Development of Doppler global velocimetry for the...
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18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
Development of Doppler global velocimetry for the measurement of eddy convective velocities
Tobias Ecker1,*, K. Todd Lowe2, Wing F. Ng3
1: Institute for Aerodynamics and Flow Technology, German Aerospace Center (DLR), Göttingen, Germany 2: Department of Aerospace and Ocean Engineering, Virginia Tech, VA, USA
3: Department of Mechanical Engineering, Virginia Tech, VA, USA
* Correspondent author: [email protected]
Keywords: DGV, instrumentation development, supersonic jet
ABSTRACT
A new Doppler global velocimeter (DGV) for high-speed flow applications was developed for the measurement of
eddy convective velocities in supersonic jet flows for aeroacoustic studies. The key technologies for this instrument
are spatially resolving, high sensitivity photomultipliers, field programmable gate array (FPGA)-based signal
processors and a continuous wave laser, which can be frequency tuned. The instrument presented in this study is
based upon the classic DGV technique but with technology enhancements that allow instantaneous and long time
span velocimetry at a 250 kHz sampling rate. The instrument has been matured to deliver a robust performance
even with low signal-to-noise ratios, as may be encountered in high-speed flows and large-scale facilities.
Measurements in both a small scale supersonic jet facility as well as a large-scale multi-stream nozzle at NASA
Glenn are presented to demonstrate the instrument capabilities in delivering robust measurement of convective
velocities within high-speed turbulent shear flows.
1. Introduction
Doppler global velocimetry is a technique based on the selective absorption characteristics of
molecular gases that is capable of direct measurement of the Doppler effect. DGV relies on the
detection of the Doppler shift of laser light scattered off tracer particles within the local flow. Its
capability to detect multiple particles within the measurement volume, while its temporal
resolution and scale up is only limited by particle response and signal to noise ratio, make it a
suitable instrument for time-resolved measurements of high speed flows.
The measurement of convective velocities, the speed at which eddies within the turbulent
flow convect, has been of particular interest due to its significance in the interpretation and the
development of aero-acoustic analogies (Lighthill, 1952). While hotwire anemometry, due to its
frequency response, can still be considered a standard for experiments in many experimental
studies on turbulent flow, its use in heated, supersonic shear layers has become obsolete with the
advent of optical and non-intrusive measurement techniques. The use of laser based instruments
like laser Doppler velocimetry (LDV), particle image velocimetry (PIV) and Doppler global
velocimetry DGV or other optical instruments (Kuo et al., 2011) avoid many of the issues
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
intrusive instrumentation in supersonic flows have. LDV has been used for doing two-point
correlations, which is the basis for the determination of the convective wavespeed, in hot and
cold high-speed jets in early studies by Lau et al. (1980), and most recently by Kerhervé et al.
(2004a, 2004b) in a cold supersonic jet. As LDV is a single point instrument, two LDVs had to be
operated simultaneously at varying separation distances within the flow. PIV, being a planar
technique, allows the correlation of the voxels with one another, reducing the complexity in the
measurement and determination of correlation functions and convective wavespeeds. Wernet
(2007) has successfully demonstrated the of use temporally resolved PIV for space-correlations in
cold and hot jet flows within a large scale facility, usually operating at 25-50 kHz. Like LDV,
DGV is a velocimetry technique based on the direct measurement of the Doppler effect. The
technique was first invented by Komine (1990) and subsequently refined by Komine et al. (1991)
and Meyers and Komine (1991). Detailed uncertainty analyses and flow studies have been
carried out by Meyers et al. (2001) in the early 1990s at NASA Langley, mainly for the
measurement of mean flow components. Smith (1998) and Clancy et al. (1999) first showed the
application of DGV to obtain instantaneous velocity vectors in several continuous frames in a
supersonic flow. However while these measurements were instantaneous, they were not
continuous and did not allow the capture of time-resolved flow features. The first to
demonstrate the application of a high repetition rate DGV system in a high-speed facility was
Thurow et al. (2005). His implementation used a high-speed camera (128 x 64 pixels) to obtain
one-component time-resolved velocity data as well as convective velocities in a rectangular
Mach 2.0 jet. Based upon this previous work Ecker et al. (2014) introduced a point Dopper
velocimeter (Cavone, 2006) operated in a cold supersonic jet at 100 kHz effective data rates and a
reduced number of sensors using a time-multiplexed technique. This work has been matured to
include a novel sensor system based on a 64 channel Photomultiplier (PMT), enabling spatially
and temporally resolved measurements in high-speed flows. The instrument has since been
applied to the measurement of convective velocities within heated small (Ecker et al. 2015b,c,d)
and large scale facilities (Ecker et al., 2016a,b) and is described in detail in this paper.
