Image Analysis as Measuring
Euromech 1991
Technique in Flows
11 1 1 « • • l i . . , , / / / - : ? : c e * i I 1 V I * * > * * * » » i f ' ' ' ' - \ \ \ S \
1
\ \ t t ) V » \ \ - / / / / / ' I i • it'U Delft
Delft U n i v e r s i t y of T e c h n o l o g y1 Articles and e x t e n d e d abstracts of the E u r o m e c h C o l l o q u i u m 279, 2-5 J u l y 1 9 9 1 , J . M . Burgers Centre, Delft University of T e c h n o l o g y , Delft, t h e Netherlands
Measuring Technique i n Flows
I n v i t e d L e c t u r e s a n d E x t e n d e d A b s t r a c t s o f t h e E u r o m e c h C o l l o q u i u m 279 2-5 J u l y 1991 J . M . Burgers Centre L a b . Aero and H y d r o d y n a m i c s D e l f t University of Technology Rotterdamseweg 145 2628 A L D e l f tSponsors
Akzo Dantec
Delft Hydraulics Deltalab
D.S.M. Research Kluwer Academic Publishers
Min. of Transport and Public Works Philips Research
Shell Nederland B.V T.S.I.
C o l l o q u i u m chairmen
p r o f . d r . i r . F . T . M . Nieuwstadt and p r o f . d r . M . L . R i e t h m u l l e r Local Organizing C o m m i t t e e
i r . J . M . Bessem H . J . van der Brugge-Peeters C . J . A . van Leeuwen M . Oemar-Gemerts P. Baan H o f m a n i r . J. Westerweel
Image analysis as measuring technique i n flows
T u e s d a y 2 J u l y . P I V — m e t h o d s : t e c h n i q u e s a n d a p p l i c a t i o n s C h a i r m a n : F . T . M . N i e u w s t a d t
08.30-09.30 r e g i s t r a t i o n and opening
09.30-10.00 I n v i t e d lecture: Past and F u t u r e of PTV
R . M e y n a r t
European Space Agency, the Netherlands
10.00-10.50 I n v i t e d Lecture: Recent advances i n the a p p l i c a t i o n o f P I V ( P a r t i c l e Image l o c i m e t r y ) , L S V (Laser Speckle Velocimetry) and P T V ( P a r t i c l e T r a c k i n g Ve-l o c i m e t r y )
L . L o u r e n g o
Florida State University, USA
10.50-11.10 Coffee
11.10-11.35 Stochastic o p t i m i z a t i o n of processing techniques f o r P I V
C . S . M o r a i t i s a n d M . L . R i e t h m u l l e r
Von Karman Institute for Fluid Dynamics, Belgium
11.3512.00 Development o f p a r t i c l e image velocimetry: a new c o m p u t a t i o n m e t h o d w i t h d i -r e c t i o n a l -r e s o l u t i o n
P . G u i b e r t , M . M u r a t , Q . C . D u d a n a n d J . J u l i e n Université de Paris, France
12.00-12.25 A n a u t o m a t i c evaluation system f o r PTV record and specklegrams based o n trans-puters
H . W i n t r i c h a n d W . M e r z k i r c h . University of Essen, Germany
12.00-14.00 L u n c h C h a i r m a n : R . M e y n a r t
14.00-14.25 A P I V system f o r a p p l i c a t i o n i n transsonic w i n d tunnels
J . K o m p e n h a n s a n d R . H o e k e r DLR Götingen, Germany
14.25-14.50 A n experimental comparison between o p t i c a l and d i g i t a l p a r t i c l e v e l o c i m e t r y
J . M . C o r n e l i s s e , H . W . H . E . G o d e f r o y , F . K o o p m a n s , R . E . U i t t e n b o g a a r d a n d J . W e s t e r w e e l
Delft Hydraulics, the Netherlands
14.50-15.15 P a r t i c l e image v e l o c i m e t r y at T . U . D e n m a r k
P . B u c h h a v e a n d M . L . J a k o b s e n
Technical University of Denmark, Denmark
15.15-15.40 A 3 D PTV system and its a p l i c a t i o n t o the measurement o f acoustic s t r e a m i n g
P . A r r o y o , C . G r e a t e d a n d D . H a h n Stanford University, USA
15.40-16.00 Tea
16.00-16.25 I n v e s t i g a t i o n of turbulence s t r u c t u r e using i m a g i n g methods
R . J . A d r i a n , P . W . O f f u t t a n d C . E . W a r k University of Illinois, USA
16.50-17.15 A p p l i c a t i o n of p a r t i c l e image displacement velocimetry t o i n d u s t r i a l flows
S . A r e f l , M . C h a n c o n n e , B . B r o u s s o u l o u x a n d M . M o i n e Centre Technique des Industries Aerauliques et Thermiques, France
17.30-18.30 C o c k t a i l p a r t y
W e d n e s d a y 3 J u l y . P I V a n d P a r t i c l e T r a c k i n g : t e c h n i q u e s a n d a p p l i c a t i o n s C h a i r m a n : H . R o y e r
09.00-09.50 I n v i t e d Lecture: T h e o r y and priciple of P I V (Particle Image V e l o c i m e t r y ) methods
R . J . A d r i a n
University of Illinois, USA
09.50-10.15 Development and p o t e n t i a l of a p h o t o g r a m m e t r i c 3-D p a r t i c l e t r a c k i n g velocimeter
H . G . M a a s a n d T h . D r a c o s
Swiss Federal Institute of Technology, Switzerland
10.15-10.40 P I V technique i n the t u r b u l e n t intensity and space derivative evaluation
A . C e d e n e s e a n d G . P . R o m a n o Universitd degli Studi di Roma, Italy
10.40-11.00 Coffee
11.00-11.25 A colored m e t h o d f o r P I V technique
