Large scale structures in jets and in flows over cavities were investigated experimentally to determine their role in entrainment, mixing, and noise production. The presence of these structures resulted in growth of the shear layer and entrainment. Merging of adjacent large scale structures caused the near field pressure signal in excited flows. It is believed that both the entrained fluid as well as its eventual mixing with the jet flow can be controlled by introducing pulsation in the jet flow at a frequency for which the flow is most unstable

Massier, P. F.

Sarohia, V.

English

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LARGE-SCALE TURBULENT STRUCTURES IN JETS AND IN FLOWSOVER CAVITIES AND
THEIR RELATIONSHIP TO ENTRAINMENT AND MIXING
V. Sarohia and P. F. Massier
Jet Propulsion Laboratory
California Institute of Technology
Introduction
Results of an experimental study are presented elucidating the presence of
large-scale turbulent structures both in free jets and in axisymmetric flows
over cavities. The purpose was to determine their role in entrainment, in mix-
ing, in their growth, and in the production of noise. Although the flow con-
figuration in the experiments was not that which exists in a thrust augmentor,
the results nevertheless are of importance in understanding the various mixing
and entrainment processes associated with the performance of such a device.
Recent realization of the presence of large-scale structures (I-5) in
turbulent shear flows has generated an interst in advancing the understanding
of their role in entrainment, mixing, production of noise and of the Reynolds
stresses. In the present investigation the large-scale structures in jet flows
were observed by simultaneous flow visualization and measurements of physical
flow quantities to determine their development and their interaction with each
other. Near field pressure signals as sensed by microphones were analyzed and
matched with motion picture frames in an attempt to establish any link between
the dynamics of these large-scale structures and the production of jet noise.
The experimental results on cavity shear layers were obtained under oscil-
lating and non-oscillating flow conditions, i.e., with and without the presence
of strong organized large-scale structures in the shear flow. Oscillations in
flows over an axisymmetric cavity are caused by strong feedback from the down-
stream cavity corner (6-7) They are accompanied by large-scale structures in
295
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the shear layer and can be altered by changing the cavity geometry or by chang-
ing the freestream flow conditions.
Results of Jet Flow Measurements
Subsonic jet flows were generated by expanding air at room temperature
through a convergent nozzle which had an exit diameter of 4.2 cm. The jet dis-
charged into an anechoic chamber. The Mach number Me ranged between 0.I and
0.9. The flow in the plenum chamber could also be modulated between I00 and
2000 Hz by first passing the flow through a pneumatic transducer, thereby excit-
ing the jet flow. The rms velocity fluctuations at the center of the nozzle
exit plane could be varied from 2% to as high as 8% of the mean jet velocity.
These periodic flow fluctuations produced organized large-scale vortex structures
in the jet shear layer. The interaction of these large-scale structures as they
convected downstream and their role in the production of jet noise was investi-
gated using the instrumentation system shown in Figure I. Both still and high-
speed Schlieren motion picutres up to 7000 frames/sec were taken. The duration
of the flash used for the movies was 0.3 _s. The jet flow was made visible by
mixing a small amount of CO2 gas with the airflow in the plenum chamber. A
series of microphones were placed in the near field and their output was synch-
ronized with the high-speed motion picture frames so that near field pressure
fluctuations could be analyzed simultaneously with the movies on a frame-by-
frame basis.
A spark shadowgraph showing the large-scale sctructures in a non-excited
jet flow at a nozzle exit Mach number Me , of 0.69, and at a Reynolds number based
on nozzle exit dia. Re D = 0.4 X 106 is shown in Figure 2. This shadowgraph
296
clearly indicates the presence of large-scale structures as far as 5 to 7 jet
dia. downstream of the nozzle exit. On close scrutinity of Figure 2 one can
infer that the spacing between the structures approximately doubles as they
propagate downstream. Because the merging of the large-scale structures occured
many times at a relatively fast rate within a few jet diameters, it was not
possible to relate the convection of these large-scale structures and their
merging with each other to the near-field pressure signal. The process of merg-
ing of these structures was slowed down by exciting the jet flow, thereby intro-
ducing large-scale structures whose behavior could be studied visually and
hence, merging identified.
A set of a sequential Schlieren motion-picture frames showing the behavior
of artificially introduced large-scale structures is shown in Figure 3. The
time duration between the frames was approximately 140 Us. The lower part of the mixing
layer in frame 2 of Figure 3 shows a vortex structure which can be seen subse-
quently in frames 3, 4 and 5. In these f_ames the vortices that were shed earlier
can also be seen. In frame 5, there are two vortices side by side which merged
with each other sometime before frame 6 was taken. The merged structure then
propagated downstream as can be seen in frames 7 and 8. From these high-speed
motion picture observations, it was concluded that the time taken for the merging
process of two artifically generated vortex structures to occur was about 10% of their
life span in the jet flow. Under similar mean flow conditions in the nozzle
an increase of 15 to 20% in the radial growth of the jet was observed when the