In this paper, we present the details of a novel 64-simultaneous-point, 250 kHz repetition
rate Doppler global velocimeter. The attributes of this development are particularly well suited
for measurements of turbulent eddy convective velocity in high speed flows. Given the
aeroacoustics interest in convective velocity due to the impact this parameter has on radiation
efficiency, the applications are those of relevance to jet noise for high speed propulsion. An
analysis of the operating principles and measurement uncertainties of this technique for
convective velocities is provided. Results are presented for convective velocity measured in
supersonic heated jets and in three-stream subsonic jets near Mach 1. These results reveal the
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
value of the method for obtaining quantitative turbulence structure information of developing
shear layers of relevance to the jet noise community.
2. Doppler global velocimetry
For the classic implementation of DGV (as first proposed by Komine (1990)), the
frequency of a single-longitudinal-mode laser is set so that the Doppler shifted frequency lies
optimally in the absorption spectrum of a molecular gas filter. A reference light path is used to
normalize the intensity of the filtered scattering by the intensity of the local scattering based on
particle density and laser light amplitude. The Doppler shift of the scattered light depends upon
the laser frequency, vector particle velocity, and laser propagation and observation directions
(see Fig. 1) and is related by the Doppler equation (Ecker et al., 2014) as:
𝑓𝐷 = 𝑓0�� ∙𝑒
𝑐 (1)
Fig. 1 Vector definitions of the DGV principle.
The Doppler shifted frequency is obtained by applying the intensity-frequency transfer function
of the molecular as filter to the intensity based sensor signals. An experimental scan compared to
a model fit by Forkey et al. (1997) is shown in Fig. 2 (Ecker et al., 2014).
Fig. 2 Experimental Iodine cell transmission scan (black) compared to numerical model fit (red).
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
3. Instrumentation
The presented instrument uses Doppler shift measured from a pair of co-planar laser sheets at 45
deg to the streamwise axis, which are time multiplexed to achieve three-velocity component
operation. Two receiver systems using novel 64 channel PMT cameras in a Doppler sensitive and
reference layout to record the light scattered from the particles in the measurement volume.
Recent studies have successfully shown the scale up of the instrument for the use in mid and
large-scale jet geometries.
(a)
(b)
Fig. 3 TR-DGV basic operating principle (a), sensitivity vectors within the facility coordinate
system for TR-DGV instrument (b). Image courtesy Ecker et al. (2015b).
To time multiplex the arrangement, acousto-optical modulators (AOMs) are used to chop
the incident laser beam into 4 μs, or shorter, pulses. The pulsed laser light is routed via free space
optics to laser sheet generating optics. The laser optics are arranged in a 45-degree configuration
to the jet centerline and create the measurement. Light receiver units are placed on both sides of
the flow, perpendicular to the measurement plane. In each receiver unit, the light collected by a
camera lens is first split into two paths—a signal path passing through a starved iodine cell and
-1-0.5
00.5
11
0.5
0
-0.5
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8-1
unit vectorssensitivity vectorsobservation vectorslaser direction vectors
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
a reference path with no cell. The basic sensor configuration and laser sheet timing is shown in
Fig. 3 (a) and the four measurement vectors for this instrument are shown within the facility
coordinate system in Fig. 3 (b). For a receiver system with a measurement plane parallel to the
stream-wise axis the maximal sheet width is dictated by the 45 deg angle position (for a square
sensor), which was also found as the angle with the lowest overall uncertainty by Ecker et al.