J . S t e f a n i n i , G . C o g n e t , J . C . V i l a , B . M e r i t e CEA-Cadarache, France
11.25-11.50 A camera f o r measuring density, size and velocity of rising air bubbles and water velocity i n a bubble p l u m e
A . M ü l l e r a n d C . H u g i
Swiss Federal Institute of Technology, Switzerland
11.50-12.15 T i m e v a r y i n g turbulence: some particle t r a c k i n g experiments
S . D a l z i e l
University of Cambridge, Engeland
12.15-12.40 Stereometry i n studies of f r i c t i o n wakes
A . A . B o r i s o v , B . G . N o v i k o v a n d V . D . F e d o s e n k o Institute of Thermophysiks, Novosibirsk, USSR
12.40-14.00 L u n c h C h a i r m a n : R . J . A d r i a n
14.00-14.25 Coherent structures i n two-dimensional flow: measurement and results
J . B . F l ö r a n d C . W . M . W i l l i a m s
Eindhoven University of Technology, the Netherlands
14.25-14.50 P a r t i c l e image velocimetry measurements of the aerodynamics o f a w i n d t u r b i n e
I . G r a n t , G . H . S m i t h , A . L i u , D . I n f i e l d , T . E i c h Heriot-Watt University, Scotland
14.50-15.15 A c r i t i c a l analysis of p a r t i c l e image v e l o c i m e t r y technique as a p p l i e d t o waves on beaches
P . A . Q u i n n , C . G r a y , D . J . S k y n e r , C . A . G r e a t e d a n d W . J . E a s s o n Edinburg University, Scotland
T h u r s d a y 4 J u l y . Scalar f i e l d s a n d d a t a analysis C h a i r m a n : L . L o u r e n g o
09.00-09.50 I n v i t e d Lecture: V i s u a l i z a t i o n methods f o r f l u i d f l o w d a t a sets
L . H e s s e l i n k
Stanford University, USA
09.50-10.15 D i g i t a l Image processing f o r Schlieren optics
G . H . S c h n e r r , R . R . H u b e r a n d H . A . J a n t z e n University of Karlsruhe, Germany
10.15-10.40 A s t u d y o f pressurized gas release by laser induced fluorescence and image analysis techniques
S . G . B r y c e a n d R . J . F r y e r - T a y l o r Shell Research Thornton, England
10.40-11.00 Coffee
11.00-11.25 A p p l i c a t i o n s of an a u t o m a t e d P I V and post-processing techniques system
T . B r u c e , C . E l g a a r d a n d D . R . M c C l u s k e y University of Edinburgh, Scotland
11.25-11.50 D e t e r m i n a t i o n o f velocity - concentration correlations b y Particle image velocime-t r y and laser induced fluorence i m a g i n g
S . S i m o e n s a n d M . A y r a u l t Ecole Centrale de Lyon, France
11.50-12.15 A n a p p l i c a t i o n of image processing methods t o determine the c r i t i c a l shear stress i n sewer systems
W . B e c h t e l e r , V . B a i e r a n d S. H a r t m a n n
University of the Armed Forces Munich, Germany
12.15-12.40 S t u d y o f v o r t e x m u l t i p l e s i n a s t r a t i f i e d fluid b y p a r t i c l e displacement analysis
S . I . V o r o p a y e v a n d Y a . D . A f a n a s y e v Institute of Oceanology, Acad, of Sci., USSR
12.40-14.00 L u n c h
C h a i r m a n : L . H e s s e l i n k
14.00-14.25 Laser t o m o g r a p h y o f a buoyant t u r b u l e n t d i f f u s i o n flame
G . W h e a t e r a n d K . N . B r a y Cambridge University, England
14.25-14.50 Measurement of p r o d u c t concentration fluctuations of t w o parallel reactive jets using d i g i t a l image processing
H . S t a p o u n t z i s , J . M . B e s s e m , F . T . M . N i e u w s t a d t a n d J . W e s t e r w e e l University of Thesssaloniki, Greece
Université de St Etienne, France 15.15-15.35 Tea 15.35-17.00 P o s t e r s e s s i o n 19.30-21.30 Conference D i n n e r F r i d a y 5 J u l y . H o l o g r a p h i c T e c h n i q u e s C h a i r m a n : M . L . R i e t h m u l l e r
09.00-09.50 I n v i t e d Lecture: C o n t r i b u t i o n of microholography t o 3-D flow measurements
H . R o y e r
Institut Saint-Louis, France
09.50-10.15 O p t i c a l processing of in-line and off-line holograms f o r 3D instantaneous velocity measurements
C . S . M o r a i t i s a n d M . L . R i e t h m u l l e r
von Karman Institute for Fluid Dynamics, Belgium
10.15-10.40 D i g i t a l processing of multipass holograms
R . D e r o n
Office National d'Etudes et de Recherches Aerospatiales, France
10.40-11.05 Extension o f p a r t i c l e image Velocimetry ( P I V ) to a 3-D measuring m e t h o d
H . H i n r i c h s , K . H i n s c h , G . K u h f a l a n d P . M e i n l s c h m i d t Carl-von-Ossietzky University, Germany
11.05-11.30 Coffee
11.30-11.55 Particle detection c r i t e r i a f o r a u t o m a t i c d a t a r e d u c t i o n f r o m micro-holograms
M . S t a n i s l a s , C . G e i l e r , H . R o y e r , M . C o u r b o n , J . A z e m a a n d J . D a n i e -r e s
Institut de Mecaniques de fluides, Lille, France
