas from its periphery in ambient space.
 It is revealed folding of ejections into the counter rotating vortex pair.
 It is shown, that tangential ejections are subjected to high – frequency secondary
instability.
Parabolic round jet in a shear cross flow (flat plate boundary
layer)
Direct numerical simulation (DNS) by S. Bagheri et al.
Global stability of jet in cross flow // J Fluid Mech., vol. 624, pp. 33-44
Experimental set - up
Boundary layer mean velocity
profile, x = 250 mm
Round jet mean velocity profile at
the nozzle exit
Smoke visualization of the jet in a flat plate boundary layer at
K = Ujet/U0  3 (acoustic field frequency from 30 up to 180 Hz)
Double click
here
Video file No. 7.3
Cross flow direction
Streamwise jet section
Smoke visualization of the jet in a flat plate boundary layer at
K = Ujet/U0  3 (acoustic field frequency F = 180 Hz)
Double click
here
Video file No. 7.4
Cross flow direction
Jet cross section at different distances from the flat plate
Smoke visualization of the jet in a flat plate boundary layer at
K = Ujet/U0  3 (acoustic field frequency F = 180 - 1000 Hz)
Double click
here
Video file No. 7.5
Cross flow direction
Jet cross section at different distances from the flat plate
KEY POINTS:
 Characteristics of development of a round jet with a top hat and parabolic mean velocity
profile at the nozzle exit essentially differ.
 Instability of a round jet with a parabolic mean velocity profile leads to its deformation in
the shape of the tangential ejections of gas from the jet periphery by means of cross flow into
the ambient space, folding of ejections into the counter rotating vortex pair and thereof to
reduction of the jet core size.
 The round jet with a parabolic mean velocity profile in a cross shear flow is subjected by
folding into the counter rotating stationary vortex pair.
 The most unstable global modes with high frequencies represent wave packets on the
counter rotating stationary vortex pair. These modes are connected with W- like vortex
structures on a flow shear layer.
 Global modes at low frequencies also have considerable amplitude in a jet wake closer to
the wall.
 Growth in jet penetration and reduction in the near-field entrainment of cross-flow fluid
by a parabolic jet in cross flow is found.
 The jet/crossflow interfaces of the parabolic jet in cross flow might have undergone a
‘‘stretching and thinning’’ process caused by the cross-flow.