(2014) for this type of sensor geometry. This configuration could be extended to volumetric
operation by using a moving light sheet and a lens configuration with sufficient depth of field.
Using three of the four measurement velocity vectors the three component velocity vector
within the facility coordinate system can be extracted by using a transformation matrix:
�� = [
𝑒 1𝑇
𝑒 2𝑇
𝑒 3𝑇
]
−1
(
𝑢1
𝑢2
𝑢3
) = [𝑅11 𝑅12 𝑅13
𝑅21 𝑅22 𝑅23
𝑅31 𝑅32 𝑅33
] (
𝑢1
𝑢2
𝑢3
) (2)
The main sources of uncertainty of DGV are attributed to the characterization of the molecular
gas cell, laser line stability and signal to noise ratio of the sensor. Progresses in the development
of advanced DGV techniques, which address these issues, have been made since and lead to new
derivate DGV technologies (Fischer et al. 2007, 2008, Charret et al. 2004, Cadel et al. 2015). The
uncertainty of the velocity components along the sensitivity vectors is found to be a combination
of systematic and random uncertainties, and can be expressed as:
𝛿𝑢𝑖 =𝑐
𝑓0√(
𝑑𝑓
𝑑𝑇)𝑖
2(𝛿𝑇𝑖)2 + 𝑇𝑖
2 [𝛿 (𝑑𝑓
𝑑𝑇)𝑖]2
+𝑓𝑖
2
𝑓02 {(
𝑑𝑓
𝑑𝑇)0
2(𝛿𝑇0)2 + 𝑇0
2 [𝛿 (𝑑𝑓
𝑑𝑇)0]2
} (3)
Applying the transformation matrix to this analysis allows obtaining uncertainties within the
facility coordinate system as:
∆�� = (
𝛿𝑢𝑥
𝛿𝑢𝑦
𝛿𝑢𝑧
) = √([𝑅11 𝑅12 𝑅13
𝑅21 𝑅22 𝑅23
𝑅31 𝑅32 𝑅33
])
2
(
𝛿𝑢1
𝛿𝑢2
𝛿𝑢3
)
2
+ ([
𝛿𝑅11 𝛿𝑅12 𝛿𝑅13
𝛿𝑅21 𝛿𝑅22 𝛿𝑅23
𝛿𝑅31 𝛿𝑅32 𝛿𝑅33
])
2
(
𝑢1
𝑢2
𝑢3
)
2
(4)
For the instrument used in this study, two receiver systems using 64 channel PMT
cameras are used to record the light scattered from particles in the measurement volume. Recent
studies have successfully shown the scale up of the instrument for the use in mid and large-scale
jet geometries. The system consist of three essential subsystems:
1. Laser system, which enables conditioning of the Laser beam
2. Camera system, which receives the Doppler shifted light
3. DAQ system, which records the data and interfaces all subsystems
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
Fig. 4 DGV system electric and optical signal overview (Ecker et al., 2016b).
The signal flow and functional integration of the Laser multiplexing system (1), as well as
of the two camera systems (2) and the data acquisition system (3) for the DGV instrument are
shown in Fig. 4. The laser system is based on a Verdi V18 single frequency diode pumped
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
continuous wave laser which is time multiplexed using two IntraAction Corp. 80 MHz AOMs.
This laser has a line width of approximately 5 MHz over 20 ms (Coherent Inc white paper). The
time multiplexing of the laser sheets is controlled via a BNC 565 delay generator, which is
triggered by a digital output from the DAQ system. Continuous triggering from the FPGA
Adapter module, which is also used for recording of the PMT data, ensures a maximum of
synchrony between laser multiplexing and data recording.
The 1 MHz trigger signal is recorded by the multipurpose DAQ card as a reference. A reference
beam is extracted from the multiplexing system via a beamsplitter and used to record the light
intensity from two Thorlabs PDA100A photodiodes which are setup around a reference Iodine
(ISSI Inc.) cell in a similar configuration as described for the DGV camera system.
