Abstract:
The flow characteristics of symmetric and asymmetric free air jets in the near
field have been studied experimentally. Jets produced by circular parabolic
nozzle with different exit and upstream flow conditions were investigated to
determine their influence on the flow field. Different exit conditions were
obtained by the attachment of exit lengths, vertical flanges, wedge shape
sections and by changing the Reynolds number. The upstream flow condition
was changed by acoustic excitation of frequency 0 - 300 Hz. Measurements
were taken in the Reynolds number range Red = 2 x 104 to 1 x 105. For each
case, axial mean velocity and static pressure were measured by the pitot-static
tube and axial turbulent intensity by the constant temperature hot-wire
anemometer.
The mean static pressure within the potential core was positive and that in
the mixing region negative. The centerline mean velocity near the exit
decreased slightly and corresponding mean static pressure increased and these
were suppressed due to the increase of the exit length, Reynolds number and
wedge angle. This decay of centerline mean velocity was influenced by
upstream excitation and maximum occurred at the preferred mode. In the
case of nozzle with exit vertical flange, the centerline mean velocity near the
exit increased sharply and corresponding centerline mean static pressure
decreased. The pref~rred and suppression mode of excitation were at Std = 0.58
and 1.85 for normal nozzle and those for wedge shaped nozzle 0.31 and 1.8
respectively. :rhe centerline turbulent intensities decreased with the increase
of exit vertical flange, length, wedge angle and Reynolds number. And these
intensities further decreased at the suppression mode of excitation. On the
other hand, at the preferred mode of excitation, the centerline turbulent
intensities amplified much more than it's unexcited value.
The stream-wise mean velocity profile showed saddle shaped behavior,
which moved towards the downstream distance and disappeared at the end of
the potential core. Corresponding mean static pressure dropped significantly,
but the location of minimum negative pressure did not correspond to that of
the maximum peak of mean velocity. The pe~k value of the saddle shaped
profile increased due to exit vertical flange and decreased due to the increase
of the exit Reynolds number, length and wedge angle. The peak turbulent intensity along the transverse direction was found to be suppressed with the
increase of exit vertical flange, length, Reynolds number and wedge angle.
With the upstream excitation the turbulent intensity further decreased below
it's unexcited value.
The increase of nozzle exit length, Reynolds number and the presence of
vertical flange were found to increase the potential core. The entrainment
rate increased with the increase of Reynolds number but decreased in the
presence of exit vertical flange, length and wedge angle. The viscous shear
stress decreased and the Reynolds shear stress increased towards the boundary
layer. This was due to the continuous transformation of energy from the
mean flow field to the generation of turbulence.
