Planar Doppler Velocimetry Activities at DLR Cologne

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Planar Doppler Velocimetry - Activities at DLR
Cologne

• C. Willert Institute of Propulsion
  Technology,German Aerospace Center (DLR),
  Cologne, Germany

• Topics
• research group at DLR
• overview PDV-technique
• applications of PDV
• current / future activities

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Section Engine Measurement Technique
Section Head Richard Schodl Staff 13-15 people,
8 permanent

• Objectives / areas of research
• development of laser-based diagnostic techniques
  for application in turbomachinery and combustor
  research
• planar measurement techniques based on Mie
  scattering PDV (DGV), PIV, tracer shock
  visualization, quantitative light sheet imaging
  (mixing fraction)
• laser-2-focus anenometry (L2F, Doppler L2F)
• spectroscopy OH-LIF, CARS
• technology transfer to other fields (automotive,
  wind tunnels, )

Status July 2001
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Historic development of PDV at DLR
1992-1993 Diploma work of Ingo RoehleLaser-Doppl
er-velocimetry on the basis of frequency-selective
absorption(assessment and application of
point-wise and image-based DV) 1993-1996 Developme
nt of planar Doppler velocimetry system Various
applications - swirl nozzle- car model wake-
isothermal combustor rig- flow inside aircraft
model in wind tunnel 1994-1998 Development of
Doppler-L2F system 1997-1999 Design and
construction of long-pulse, frequency-stabilized
NdYAG for PDV in reacting (hot) flows
(cooperation with Laser Center Hannover) since
1997 Technology transfer project- second
generation PDV system- PDV system for
PTB-Braunschweig- automotive applications 2001 Pr
att Whitney / EREA Award to Dr. Roehle for his
work on PDV (PhD. Thesis) 2002 Agreement between
IAV and DLR on PDV-application to automotive
engine research (PDV system delivery in October
2002)
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Main aspects of DLR-Colognes PDV-Systems
DLRs PDV-Systems are optimized for measurement
of time- or phase-averaged velocity

• Advantages
• use of CW-lasers (easier to stabilize)
• sweeping beam light sheet generation (uniform
  intensity distribution)
• single camera / triple light sheet configuration
  (cost effective)
• illumination using fiber optics (easy handling,
  user safety)
• on-chip signal integration
• reduced speckle effects
• Disadvantages
• no single-shot capability, no RMS-values
• flow must be steady or cyclic
• long signal integration -gt background light
  suppression necessary

• Use of sealed, vapor-starved iodine cells
• cell transmission profile insensitive to small
  temperature changes
• stable over long periods (years)

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Principle of Planar Doppler Velocimetry (PDV)

• also known as Doppler Global Velocimetry (DGV)
• laser light sheet based
• obtains velocities of tracers suspended in
  flow(Mie scattering technique)
• uses iodine cell as frequency to intensity
  converter

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Principle of Planar Doppler Velocimetry (PDV)
PDV measures the magnitude of projection of the
velocity vector on the difference vector ( o-l )
between observation vector ( o ) and illumination
vector ( l )
Light sheet
?/2
l
o
-
l
o
V
At ?90 2,7MHz ? 1m/s
o-l
Measured velocity component
V
Observer
(camera)
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Iodine absorption bands near 514.5nm
tuning range of Ar 514 line
most frequently used profile
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Calibration of iodine cells
Raw data from rotating wheel
Rotating wheel calibration system
wheel with retro-reflectors or mirrors (saw)
w
n
0
Reference
Iodine cell
Signal
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Principle of Planar Doppler Velocimetry (PDV)
Use narrow absorption bands of iodine as
frequency to transmission filter.
800 MHz (300 m/s)
1
T
meas.
Iodine cell transmission
Iodine Cells without heater casing
T
0
0
nmeas
n?
Frequency
??
Transmission (intensity ratio) is measureable
with light detectors, e.g. CCD sensors.
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Principle of Planar Doppler Velocimetry (PDV)
Normalized Image (sig ref)
Signal Image (view through iodine cell)
1
T
meas.
T
0
0
nmeas
n?