The scattered light is sensed by two photomultiplier (PMT) array detectors, one for each
the signal and reference paths. A 2 in beamsplitter cube is used to divide the received between
light paths. The PMT array detectors and the high bandwidth data acquisition system used to
measure the PMT anode signals form a unique system for very high repetition rate, low light
measurements. The PMT array detectors are Hamamatsu H8500C PMT modules, each with 64
anodes. Custom 16-channel, high-speed amplifier boards from Ectronic GmbH amplify the
anode signals and transmit them to the adapter module of the data acquisition system. On each
module, 32 anodes were used and the flow was imaged such that the points were distributed in
a rectangular grid that was 30.32 mm in the stream-wise direction and 61.22 mm in the radial
direction.
Fig. 5 CAD of TR-DGV instrument configured for small-scale supersonic hot jet. Image courtesy
Ecker et al. (2015b).
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
Depending on the application and required laser output, the optical routing of the laser
beams can be done via fiber optics or mirror configurations. Fig. 5 portraits the basic instrument
geometry for the use in a small-scale facility using 8 μm telecommunication grade fiber optics.
Pictures of the scaled up system using a mirror configuration are shown in the applications
section.
4. Data processing for convective velocities
The multi-point PMT (effective area 24.12 x 48.7 mm2, configuration is displayed in Fig. 6) (Ecker
2015a) allows space/time cross-correlation processing of the experimental data over all 32 sensor
pixels. The time-resolved data s(x,r,t) acquired on each sensor pixel were processed and
correlated to reference locations throughout the shear layer. The second-order cross-correlation
(Morris and Zaman, 2010) is defined as
𝑅𝑖𝑗(𝑥, 𝑟, 𝜍, 𝜉, 𝜏) =1
𝑇∫ 𝑠𝑖(𝑥, 𝑟, 𝑡)𝑠𝑗(𝑥 + 𝜍, 𝑟 + 𝜉, 𝑡 + 𝜏)𝑑𝑡 =
𝑇/2
−𝑇/2𝑠𝑖(𝑥, 𝑟, 𝑡)𝑠𝑗(𝑥 + 𝜍, 𝑟 + 𝜉, 𝑡 + 𝜏) (5)
where 𝜍 and 𝜉 are the spatial separation and 𝜏 the delay timescale from the reference location. As
used in equation (2), further uses of the ( ) notation indicate time averaging.
The correlation function exhibits maxima along the convective ridge. These points lie on the
locus of 𝜍𝑟 = 𝑢𝑐𝜏𝑟, and the correlation function along this locus,
𝑅(𝜏𝑟) = 𝑅(𝜍𝑟 = 𝑢𝑐𝜏𝑟) (6)
characterizes the decay of the spatio/temporal extent of integral scale eddy correlation.
The space/time correlations maps measured allow the determination of equation (7) from which
the integral convective velocity (Fisher and Davies, 1964) can be determined, given the known
stream-wise measurement position spacing,
𝑢𝑐 = 𝜍𝑟 𝜏𝑟⁄ = 𝑘Δ𝑥/𝜏𝑟 (7)
Fig 6. PMT sensor configuration (left), example space/time correlation across the sensor (right).
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
Processing for frequency dependent convective velocities as well as intermittency has been
demonstrated by Ecker et al. (2015c,d) in previous studies. The uncertainty within the
measurement of the convective wavespeed behaves much differently than the uncertainty within
the instantaneous velocity component. The uncertainty within the determination of the
convective wavespeed from the correlation function Rij is primarily dependent on the time
accuracy of the data acquisition and the sensor spacing.
