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Optical techniques for density (andconcentration) measurements in
fluid mechanicsJ. Kristian Sveen
jks@math.uio.no
Dept. of Mathematics, University of Oslo
Optical density meas. tech. – p.1/35
Overview
1. Density/concentration measured optically?
2. Focus on applications in Fluid Mechanics
Concept
Refractive index - densityMean density along line of sight
Use a passive tracer � concentration of tracer givesdensity/concentration of fluid
2D slice (or 3D using a scanning system)
Optical density meas. tech. – p.2/35
Example
Laser light passing through fluid w. strong densitygradient
see Merzkirch [1974], Merzkirch and Peters [1992]
Optical density meas. tech. – p.3/35
Deflection of light rays
Constant second derivative
Constant density gradient
Constant density
see Merzkirch [1974]
Optical density meas. tech. – p.4/35
What do we see?
Looking through a fluid
Incident light disturbance
Photographicdevice
∆yθ t
t*
Shadowgraph - measure
���
Schlieren - measure
�
Mach-Zendner Interferometer - measure phase-shift��� � ��� � � � �� � ��
Optical density meas. tech. – p.5/35
See the light
Method
Shadowgraph
Schlieren
Interferometry
Easy
Difficult
What we measure
Second derivative
of densityFirst derivative
Density
of density
Optical density meas. tech. – p.6/35
Shadowgraph
Optical density meas. tech. – p.7/35
Shadowgraph
Pionered by Dvorak (1880)
*Light Source
Lens Lens
Measurementsection (w optical disturbance)
Lens
Photographicdevice
Shadowgraph – p.8/35
Shadowgraph
Picture copied from Merzkirch (1974)Shadowgraph – p.9/35
Schlieren
Shadowgraph – p.10/35
Schlieren
Attributed to Focault (1859) and Toepler (1864) (dep.on national preference)
Used by Focault to check quality of lenses and mirrors
Elaborated by Toepler
Now often referred to as the “Toepler method”
Schlieren – p.11/35
Schlieren
Idea: Introduce knife edge at focal pointlight bent down is removed � darker-spotslight bent up is kept � brighter-spots
Direction of density gradient known
*Light Source
Lens Lens
LensPhotographicdevice
Knife edge
Measurementsection (w optical disturbance)
Schlieren – p.12/35
Schlieren - example
Schlieren – p.13/35
Schlieren in the digital world
Shadowgraph - sensitive to changes in secondderivative of density
Idea: compare two subsequent images
Construct first derivative by “integration”
Schlieren – p.14/35
Schlieren in the digital world contd
Motion in density field distorts background pattern
Schlieren – p.15/35
Synthetic Schlieren
Dalziel et al. [2000], Sutherland et al. [1999, 2003],Onu et al. [2003]
Using different background patterns1. Lines (horizontal or vertical)2. Regular pattern (dots individually tracked)3. Irregular pattern (using pattern matching)
Using different algorithms for pattern matching
Problem - regularity in pattern limits maximal recoverable dis-
placement
Schlieren – p.16/35
Synthetic Schlieren
Examples of background patterns:
1 2
Schlieren – p.17/35
Synthetic Schlieren
The relationship between the displacement and thedensity is given by (Dalziel et al. [2000]):
��� � ��� � � � ���
� �
��
and
� � � ��� � � � ���
� �
��
where - width of tank,
�distance from tank to
pattern (to increase sensitivity),
� � ��� �� �
,
� � �
g cm � � and � the density fluctuations
Schlieren – p.18/35
Synthetic Schlieren
Applied to measurements of internal waves in a linearstratification
2ρ
ρ1
Density field (perturbation)Setup
Schlieren – p.19/35
Background Orientated Schlieren (BOS)
Meier [1999], Richard and Raffel [2001], Klinge et al.[2003]
Essentially the same as Dalziel et al. [2000]
Cross correlation (w FFT) for pattern matching
Applied to measurements in air
Schlieren – p.20/35
BOS - continued
Displacement fields proportional to
� � �� and
� � �� .
