Results from mean flow field surveys are reported. Flow fluctuation amplitude measurements and acoustic measurements are presented. The organized structure was characterized in terms of axial flow and radial flow

Morrison, G. L.

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https://ntrs.nasa.gov/search.jsp?R=19810014300 2020-03-21T14:18:23+00:00Z
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Department of Mechanical Engineering
Texas A&M University
Coliege Station, Texas
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SEMI-ANNUAL REPORT TO
NASA LANGLEY RESEARCH CENTER
FOR GRANT NAGi-112
THE ROLE OF COHERENT STRUCTURES IN THE
GENERATION OF NOISE FOR SUBSONIC JETS
MAY 1, 1981
1
G. L. MORRISON
MECHANICAL ENGINEERING DEPARITlENT
TEXAS A&M UNIVERSITY
COLLEGE STATION, TEXAS "17843
on tLLBSTaA^i
The objectives of this work were categorized into four main topics:
1. Mean flow field surveys
2. Flow fluctuation amplitude measurements
3. identification and characterization of the organized structure
4. Acoustic measurements
This report will briefly restate the specific objectives of each topic and the
progress to date on each.
1. MEAN FLOW FIELD SURVEYS
Radial and axial distributions of the Pitot and static pressures as well as
the local stagnation temperature are to be measured and the axial and radial
distributions of the velocity are to be caluclated. In addition, hot-wire
anemametry is to be used to directly measure the axial and radial distributions
of the -pan flow.
All of the measurements for this topic have been performed. Sane reduction
of the data remains to be done.
2. FLOW FLUCTUATION AMPLITUDE MEASUREMENTS
Radial and axial distributions of the amplitude of the axial and radial
flow fluctuations and their product are to be measured for the full spectrum and
for individual spectral components.
We have been emphasizing the measurement of the axial flow fluctuations for
both the full spectra and for individual spectral components. The data
reduction technique for the normal hot-wire probes used to measure the axial
flow fluctuations has been our major concern in this topic. We have been trying
to get a better estimate of the stagnation temperature fluctuations by directly
calibrating the hot-wire probes sensitivity to stagnation temperature variations
and by calculating this sensitivity from analytical considerations. We are
continuing to work on this in an effort to get consistant results from both
measurements. Figure 1 shows profiles of the RMS values of the normal hot-wire
voltage fluctuations for the full spectrum (1-20 KHz, St = 0.05 to 1.26) and for
selected spectral components (1/10 octave bandpass filtered). Figure 2 shows
the RMS value of the axial mass velocity fluctuation level nondimensionalized by
the local mean mass velocity. These measurements were made at the radial
location of maximum fluctuation level for the 1-20 KHz frequency band.
No measurements have been made to determine the radial flow fluctuations or
their product with the axial quantities. This is planned to be the major thrust
of the summer.
3. IDENTIFICATION AND CHARACTERIZATION OF THE ORGANIZED STURCTURE
The characterization of the organized structure is to be done for both the
axial and radial flow fluctuations and their product in both natural and excited
jets. This characterization includes the measurement of spectra at different
locations in the flow, axial wavelengths for individual spectral conWnerts and
azimuthal mode numbers for each spectral oomponent investigated.
Characterization of the organized structure has been done for the axial
portion of the flow fluctuations in the natural jet. Spectra have been measured
at several different locations in the jet (See Figure 3) and axial wavelengths
determined for several different spectral components (See Figure 4). The
spectra of the hot-wire fluctuations were made at the radial location of maximum
fluctuation level and show the frequency content shifting to the lower
frequencies as the flow progresses downstream. Note that the vertical linear
scale in each spectrum is the same for all axial locations shown and 20 KHz
corresponds to a Strouhal number of 1.26. Figure 4 shows axial wavelengths of
the coherent structure in three different high Reynolds number subsonic jets (M
= 0.6, 0.7, and 0.8) and for several transonic and supersonic low Reynolds
number. jets. The low Reynolds number jets were undergoing transition from
laminar to turbulent flow. This figure shows that the axial wavelength of the
coherent structure varies in the same manner regardless of the Mach number or
Reynolds number of the flow for these high speed ca► Vressible flows. This is a
very important discovery.
Attempts to measure the mode numbers in the natural jet have resulted in
only the zero mode being found. However, these measurements do not preclude the
existance of higher modes in the jet since the antisymnetric modes may not be
locked into one plane of oscillation. This would tend to cancel out the
contribution of these modes in our measurements due to the long time period
needed to perform the cross correlation measurements at the different azimuthal
locations. In order to determine if these higher modes are present it will be
necessary to artificially excite the jet and "lock" the modes into one plane of
oscillation. This work is currently being pursued.
4. ACOUSTIC MEASUREMENTS
Sound pressure level contours are to be measured in the acoustic field of
both the natural and artificially excited jets. When the jet is excited,
correlations between the exciter and the microphone signals are to be measured
to determine if the organized structure is directly radiating noise.
The sound pressure level contours have been measured for the natural jet
(See Figure 5). Problems with electronic noise induced when more than one
exciter electrode is operating have delayed these measurements and the other
measurements in the jet which require excitation. We are currently working on
shielding the exciter circuitry and cables in an effort to reduce or eliminate
this electronic interference.
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In summary, we have completesd, all of the measurements of the mean flow and
nearly all of the measurements dealing with the axial flow fluctuations in the
natural jet. Radial velocity fluctuation measurements are to be performed this
summer. All data concerning the excited j^t are pending upon our ability to
reduce the electronic interference. It may be necessary to perform the
experiments with only one (i.e. a point exciter) electrode.
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The role of coherent structures in the generation of noise for subsonic jets

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