Propagation of error analysis yields the following uncertainty for the convective velocity:
𝛿𝑢𝑐 = ±√(1
𝜏)2(𝛿Δ𝑥)2 + (
Δ𝑥
𝜏2)2(𝛿𝜏)2 = ±𝑢𝑐√(
𝛿Δ𝑥
Δ𝑥)2
+ (𝛿𝜏
𝜏2)2
(8)
Ecker et al. (2015b) analyzed the uncertainty within the processing algorithms used to obtain the
lag time. By applying a Monte-Carlo simulation using synthetic flow data they found that noise
has a very low impact on the lag time estimation. For a case with mean flow and turbulence
properties representative to supersonic shear layers, they found the error within the lag time
estimate to approach values of about 4% at signal to noise ratios (SNRlog) above 0. As can be
seen from equation 8, the normalized uncertainty for convective velocity is a combination of the
relative uncertainties of the sensor spacing and lag time estimate. As long as the relative
uncertainties can be held constant, scale up of the instrument for the use in larger scale facilities
does not lead to increased uncertainties.
5. Applications
The TR-DGV instrument has to date been used for the measurement of the eddy convective
velocity in two facilities, the Virginia Tech hot supersonic jet facility (Ecker et al. 2015b) and the
High Flow Jet Exit Rig (HFJER) at the NASA Aeroacoustic Propulsion lab (AAPL). An overview
of the very different optical instrument parameters is shown in table 1, demonstrating the
scalability of the TR-DGV technique.
Table 1. Instrumentation parameters for two facilities used.
Small scale jet facility NASA AAPL
Lens focal length 85 mm 200 mm
𝛥𝑡 (Laser) 4 𝜇𝑠
Measurement area (PMT coverage) 12 x 24 mm2 30.32 x 61.22 mm2
𝛥𝑥 (horizontal) 3 mm 7.58 mm
As shown by Ecker et al. (2014), the instantaneous measurement uncertainty can be estimated for
this configuration as ±9.2 m/s for the streamwise velocity component from equation (3) and (4),
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
similarly the mean bias uncertainty is ±1.5 m/s for all components. The uncertainties, as well as
the condition number for the transformation matrix and the minimally required Iodine line
frequency width are summarized in table 2.
Table 2. Uncertainties and Doppler frequency range.
Uncertainty (inst.) 9.2 / 6.5 / 6.2 m/s
Uncertainty (mean) 1.5 / 1.5 / 1.5 m/s
Uncertainty [𝛿𝑢𝑐/𝑢𝑐] 0.065
Uncertainty [𝛿𝛥𝑥 /𝛥𝑥 ] 0.05
Uncertainty [𝛿𝜏 /𝜏 ] 0.04
Max Condition number 4.2
Range (Doppler freq) 0.12 GHz
The properties of the Iodine cells of the DGV instrument were optimized for their respective use.
The reference cell, which is located on the optical table and used to obtain the reference
frequency, was operated at 65 °C. At this temperature and the relatively low filling pressure, this
cell is ideally suited to track the initial Laser frequency due to its gentle transmission slopes. The
measurement cells, which are located in both DGV camera units inhibit a much higher
sensitivity to frequency change and therefore to the velocity fluctuations within the flow. The
properties of the Iodine cells used are summarized in table 3.
Table 3. Iodine cell properties.
reference cell measurement cell (O1/O2)
Pressure 0.1 Torr 1.0 / 1.2 Torr
Body temperature 65 °C 35 / 35 °C
Small-scale supersonic hot jet facility at Virginia Tech
The TR-DGV setup for this particular experiment is displayed in Fig. 5 and Fig 7 (a). The
distribution of the convective velocity and Mach number for several streamwise stations was
measured by Ecker et al. (2015b) for two heated conditions (TTR = total temperature / ambient
temperature = 1.6 and TTR = 2.0) in a small scale (d = 2 in) supersonic hot jet facility.