Schlieren – p.21/35
BOS example
Rising Helium from a container (or: the coffee cupexperiment)
Courtesy of F. Klinge, DLR
Schlieren – p.22/35
Industrial applications
Example: Structured Light - used to measure 3Dobjects:
Method currently in use at Kongsberg Automotive -Automatic inspection of aluminium car-parts.
Picture from Sintef Materials Technology - used without permissionSchlieren – p.23/35
Structured light continued
Should be applicable in the fluid mechanics lab:High speed camera (2000 fps, 1Mpix), viewing fromabove3D, high resolution, surface wave measurements
Schlieren – p.24/35
Industrial applications
Pattern matching approaches used widelyMpeg encoding of digital moviesObject recognitionRobotics Vision - automatic car parking:
Schlieren – p.25/35
Comments on the pattern matching
So far:
Absolute differences
Cross correlation (full or FFT-based)
Squared differences
Other approaches may be applicable:
Differential approaces (often used in Optical Flow):
��� � � � �� � � �
Intensity (
�
) conservedSmall displacements (generally <1 pixel)
Schlieren – p.26/35
Laser Induced Fluorescence
Schlieren – p.27/35
Laser Induced Fluorescence (LIF)
Idea: Add a visual tracer (indicator) � function ofconcentration
Pankow et al. [1984], Asher and Pankow [1989]
Koochesfahani [1984], Koochesfahani and Dimotakis[1985, 1986]
Walker [1987]
LIF – p.28/35
LIF - howto
Flow illuminated by thin sheet of light formed by a laserand optics
Add fluorescent dyeDye absorbs laser lightre-emits light at longer wavelengthcommonly used dyes: Rhodamine 6G, RhodamineB, Fluorescein Disodium, ++
CCD camera w filter records flowFilter removes all light except the fluorescentThe light intensity directly related todensity/concentration
LIF – p.29/35
LIF - calibration and use
At low concentration levels ( � � ��� � � � �) there is a linearrelationship between signal and dye concentration:
� � � � � where
�
is signal, � a constant,�
concentration and
�
laser light intensity
Calibration consists of determining � at each camerapixel
In use:� � � � � �
The above holds for density measurements - slight difference for temperature
LIF – p.30/35
LIF - some references
Papantoniou and List [1989], Law and Wang [1999] -Buoyant jet. Related to mixing and entrainment of arising buoyant jet (for example discharges fromoil-platforms)
Turbulent jets: Guillard et al. [1998], Westerweel et al.[2002]
Concentration profiles on the air/seainterface: Münsterer and Jähne [1998]
Oxygen concentration in air: Miles and Lempert [1997]
LIF – p.31/35
LIF - some more references
Deusch and Dracos [2001] - 3D field using a scanningmirror system.
Sakakibara and Adrian [1999]:
LIF – p.32/35
LIF - example
Liao and Cowen [2003], motivated by the workof Koehl et al. [2001]
Purpose: how can lobsters smell?How to find the source of polution etcSolve the “inverse problem” of turbulent diffusion inthe environment
Image showing the far-field of a plume:
LIF – p.33/35
LIF - example contd
Koehl et al. [2001] - Lobster sniffing
Rapid flickagainst current
Slow returnalong current
Current
LIF – p.34/35
Summary
Briefly reviewed Schlieren (BOS and ’synthetic’)methods and LIF
Provide quantitative information aboutconcentration/density fields
Non-intrusive, whole-field methods
Generally 2D - but extensions to 3D possible for LIF
LIF – p.35/35
References
W. E. Asher and J. F. Pankow. Direct observation of con-centration fluctuations close to a gas/liquid interface.Chem. Engrg. Sci., 44:1451–1455, 1989.
S. B. Dalziel, G. O. Hughes, and B. R. Sutherland.Whole-field density measurements by ’syntheticschlieren’. Exp. Fluids, 28(4), 2000.
S. Deusch and T. Dracos. Time resolved 3d passive scalarconcentration-field imaging by laser induced floures-cence (lif) in moving liquids. Meas. Sci. Tech., 12,2001.