11.55-12.20 A l g o r i t h m s f o r a u t o m a t i c meaurement of size and velocity o f spray droplets f r o m holographic reconstructions
A . C h a v e z a n d F . M a y i n g e r
Technische Universtdt Miinchen, Germany
12.20-12.45 D i g i t a l holographic i n t e r f e r o m e t r y i n the investigation o f unsteady flows
T . A . W . M . L a n e n
Delft University of Technology, the Netherlands
C h a r a c t e r i z i t i o n of Savonius r o t o r wake using image processing techniques
J . M a s s o n s , J n a . G a v a l d a , J . E s c o d a , X . R u i z a n d F . D i a z University of Barcelona, Spain
Flow experiments and visualization i n a h y d r o s t a t i c j o u r n a l bearing
M . J . B r a u n a n d R . C . H e n d r i c k s University of Akron, USA
L S V applied t o the flow i n a r o t a t i n g channel
S . U e l l n e r
Technische Hochschule Darmstadt, Germany
A n a u t o m a t i c image analysis m e t h o d f o r d e t e r m i n i n g each sense of velocity vectors using a p a t h l i n e p i c t u r e
K . H o j o
Osaka Institute of Technology, Japan
Unsteady flows velocity field restoration by a specific d i g i t a l image processing
P . de R a m e f o r t , A . T e x i e r a n d M . C o u t a n c e a u Université de Poitiers, France
Use o f f r a c t a l analysis i n t u r b u l e n t flows
J . M . R e d o n d o
Universitad Politechnica de Catalunya, Spain
I n v e s t i g a t i o n o f the free edge of an unsteady axisymmetric t u r b u l e n t j e t
F . Z . L a h b a b i , J . B o r e e , H . J . N u g l i s c h a n d G . C h a r n a y Institut Mecaniques des Fluides Toulouse, France
S t u d y o f h o r i z o n t a l j e t i n a s t r a t i f i e d fluid using t h y m o l blue velocimetry and weak disturbance p r o p a g a t i o n m e t h o d
S . I . V o r o p a y e v a n d Y . D . A f a n a s y e v Institute of Oceanology, Acad, of Set., USSR
A c o m p a r a t i v e s t u d y of optical autocorrelators for P I V
C . S . M o r a i t i s a n d M . L . R i e t h m u l l e r
von Karman Institute for Fluid Dynamics, Belgium
D e t e c t i o n o f low speed streaks over a drag reducing surface
A . D . S c h w a r z - v a n M a n e n , H . P . S p a r i d a n s , K . K r i s h n a P r a s a d , F . T . M . N i e u w s t a d t Eindhoven University of Technology, the Netherlands
D i g i t a l image processing f o r the analysis of hydrogenbubble timeline visualizations i n a t u r -bulent b o u n d a r y layer
A . J . B r a n d a n d F . T . M . N i e u w s t a d t
Delft University of Technology, the Netherlands
Analysis of a b u b b u n g 2D-gas-fluidized bed using Image Processing
H . B . M . S c h u l t e , R . F . M u d d e , H . E . A . v a n de A k k e r Delft University of Technology, the Netherlands
D e t e r m i n a t i o n o f flow profiles i n the die-region of an extruder
F . B u i t e n d i j k
O i l f i l m i n t e r f e r o m e t r y ; A n o p t i c a l m e t h o d o f measuring w a l l shear stress
H . A . S i l l e r , R . J . P e r k i n s a n d G . J a n k e DAMPT, University of Cambridge, England
Roland Meynart
European Space Agency - European Space Research and Technology Centre Instrument Technology Division
2200AG Noordwijk - The Netherlands
The historical development of the technique known under the names Laser Speckle
Velocimetry or, more recently, Particle Image Velocimetry will be described and
illustrated by examples and results dating back to the early times when only a few
"pioneers" were believing in the potential capabilities of this technique.
It will be shown how the technique inherited from the so-called speckle techniques of
stress analysis in solid mechanics and why the analysis of optical fringes for data
reduction in place of direct inspection of the recorded images was considered as a
major advance.
Various types of flows (Poiseuille and convection flows, gas jets) were used in the first
years to validate the technique and show how it can provide whole-field velocity
information difficult or sometimes impossible to obtain by other techniques.
The inherent limitations of the technique due to basic physical principles will be
covered as well as the rapidly improving hardware limitations. The importance of
optimum recording of the particle field images will be emphasized.
Finally, the prospects of improvement of the technique will be outlined, with
particular attention drawn to the applications of the technique. Two examples of
experiments considered by the European Space Agency for possible implementation
in the International Space Station will be shortly described: a multi-purpose facility for
the study of the structure of liquid flows and a combustion flow diagnostics facility.
