| Streamwise
Vorticity Formation in a Transverse Jet
Transverse jets have a wide range of applications in engineering,
from propulsion and power generation to exhaust dispersion.
The mixing properties of the transverse jet significantly
impact the performance of these systems. Experimental studies
have identified a small number of coherent flow structures
which appear to control the evolution of the jet boundary,
suggesting that the mixing characteristics of the transverse
jet can be altered by manipulating these structures.
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Figure
3 Total velocity magnitude in the
centerplane z = 0, 
= 4.50. |
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The complexity of the flow renders detailed numerical simulation
nearly intractable, but Marzouk and Ghoniem have attempted
a careful reduction of the physics in order to reproduce the
essential dynamics of the underlying flow structures. They
used a vortex filament method to model a transverse jet at
high jet-to-crossflow velocity ratio, capturing the mechanisms
leading to the formation of streamwise entraining structures—namely,
the counter-rotating vortex pair (CVP).
Results show that the formation of the CVP is initiated very
close to the nozzle, fed by jet boundary layer vorticity.
Large-scale vortex rings form in the early stages of the jet
shear layer due to rollup. As each ring convects into the
flow, its lee side is lifted upwards and folded backwards,
eventually transforming each ring into two crescent-shaped
sections connected by counter-rotating vortices aligned along
the jet trajectory. Lee and windward sides of neighboring
vortex rings merge, canceling vorticity in a plane normal
to the jet trajectory and leaving only the counter-rotating
vortex pair. The instantaneous vorticity field supports the
assertion that the counter-rotating vortices are driven by
the instantaneous dynamics of the flow (Figure 3).
INVESTIGATORS
A. F. Ghoniem and Y. M. Marzouk, Massachusetts Institute of
Technology.
PUBLICATION
Y. M. Marzouk and A. F. Ghoniem, “Mechanism of streamwise
vorticity formation in a transverse jet,” American Institute
of Aeronautics and Astronautics paper AIAA 2002–1063
(2002).
URL
http://centaur.mit.edu/rgd/ |
