The mixing of the round jet normal to a uniform crossflow is studied
for a range
of jet-to-crossflow velocity ratios, r, from 5 to 25.
Planar laser-induced fluorescence
(PLIF) of acetone vapour seeded into the jet is used to acquire quantitative
images of the scalar concentration field. Emphasis is placed on r=10
and r=20 and a few select images are acquired up to r=200.
The Reynolds number based on the jet exit diameter, d,
and the exit velocity varies from 8400 to 41 500.
Images are acquired for conditions in which the product rd is
held constant, requiring decreasing d for increasing r.
Results from this experimental study concern structural events of the
interaction region, and mixing and mean centreline concentration decay
in the near and
far fields. The results cover all three regions of the transverse jet,
and suggest that the
jet scales with three length scales: d, rd and
Events within the vortex interaction region display d-scaling,
including the crossflow boundary layer separation and roll-up. Over
the range of velocity ratios studied, the
vortex interaction region shows r-dependent variations in the
flow field, including the
emergence of jet fluid in the wake structures for r>10 and
a slower development of
the counter-rotating vortex pair (CVP) in higher-r jets.
The trajectory and physical dimension of the jet in both the near and
display rd-scaling. The near field is characterized
by a centreline concentration decay
along the centreline coordinate s of s−1.3,
from the decay rate (s−1) of the free
jet. When normalized by rd, the decay of each velocity-ratio jet
branches away from
the s−1.3 decay, approaching a decay of
s−2/3, a rate predicted by modelling efforts.
The branch points represent a transition in the flow field from enhanced
reduced mixing compared to the free jet. When normalized by
r2d, the branch points
occur at a uniform jet position, s/r2d=0.3,
which is viewed to be the division between
the near and far fields. Self-similarity is not seen in the near field,
but may be present
in the far field.
The view of the branch points as a place of transition in the flow is
the probability density function (p.d.f.) of concentration along the upper
edge of the
jet. Before the branch points, the p.d.f.s are non-marching in character,
and after the
branch points, they are tilted in character.
Instantaneously, the CVP is asymmetric in shape and concentration. End
reveal extensive motion of the CVP and plan views show this motion can
both axisymmetric and sinusoidal motion. Ensemble-averaged images show
concentration is asymmetric about the centreline plane.