P a p e r t o be p r e s e n t e d a t t h e E u r o M e c h C o l l o q u i u m 279
D e l f t 2 - 5 , J u l y , 1 9 9 1
W h o l e f i e l d v e l o c i m e t r y t e c h n i q u e s s u c h a s L a s e r
S p e c k l e ( L S V ) P a r t i c l e I m a g e ( P I V ) a n d P a r t i c l e T r a c k i n g
V e l o c i m e t r y ( P T V ) a r e d i f f e r e n t i m p l e m e n t a t i o n s o f t h e s a m e
b a s i c p r i n c i p l e s m e a s u r i n g i n s t a n t a n e o u s f l u i d v e l o c i t i e s b y
r e c o r d i n g t h e p o s i t i o n o f i m a g e s b y s m a l l t r a c e r s s u s p e n d e d
i n t h e f l u i d a t s u c c e s s i v e t i m e i n s t a n t s . T h e common
u n d e r l y i n g a s s u m p t i o n i s t h a t t h e s e t r a c e r s f o l l o w c l o s e l y ,
a n d w i t h m i n i m a l l a g , t h e f l u i d m o t i o n . T h i s a s s u m p t i o n
h o l d s t r u e f o r a w i d e v a r i e t y o f f l o w s o f i n t e r e s t p r o v i d e d
t h a t t h e t r a c e r s a r e s m a l l e n o u g h a n d / o r t h a t t h e i r d e n s i t y
a p p r o a c h e s t h a t o f t h e f l u i d .
B e s i d e s t h e i r common g o a l L S V a n d P I V , o n o n e h a n d , a n d
PTV o n t h e o t h e r h a n d do n o t s h a r e t h e s a m e h i s t o r i c a l
d e v e l o p m e n t a n d p r a c t i c e . An i m p o r t a n t d i f f e r e n c e i s t h a t i n
L S V a n d P I V , w h i c h h a v e b e e n e s t a b l i s h e d a s t w o o p e r a t i n g
m o d e s o f t h e same m e t h o d ( M e y n a r t a n d L o u r e n c o , 1 9 8 4 ) a n d
( A d r i a n , 1 9 8 4 ) , t h e c o n c e n t r a t i o n o f t r a c e r s i s r a t h e r h i g h
a n d t h e m e a s u r e m e n t o f a " l o c a l " f l u i d v e l o c i t y r e s u l t s f r o m
a n a v e r a g e o v e r many t r a c e r s c o n t a i n e d i n a i n t e r r o g a t i o n
c e l l . T h e c e l l i s r e g u l a r l y s p a c e d a n d i t s s i z e d e t e r m i n e s
t h e s p a t i a l r e s o l u t i o n . T h i s i s i n c o n t r a s t w i t h PTV w e r e
t h e v e l o c i t y i s d e t e r m i n e d a t r a n d o m l o c a t i o n s u s i n g t h e
i m a g e s p r o d u c e d b y a s i n g l e t r a c e r .
P a r t i c l e T r a c k i n g V e l o c i m e t r y h a s e v o l v e d a s a m e a n s
o f e x t r a c t i n g q u a n t i t a t i v e i n f o r m a t i o n f r o m c o n v e n t i o n a l
f l o w v i s u a l i z a t i o n d a t a , s u c h a s s t r e a k p h o t o g r a p h y o r
m u l t i p l e e x p o s u r e p h o t o g r a p h y ( E l k i n s e t a l , 1 9 7 7 ) ,
( K o b a y a s h i e t a l , 1 9 8 3 ) , ( U t a m i a n d U e n o , 1 9 8 4 ) a n d ( G h a r i b
e t a l , 1 9 8 5 ) . W h i l e t h e s e m e t h o d s e x c e l a s a m e a n s f o r f a s t
a n d e a s y m a p p i n g o f t h e f l o w b a s i c s t r u c t u r e s , t h e y f a i l i n
p r o v i d i n g a n a c c u r a t e v e l o c i t y f i e l d map w i t h h i g h s p a t i a l
d e n s i t y . T y p i c a l l y t h e e v a l u a t i o n o f t h e p a r t i c u l a t e
d i s p l a c e m e n t s d i r e c t l y f r o m t h e i r i m a g e s , s u c h a s
e v a l u a t i n g s t r e a k l e n g t h o r s p a c i n g b e t w e e n s u c c e s s i v e
i m a g e s o f t h e same t r a c e r , r e q u i r e s p r o c e s s i n g l a r g e
a m o u n t s o f d a t a a n d r a t h e r s o p h i s t i c a t e d s o f t w a r e . T h e s e
t e c h n i q u e s t e n d t o b e q u i t e l a b o r i o u s a n d u s u a l l y n o t v e r y
a c c u r a t e , a s t h e m e a s u r e m e n t e r r o r s b e c o m e l a r g e w h e n t h e
mean d i s t a n c e b e t w e e n t r a c e r s i s o f t h e s a m e o r d e r o f
m a g n i t u d e o f t h e s t r e a k l e n g t h , o r t h e s p a c i n g b e t w e e n
s u c c e s s i v e p a r t i c l e i m a g e s , a n d / o r t h e s e b e c o m e s m a l l . A s a
c o n s e q u e n c e t h e t r a c e r d e n s i t y i s k e p t s m a l l , r e s u l t i n g i n
v e l o c i t y m e a s u r e m e n t s w i t h p o o r s p a t i a l r e s o l u t i o n . T h e
p r o b l e m o f h a v i n g s p a r s e v e l o c i t y i n f o r m a t i o n a t r a n d o m
l o c a t i o n s h a s b e e n a d d r e s s e d w i t h t h e i n t r o d u c t i o n o f r a t h e r
s o p h i s t i c a t e d i n t e r p o l a t i n g s c h e m e s , b u t t h e v a l i d i t y o f