Reference Image
Low cell transmission (lower velocity)
High cell transmission (higher velocity)
1 velocity component
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PDV-system laser stabilization
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PDV-system laser stabilization
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Light sheet arrangement for3-component time- /
phase-averaging PDV
sequential light sheet access ? 3 PDV image
pairs ? 3-component time-averaged velocity data
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Light sheet 1 active
Light sheet 1 active
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Light sheet 2 active
Light sheet 2 active
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Light sheet 3 active
Light sheet 3 active
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Light sheet 3 active
sequential light sheet access ? 3 PDV image
pairs ? 3-component time-averaged velocity data
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PDV image processing
Light Sheet 1
Light Sheet 2
Light Sheet 3



Processing
V
W
U
Velocity data
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PDV Data Processing Final Data Set
Acquisition time 3 x 10 seconds Processing
time lt 10 s Total time per data set lt 1 minute
Data is 4 x desampled! 1 Vector for each pixel gt
380 x 170 vectors
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From planes to volumes
isothermal flow inside double-staged annular
combustor segment
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Example volumetric PDV in combustor rig
camera
mirror
Camera and light-sheet generators mounted on
common traverse
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Example volumetric PDV in combustor rig
50 parallel planes
isothermal flow inside double-staged annular
combustor segment
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Recent PDV system improvements
camera system

• use of newer generation, cooled CCD cameras with
  higher quantum efficiency higher sensitivity,
  better linearity, better SNR (2001 at 95
  saturation)
• replacement of beam splitter plate by beam
  splitter cube
• treatment of camera non-linearity
• imaging via fiber optic bundles (Cranfield Univ.)

software

• complete make-over of software object oriented,
  modular architecture capable of handling
  over-determined imaging configurations (4 camera
  views, 6 light sheets) use of established
  netCDF data format as for PIV (for data exchange
  data merging with PIV community)

technique / other

• single-mode monitoring of laser
• improved light sheet scanning optics

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PDV system image acquisition system
original setup
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PDV system image resolution
reference camera
signal camera
Astigmatism due to finite thickness of beam
splitter plate at 45 angle
ghost image due to second plate surface
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Improvements of PDV camera system

• new camera with
• beam splitter cube
• no astigmatism? higher spatial resolution
• split ratio 65T-35R? no neutral density
  filter? higher sensitivity

65T - 35 R
T 50
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Improvements of PDV camera system
Problem different PDV results of same flow
depending on signal intensity
Nonlinear camera response !
Solution use intensity-correction look-up table
(LUT)
(applied to all images)
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PDV system fiber image bundles
idea of R.Tatam/D.Nobes, Cranfied University 4
individual bundles merged into common
exit length 3.8m / bundle image size 6 x 5
mm / bundle resolution 600 x 500 fibers /
bundle
2x2 bundle exit
bundle entries (without imaging lenses)
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PDV system fiber image bundles
image transfer optics using conventional camera
lenses
bundle entries (4)
illumination source
bundle exit
85mm/f1.4
50mm/f1.2
iodine cell
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PDV system fiber image bundles (exit plane)
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Summary

• Characteristics of Planar Doppler Velocimetry
• high spatial resolution 1 data for each pixel
• high dynamic range 150 m/s (or more)
• fast data acquisition and processing lt 1 minute
  per data set
• no need to resolve individual particles
• scaleable to larger viewing areas
• not directionally sensitive to flow vector
  (e.g. PIV is sensitive to in-plane flow)
• Characteristics specific to time- /
  phase-averaging PDV
• low relative error better than 1 m/s
• 3-C velocity data through sequential light sheet
  access
• no noticeable speckle effects

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PDV-Applications

• Aerodynamics, Turbomachinery
• swirled fuel spray nozzle
• engine intake of aircraft model in wind tunnel
• axially staged combustor sector
• swirl nozzle with precessing vortex core
  (phase-averaged)
• atmospheric combustion (kerosene, atmospheric)
• Automotive
• wake of model car
• engine-exhaust manifold (phase averaged)
• catalytic converter (downstream of turbocharger)
• turbocharger
• in-cylinder flow of gasoline diesel engines

• in the near future
• cryogenic wind tunnel flow (ETW, M-DAW
  EC-project)
• large-scale wind tunnel (i.e. DNW-LLF)
• combustion (pressurized, thermo-acoustics,
  phase-averaged)