F. Guillard, R. Fritzon, J. Revstedt, C. Tragard, M Alden,and L. Fuchs. Mixing in a confined turbulent imping-ing jet using planar laser induced fluorescence. Exp.Fluids, 25:143–150, 1998.
F. Klinge, T. Kirmse, and J. Kompenhans. Applicationof quantitative background oriented schlieren (bos): in-vestigation of a wing tip vortex in a transonic wind tun-nel. In Proceedings of PSFVIP-4, Chamonix, June 3-5,2003.
M. A. R. Koehl, J. R. Koseff, J. P. Crimaldi, M. G. Mc-Cay, T. Cooper, M. B. Wiley, and P. A. Moore. Lobster
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sniffing: Antennule design and hydrodynamic filteringof information in an odor plume. Science, 294:1948–1951, 2001.
M. Koochesfahani. Experiments on turbulent mixing andchemical reactions in a liquid mixing layer. PhD thesis,California Institute of Technology, 1984.
M. M. Koochesfahani and P. E. Dimotakis. Laser inducedfluorescence measurements of mixed fluid concentra-tion in a liquid plane shear layer. AIAA J., 23(11):1700–1707, 1985.
M. M. Koochesfahani and P. E. Dimotakis. Mixing andchemical reactions in a turbulent liquid mixing layer. J.Fluid Mech., 170:83–112, 1986.
A. W.-K. Law and H. Wang. Simultaneous velocity andconcentration measurements of buoyant jet dischargeswith combined dpiv and plif. In Jayawardena Lee andWang, editors, Environmental Hydraulics, 1999.
Q. Liao and E. A. Cowen. Experimental investigation onthe growth of a scalar plume in a turbulent boundarylayer. To be submitted, 2003.
G. E. A. Meier. Hintergrund schlierenmessverfahren.Deutsche Patentanmeldung, DE 199 42 856 A1, 1999.
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W. Merzkirch. Flow visualization. Academic Press,1974.
W. Merzkirch and F Peters. Optical visualization of in-ternal gravity wave. Opt. Las. Eng., 16:411–425, 1992.
R. B. Miles and W. R. Lempert. Quantitative flow visual-ization in unseeded flows. Ann. Rev. Fluid Mech., 29:285–326, 1997.
T. Munsterer and B. Jahne. Lif measurements of con-centration profiles in the aqueous mass boundary layer.Exp. Fluids, 25:190–196, 1998.
K. Onu, M. R. Flynn, and B. R. Sutherland. Schlierenmeasurement of axisymmetric internal wave ampli-tudes. Exp. Fluids, ’Online first’, 2003.
J.F. Pankow, L.M. Isabelle, and W.E. Asher. Trace or-ganic compounds in rain. i. sample design and analysisby adsorption/thermal description. Environ. Sci. Tech-nol., 18:310–318, 1984.
D. Papantoniou and J. E. List. Large-scale structure in thefar-field of buoyant jets. J. Fluid Mech., 209:151–190,1989.
H. Richard and M Raffel. Principle and applications ofthe background oriented schlieren (bos) method. Meas.Sci. Tech., 12:1576–1585, 2001.
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J. Sakakibara and R. J. Adrian. Whole field measurementof temperature in water using two-color laser inducedfluorescence. Exp. Fluids, 26:7–15, 1999.
B. R. Sutherland, S. B. Dalziel, G. O. Hughes, and P. F.Linden. Visualization and measurement of internalwaves by ’synthetic schlieren’. part 1. vertically oscil-lating cylinder. J. Fluid Mech., 390:93–126, 1999.
B. R. Sutherland, M. R. Flynn, and K. Onu. Schlierenvisualization and measurement of axisymmetric distur-bances. Nonlin. Proc. Geophys., 10:303–309, 2003.
D.A. Walker. A fluorescent technique for measurementof concentration in mixing liquids. J. Phys.E. Sci. In-strum., 20:217–224, 1987.
J. Westerweel, T Hofman, C. Fukushima, and J. C. R.Hunt. The turbulent/non-turbulent interface at the outerboundary of a self-similar turbulent jet. Exp. Fluids,33:873–878, 2002.
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