t h i s a p p r o a c h i s q u e s t i o n a b l e c o n s i d e r i n g t h a t i s m o s t c a s e s
t h e s p a c i n g b e t w e e n d a t a p o i n t s i s l a r g e r t h a n t h e f l o w
2c o n v e n t i o n a l PTV m e t h o d s b e c a u s e o f t h e i r a b i l i t y t o p r o v i d e
h i g h e r t e m p o r a l a n d s p a t i a l r e s o l u t i o n o f t h e i n s t a n t a n e o u s
f l o w f i e l d . T h e s e t w o t e c h n i q u e s a r e b a s e d o n e s s e n t i a l l y
t h e same p r i n c i p l e s a n d m e t h o d o f o p e r a t i o n w i t h e x c e p t i o n
t o t h e a m o u n t o f s e e d i n g e m p l o y e d . M e y n a r t a n d L o u r e n c o
( 1 9 8 4 ) a r g u e d t h a t b e s t r e s u l t s a r e o b t a i n e d w h e n t h e
p a r t i c l e c o n c e n t r a t i o n i s k e p t a s h i g h a s p o s s i b l e a s t o
e n s u r e a maximum n u m b e r o f p a r t i c l e i m a g e s i n a n
i n t e r r o g a t i o n c e l l b u t n o t s o h i g h t o o v e r l a p a n d g e n e r a t e
s p e c k l e . G e n e r a t i o n o f s p e c k l e p a t t e r n s i s n o t d e s i r a b l e i n
m o s t a p p l i c a t i o n s d u e t o d e p e n d e n c e o f t h e s p e c k l e f o r m a t i o n
w i t h t h e f l u i d m o t i o n . O t h e r r e a s o n s t o l i m i t t h e
c o n c e n t r a t i o n a r e r e l a t e d t o t h e d e g r a d a t i o n o f t h e l i g h t
s h e e t p r o p a g a t i o n a n d p o s s i b l e i n t e r f e r e n c e w i t h t h e
m e c h a n i c s o f t h e f l u i d m o t i o n .
I n t h i s a r t i c l e t h e a p p l i c a t i o n o f t h e P I V t e c h n i q u e t o
t h e s t u d y o f p r a c t i c a l f l o w f i e l d s i s u s e d a s a m e a n s t o
i l l u s t r a t e b o t h t h e t e c h n i q u e s a b i l i t y t o map s p a t i a l l y a n d
t e m p o r a l l y d e v e l o p i n g f l o w s , a s w e l l a s t h e m o s t r e c e n t
f e a t u r e s i n c o r p o r a t e d i n t h e t e c h n i q u e .
2. A p p l i c a t i o n o f P I V t o t h e m e a s u r e m e n t o f a n u n s t e a d y
f l o w :
P i t c h i n g NACA a i r f o i l .
T h e u n s t e a d y f l o w p a s t a NACA 0 0 1 2 a i r f o i l i n p i t c h i n g
u p m o t i o n i s e x p e r i m e n t a l l y i n v e s t i g a t e d i n a w a t e r t o w i n g
t a n k u s i n g t h e P a r t i c l e Image V e l o c i m e t r y t e c h n i q u e . T h e
R e y n o l d s n u m b e r , b a s e d on t h e a i r f o i l s c h o r d a n d t h e
f r e e s t r e a m v e l o c i t y , i s 5 0 0 0 . T h e a i r f o i l p i t c h i n g m o t i o n i s
f r o m 0 t o 30 d e g r e e s a n g l e o f a t t a c k a t a d i m e n s i o n l e s s
p i t c h i n g r a t e o f 0 . 1 3 1 .
T h e w a t e r t o w i n g t a n k f a c i l i t y i s 180 cm l o n g a n d 4 3 x 5 5
cm i n c r o s s s e c t i o n a l a r e a . T h e t o w i n g c a r r i a g e i s d r i v e n b y
a D.C. a n d h a s a s p e e d w h i c h v a r i e s f r o m 0.3 t o 30 c m / s e c . A
m o t o r s p e e d c o n t r o l s y s t e m c o n t r o l s t h e t o w i n g s p e e d v i a a
d i g i t a l - t o - a n a l o g c o n v e r t e r . To e n s u r e a s m o o t h t r a v e r s e o f
t h e t o w i n g c a r r i a g e , a n a c c e l e r a t i o n r a m p a t t h e b e g i n n i n g
o f t h e t r a v e l a n d a d e c e l e r a t i o n r a m p a t t h e e n d o f t r a v e l
a r e i m p l e m e n t e d o n t h e c o n t r o l l e r . T h e NACA 0 0 1 2 a i r f o i l h a s
a c h o r d l e n g t h o f 6 cm a n d a n a s p e c t r a t i o o f 6 . 6 7 . A
r e c o r d i n g c a m e r a w i t h a s c a n n i n g m i r r o r s y s t e m a r e m o u n t e d
o n a r e i n f o r c e d , v i b r a t i o n f r e e p l a t f o r m t h a t e x t e n d s f r o m
t h e a i r f o i l c a r r i a g e . T h e a i r f o i l p i t c h i n g m o t i o n i s
p r o v i d e d b y a s t e p p i n g m o t o r w i t h a p r o g r a m m a b l e c o n t r o l l e r ,
w h i c h i s p r e - p r o g r a m m e d w i t h t h e t e s t p r o f i l e a n d a c t i v a t e d
b y t h e h o s t c o m p u t e r . T h e a i r f o i l c h a n g e s l i n e a r l y f r o m 0 t o
30 d e g r e e s a f t e r t h e a i r f o i l i s t o w e d more t h a n o n e c h o r d
l e n g t h . A DEC V a x s t a t i o n I I c o m p u t e r m o n i t o r s a l l m o t i o n s
( f i g u r e 1 ) .