The results of the colder (TTR = 1.6) condition are shown in Fig. 7 (b). The structure of the
eddy convective velocity and Mach number profiles indicates several noteworthy aspects of the
flow physics of heated supersonic jets. The convective speeds within the potential core at x/D =
4 and 6 for both TTR conditions are approximately uniform and equal to the jet exit velocity. In
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
comparison to the mean velocity distribution it was found that the convective velocity data
exhibits a clear difference in spreading of the convective velocity profile. Between both TTR
cases, a measurable reduction in the scaled eddy convective velocity is found for the hotter TTR
case at the downstream stations, which is believed to be leading to a reduction in convective
amplification and contributing to the reduced noise emissions. Specifically, the density ratio is
believed to play a direct role in scaling due to dependency of the ‘symmetric’ convective Mach
number (Papamoschou, 1997) on this quantity. More analysis and data can be found in Ecker et
al. (2015b).
(a)
(b)
Fig 7. Measurement of convective velocity in a small-scale supersonic hot jet (TTR=1.6) at several
streamwise stations.
Large-scale supersonic hot jet facility at NASA Glenn Research Center
Multi-stream nozzles are currently under investigation for application in the future supersonic
aircraft. In order to demonstrate the scalability of the TR-DGV instrument, measurements were
performed at a three-stream nozzle with and without axial offset of the third stream.
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
For these large-scale measurements, both the laser itself and the reference Iodine cell are housed
in an acoustically treated box in order to reduce the influence of noise and vibrations onto the
laser stability. Similarly the box also serves to thermally insulate the system from outside
temperature influences. Despite the insulation, the high frequency jet noise negatively impacted
the Laser stability and lead to ambiguity in estimating both laser and Doppler-shifted flow
frequencies. Notwithstanding the difficulties in computing the velocity statistics due to noise
induced laser instabilities, the determination of the convective velocity was relatively unaffected
by the same issues due to the inherent robustness of using the cross correlation function for lag
time detection.
(a) (b)
(c)
Fig 8. Measurement of convective velocity in a large-scale multi-stream nozzle at NASA Glenn.
(a) Instrument placed the in facility (b) photo of the beam operation during measurement (c)
convective velocity profiles of a jet issuing from three-stream axisymmetric nozzle at several
streamwise stations.
Detector units
HFJER
Laser and laser monitoring
Laser sheets
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
As in previous work, the results for convective velocity differ in structure from the mean
velocity field, which was obtained using PIV. The distribution of the convective velocity in the
axisymmetric (non-offset) case and the instrument configuration is shown in Fig 8. A more
detailed dissemination of the flow results can be found in Ecker et al. (2016a,b).
6. Summary
A new instrument is presented which combines for the first time several unique features into a
powerful system for measuring convective velocities with low statistical uncertainties. While the
conventional operating methodology of DGV is preserved, newly available photonics and data
acquisition technologies transform it into a system with unique capabilities, which is
complementary to established instruments like PIV and LDV. Time-shift multiplexing is an
approach to reduce the number of required sensor devices while still providing data rates
suitable for high-speed applications and low mean uncertainties. The presented geometrical
configuration in combination with time-shift multiplexing is of interest, because it can be easily
extended to allow volumetric operation in the future. The presented data in both a small scale
and large scale hot jet facility showed the presence of high momentum, high velocity large scale
eddies downstream of the potential core. These regions of locally high convective Mach numbers
are expected to have a leading role in supersonic jet noise due to high convective amplification
factors causing higher noise intensity. The main challenges in the large-scale facility were
predominantly related to laser line stability due to vibration caused by the jet noise. This study
shows the benefits time-resolved DGV can provide for understanding dominant turbulent
structures for noise generation in supersonic jets, and it provides the basis for the ongoing
development of time resolved multi-point DGV instrumentation. The application of the
instrument in both a small scale and large-scale facility with only minor modifications
demonstrate the easy scalability and robustness for the measurement of convective velocities of
the DGV technique.
Acknowledgements
This work was supported by the NASA’s Commercial Supersonic Technology Project in
the Advanced Air Vehicles Program, and the Office of Naval Research through the Hot Jet Noise
Reduction Basic Research Challenge and DURIP, grants N00014-11-1-0754 and N00014-12-1-
0803. The authors gratefully acknowledge the support provided for acquisition of the data by
Drs. Brenda Henderson and Mark Wernet of the NASA Glenn Research Center.
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
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