jet was excited as compared to a jet that was not excited.
Figure 4 shows successive Schlieren motion picture frames along with a
near-field pressure trace of microphone C, the location of which is shown above
297
frame No. I. Note the similarity of the behavior of the organized large-scale
structures in Figure 4 as compared to those discussed in Figure 3. Observe
that as a vortex structure passed below microphone C in frame 4 of Figure 4
there was no significant change in the near-field pressure signal. In frame
7, however, two adjacent organized structures interacted with each other,
and instantaneously there was a rapid change in the pressure signal. This
pressure pulse was then traced at later times by the other microphones located
farther downstream in the near-field. From these measurements it was concluded
that the near-field pressure signal of a jet is largely dependent on the
dynamics of the interaction of large-scale structures in the jet. A more
in-depth discussion on the relationship of large-scale structures and their
generation of near-field pressure signals is given in Reference 8.
The influence of jet flow excitation on the distribution of the mean
velocity along the jet centerline is shown in Figure 5. It is interesting to
note the differences in these velocity distributions for cases in which the
rms velocity fluctuations were random in nature with a magnitude of 2% as
compared to periodic velocity fluctuations of nearly the same value at 2.3%.
It can be seen in Figure 5 that the length of the potential core was reduced by
almost half for the excited (pulsated) case as compared to the non-pulsated
condition. One can infer from these results that the presence of large-scale
structures produced by exciting the jet flow are greatly responsible for the
rapid spread of the jet with greatly increased entrainment and mixing of the
flow. Furthermore, increased excitation reduces the length of the potential
core even more and also substantially reduces the mean velocity at the center-
line at any given X/D beyond the potential core.
298
Flows Over External Axisymmetric Cavities
The experiments on flows over axisymmetric cavities were performed using
a model which had an outside diameter d, of 2.0 in. as indicated in Figure 6.
The model had an ellipsoidal nose with provision for variation of depth d, in
steps together with a continuously adjustable width b. Either laminar or
turbulent boundary layers could be obtained at the upstream edge of the cavity.
Flow over the cavity was visualized by injecting CO2 gas at the base
inside the cavity. Figure 7 shows a typical shadowgraph with width b = 0.4 in.
and depth d = 0.5 in. Organized large-scale structures are observable over
the cavity. For a Reynolds number, Reoo = 1.60 X lO3 (where @o is the momentum
thickness at the upstream corner), and a depth d/O = 37.5, the growth of the
cavity shear layer is shown in Figure 8. Results are given both for a non-
oscillating (with organized large-scale structures in the cavity shear layer)
and for an oscillating shear layer over the cavity. The momentum thickness 0,
was determined by integrating the profiles of the mean velocity as determined
by constant-temperature hot-wire anemometry. That is,
j 0(u)o= l- dy (I)
In Eq. (1) y is the transverse coordinate, U is the local mean velocity and
U is the freestream velocity. It is quite clear from Figure 8 that the growth
rate dO/dx, which indicates the entrainment rate, was approximately 0.021 for
the non-oscillating shear layer. In the presence of organized large-scale
structures for an oscillating flow over the cavity the value of dS/dx increased
to as much as 0.046. This high entrainment rate of the shear layer seems to
299
result from the presence of the organized large-scale structures shown in
Figure 7.
Conclusions
From the experimental results of large-scale structures in jets and in
flows over cavities, it can be concluded that the presence of these structures
is greatly responsible for the growth of the shear layer, and for the entrain-
ment. Furthermore, the near-field pressure signal in excited jet flows is caused
primarily by the merging of adjacent large-scale structures. It is believed
that both the entrained fluid as well as its eventual mixing with the jet flow
can be controlled by introducing pulsation in the jet flow at a frequency for
which the flow is most unstable.
300
REFERENCES
l ° Bradshaw, P., Ferris, D. H., and Johnson, R. F., "Turbulence in the Noise-
Producing Region of a Circular Jet," Journal of Fluid Mechanics, Vol. 19,
1964, pp. 591-624.
2. Crow, S. C., and Champagne, F. H., "Orderly Structure in Jet Turbulence,"
Journal of Fluid Mechanics, Vol. 48, Part 3, 1971, pp. 547-591.
. Brown, G. L., and Roshko, A., "On Density Effects and Large Scale Structures
in Turbulent Mixing Layers," Journal of Fluid Mechanics, Vol, 64, 1974,
pp. 775-816.
4. Browand, F. K., and Weidman, P. D., "Large Scales in the Developing Mixing
Layer," Journal of Fluid Mechanics, Vol. 76, 1976, pp. 127-144o
, Winant, C. D., and Browand, F. K., "Vortex Pairing: The Mechanism of
Turbulent Mixing-Layer Growth at Moderate Reynolds Numbers," Journal of
Fluid Mechanics, Vol. 63, Part 2, 1974, pp. 237-255.
o Sarohia, V., "Experimental and Analytical Investigation of Oscillations in
Flows over Cavities," Ph.D. Thesis, Dept. of Aeronautics, California
Institute of Technology, March 1975.
7. Sarohia, V., "Experimental Investigation on Oscillations in Flows over
Shallow Cavities," AIAA Journal, Vol. 15., July 1977, pp. 984-991.
. Sarohia, V., and Massier, P. F., "Experimental Results of Large-Scale
Structuresin Jet Flows and Their Relation to Jet Noise Production,"
AIAA Journal, Vol. 16, August 1978, pp. 831-835.
301
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Large-scale turbulent structures in jets and in flows over cavities and their relationship to entrainment and mixing

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