3o f p r i s m s a n d p o l a r i z i n g c u b e beam c o m b i n e r s make t h e l i g h t
beams e m i t t e d f r o m t h e two l a s e r s c o l l i n e a r ( f i g u r e 2 ) . T i m e
s e p a r a t i o n b e t w e e n t h e l a s e r l i g h t p u l s e s c a n be v a r i e d f r o m
a f r a c t i o n o f one m i c r o s e c o n d t o a few s e c o n d s b y a d j u s t i n g
a p u l s e g e n e r a t o r u s e d a s a t r i g g e r . A c y l i n d r i c a l l e n s i s
u s e d t o f o r m t h e c o m b i n e d beam i n t o a s h e e t t h a t i l l u m i n a t e s
t h e m i d - s p a n s e c t i o n o f t h e a i r f o i l . M e t a l l i c c o a t e d
p a r t i c l e s ( T S I m o d e l 1 0 0 8 7 ) , w i t h a n a v e r a g e d i a m e t e r o f 4
m i c r o n s , a r e u s e d a s t h e f l o w t r a c e r s . A p h a s e - t r i g g e r e d 35
mm c a m e r a ( N i k o n F - 3 ) w i t h a 105mm M a c r o l e n s i s u s e d i n t h e
r e c o r d i n g o f t h e f l o w f i e l d . S y n c h r o n i z a t i o n b e t w e e n
c o m p o n e n t s i s a c c o m p l i s h e d u s i n g a T e k t r o n i x m o d u l a r
e l e c t r o n i c s s y s t e m . T h i s s y s t e m a l s o p r o v i d e s t h e p h a s e
-r e f e -r e n c e b e t w e e n t h e m o t i o n o f t h e a i -r f o i l a n d t h e P I V
p h o t o g r a p h i c t i m i n g s e q u e n c e .
O n c e t h e p h o t o g r a p h i c f i l m i s p r o c e s s e d c o n t a c t p r i n t s
a r e made. I t h a s b e e n shown b y M e y n a r t a n d L o u r e n c o ( 1 9 8 4 )
a n d P i c k e r i n g a n d H a l l i w e l l ( 1 9 8 4 ) t h a t w h e n u s i n g t h e
Y o u n g ' s f r i n g e a p p r o a c h t o r e t r i e v e t h e d i s p l a c e m e n t d a t a
t h i s p r o c e s s i n c r e a s e s t h e SNR a n d f r i n g e v i s i b i l i t y . T h e s e
p r i n t s a r e u s e d t o o b t a i n t h e v e l o c i t y d a t a b y p o i n t w i s e
s c a n n i n g f o l l o w i n g a C a r t e s i a n g r i d . T h e s t e p s i z e o f t h e
s c a n n i n g o p e r a t i o n was c h o s e n t o be 0.5 mm i n b o t h
d i r e c t i o n s ; t h i s c o r r e s p o n d s t o a s t e p s i z e o f 1.25 mm i n
t h e p h y s i c a l p l a n e . T h e i n t e r r o g a t i o n beam d i a m e t e r
c o r r e s p o n d s t o a n e q u i v a l e n t p h y s i c a l p r o b e s i z e o f 0 . 6 2 5
mm, w h i c h i s a p p r o x i m a t e l y h a l f o f t h e m e s h s i z e . T h e
Y o u n g ' s f r i n g e s p r o d u c e d b y t h e beam w e r e d i g i t a l l y
p r o c e s s e d t o p r o d u c e t h e v e l o c i t y a t t h e i n t e r r o g a t e d p o i n t
o n t h e n e g a t i v e , a n d t h e v o r t i c i t y was f o u n d b y n u m e r i c a l
d i f f e r e n t i a t i o n .
As m e n t i o n e d e a r l i e r , t h e a i r f o i l i s s t a r t e d w i t h a
r a m p t y p e m o t i o n a t z e r o a n g l e o f a t t a c k a n d i s a l l o w e d t o
move o n e c h o r d l e n g t h , a f t e r w h i c h t h e f l o w f i e l d i s f u l l y
e s t a b l i s h e d w i t h m i n i m a l e f f e c t s o f t h e s t a r t i n g p r o c e s s .
N e x t t h e a i r f o i l i s i m p u l s i v e l y g i v e n i t s p i t c h i n g m o t i o n
a n d a l l o w e d t o p i t c h t o 30 d e g r e e s a n g l e o f a t t a c k . D u r i n g
t h e p i t c h up m o t i o n t h e a i r f o i l t r a v e l s a p p r o x i m a t e l y f o u r
c h o r d l e n g t h s . O n c e t h e p i t c h up m o t i o n s t o p s t h e a i r f o i l i s
a l l o w e d t o t r a v e l f o r a n a d d i t i o n a l f o u r c h o r d l e n g t h s
b e f o r e i t c o m e s t o a c o m p l e t e s t o p . I n t h e p r e s e n t
e x p e r i m e n t s t h e a i r f o i l p i t c h e s a b o u t i t s q u a r t e r c h o r d
p o i n t .
F i g u r e s 3 a n d 4 show t h e t i m e d e v e l o p m e n t o f t h e f l o w
a r o u n d t h e p i t c h i n g a i r f o i l u s i n g t h e P I V t e c h n i q u e . F i g u r e
3 p r e s e n t s t h e v e l o c i t y f i e l d a t d i f f e r e n t t i m e s a s
u n i f o r m l y s c a l e d v e l o c i t y v e c t o r s . T h e c o r r e s p o n d i n g
4T h e e s s e n t i a l c o m p o n e n t o f t h e t e c h n i q u e t h a t m a k e s i t
p o s s i b l e t o m e a s u r e c o m p l e x f l o w f i e l d s , s u c h a s t h e o n e
p r e s e n t e d a b o v e i s t h e v e l o c i t y b i a s m e t h o d . Due t o l i m i t e d
r a n g e i n t h e f l u i d v e l o c i t y , a b o u t 20 c m / s e c , t h e v e l o c i t y
b i a s m e t h o d was s u c c e s s f u l l y i m p l e m e n t e d i n t h i s e x p e r i m e n t
u s i n g a s c a n n i n g m i r r o r . H o w e v e r , t h e s c a n n i n g m i r r o r
a p p r o a c h s u f f e r s • f r o m some r e s t r i c t i o n s . F i r s t l y , i t
r e q u i r e s t h e m o t i o n o f m e c h a n i c a l c o m p o n e n t s a n d p r e c i s e
s y n c h r o n i z a t i o n w i t h t h e c a m e r a s h u t t e r a n d t h e l a s e r
i l l u m i n a t i o n p u l s e s . T h i s m a k e s t h e u s e o f t h e m i r r o r v e r y
u n p r a c t i c a l , i f n o t i m p o s s i b l e , i n c a s e s w h e n a s y n c h r o n o u s
r e c o r d i n g o f f l o w e v e n t s i s n e c e s s a r y , e . g . t u r b u l e n t
e v e n t s , a n d when t h e r e q u i r e d b i a s v e l o c i t y m a g n i t u d e
e x c e e d s t h e m i r r o r ' s m e c h a n i c a l c a p a b i l i t i e s . T h e r e f o r e t h e
i m p l e m e n t a t i o n o f a p a s s i v e v e l o c i t y b i a s m e t h o d w h i c h d o e s
n o t r e q u i r e t h e movement o f m e c h a n i c a l c o m p o n e n t s i s v e r y
d e s i r a b l e .
A new m e t h o d i n t r o d u c e d by L a n d r e t h a n d A d r i a n ( 1 9 8 8 )
u s e s t h e p o l a r i z a t i o n o f l i g h t s c a t t e r e d f r o m s e e d i n g
p a r t i c l e s b y s w i t c h i n g t h e p o l a r i z a t i o n o f t h e i l l u m i n a t i o n
beam a n d p l a c i n g a b i r e f r i n g e n t c r y s t a l , e . g . c a l c i t e , i n
f r o n t o f t h e p h o t o g r a p h i c l e n s t o d i s p l a c e t h e p a r t i c l e
i m a g e b e t w e e n i l l u m i n a t i o n p u l s e s . I n t h e a r r a n g e m e n t
p r o p o s e d b y L a n d r e t h a n d A d r i a n a P o c k e l s c e l l i s u s e d t o
r o t a t e b y 90 d e g r e e s t h e p o l a r i z a t i o n o f t h e l a s e r o u t p u t
p r o v i d i n g t h e i l l u m i n a t i o n p u l s e s . T h e m e t h o d i s q u i t e
e f f e c t i v e p r o v i d e d t h a t t h e m o s t o f t h e l i g h t s c a t t e r e d b y
t h e t r a c e r s r e t a i n s i t s o r i g i n a l s t a t e o f p o l a r i z a t i o n . T h i s
h a p p e n s t o be t h e c a s e f o r t h e p a r t i c l e s a n d r e c o r d i n g
o p t i c s g e o m e t r i e s u s e d i n m o s t c u r r e n t P I V a p p l i c a t i o n s .
T h e r e a r e , h o w e v e r , some i n c o n v e n i e n c e s w h e n u s i n g t h i s
m e t h o d . T h e m a g n i t u d e a n d o r i e n t a t i o n o f t h e s h i f t c h a n g e s
i n t h e f i l m p l a n e , due t o t h e f a c t t h a t t h e l i g h t i s
c o l l e c t e d f r o m a l a r g e , f i n i t e v i e w a n g l e . A l s o , t h e s h i f t
a m o u n t f o r a g i v e n c a m e r a m a g n i f i c a t i o n c a n n o t be c h a n g e d
u n l e s s t h e c r y s t a l t h i c k n e s s i s v a r i e d o r m o r e t h a n o n e
c r y s t a l i s u s e d i n s e r i e s . I n o r d e r t o r e s o l v e t h e s e
p r o b l e m s a n o t h e r s h i f t i n g m e t h o d u s i n g t h e l i g h t
p o l a r i z a t i o n e f f e c t was p r o p o s e d b y L o u r e n c o ( 1 9 8 9 ) . T h e
b i r e f r i n g e n t c r y s t a l i s r e p l a c e d b y a p o l a r i z i n g beam
s p l i t t e r c u b e a n d two i s o l a t o r s ( 1 / 4 w a v e p l a t e ) a r r a n g e m e n t
( f i g u r e 5 a , b , c ) . The l i g h t p a t h s f o r t h e l i g h t s c a t t e r e d b y
a p a r t i c l e h a v i n g b o t h t h e s a n d p p o l a r i z a t i o n s t a t e s a r e
a l s o i l l u s t r a t e d . The p r i n c i p l e o f o p e r a t i o n o f t h e
a p p a r a t u s i s a s f o l l o w s . C o n s i d e r a s t a t i o n a r y p a r t i c l e t h a t
i s f i r s t i l l u m i n a t e d b y p - p o l a r i z e d l i g h t . T h e s c a t t e r e d
beam t r a v e l s t h r o u g h t h e c u b e u n d i s t u r b e d i n i t s p a t h a n d
c h a n g e s i t s s t a t e o f p o l a r i z a t i o n f r o m p t o s w h e n i t
p r o p a g a t e s t h r o u g h t h e 1/4 w a v e u n d e r g o e s a t o t a l r e f l e c t i o n
a n d r e v e r s e s i t s e n s e . Now h a v i n g i t s s t a t e o f p o l a r i z a t i o n
r o t a t e d b y 90 d e g r e e s , t h e l i g h t beam i s t o t a l l y r e f l e c t e d
5f o l l o w e d b y a c h a n g e i n i t s s t a t e o f p o l a r i z a t i o n w h i l e
i n t e r a c t i n g w i t h t h e 1/4 w a v e p l a t e ( f i g u r e 5 b ) . I t t h e n
p r o c e e d s t h r o u g h t h e c u b e a n d i n t o t h e c a m e r a l e n s .
P r o v i d i n g t h a t t h e l i g h t p a t h s h a v e s i m i l a r d i s t a n c e s a n d
t h a t t h e 1/4 w a v e p l a t e s a r e p a r a l l e l t o t h e c u b e ' s
s u r f a c e s , t h e i m a g e o f t h e s c a t t e r i n g s e e d w i l l a p p e a r i n
t h e f i l m p l a n e a t t h e same l o c a t i o n . A u n i f o r m d i s p l a c e m e n t
( s h i f t ) c a n b e g e n e r a t e d b y s l i g h t l y t i l t i n g o n e o f t h e 1/4
w a v e p l a t e s w i t h r e s p e c t t o t h e c u b e r ' s s u r f a c e . T h e a m o u n t
o f d i s p l a c e m e n t i s p r o p o r t i o n a l t o t h e a n g l e o f t i l t ( f i g u r e
5 c ) .
T h i s m e t h o d i s c u r r e n t l y b e i n g u s e d t o e x a m i n e b o t h
h i g h a n d l o w s p e e d f l o w s . An e x a m p l e o f i t s a p p l i c a t i o n
w h i l e s t u d y i n g t h e t r a n s i t i o n r e g i o n o f a r e c t a n g u l a r j e t i s
g i v e n b e l o w .
3. V o r t e x i n t e r a c t i o n s i n t h e t r a n s i t i o n r e g i o n o f a
r e c t a n g u l a r j e t
T h e t r a n s i t i o n r e g i o n o f a r e c t a n g u l a r j e t o f a s p e c t
r a t i o 4 a t a R e y n o l d s n u m b e r , b a s e d on t h e e q u i v a l e n t
d i a m e t e r , o f 4 5 0 0 was i n v e s t i g a t e d . A l o w s p e e d a i r s u p p l y
s y s t e m i s u s e d t o f l o w a i r i n a s e t t l i n g c h a m b e r 25 cm i n
l e n g t h a n d 7.5 cm i n d i a m e t e r . A h o n e y c o m b a n d a s e r i e s o f
s c r e e n s a t t h e i n l e t o f t h e s e t t l i n g c h a m b e r w e r e u s e d t o
r e d u c e t h e f l o w d i s t u r b a n c e s . T h e c r o s s - s e c t i o n a r e a o f t h e
n o z z l e c o n t r a c t i o n c h a n g e s g r a d u a l l y f r o m a c i r c u l a r c r o s s
-s e c t i o n , 7.5 cm i n d i a m e t e r , t o a r e c t a n g u l a r n o z z l e . T h e
l o n g ( L ) a n d s h o r t (W) d i m e n s i o n s o f t h e n o z z l e a r e 3 cm a n d
0.75 cm r e s p e c t i v e l y a n d s t r e a m w i s e c o n t o u r s o f t h e
c o n t r a c t i o n a r e f i f t h o r d e r p o l y n o m i a l s . T h e c o n t o u r e d
n o z z l e h a s a c o n t r a c t i o n r a t i o o f 1 9 . 6 : 1 o v e r a l e n g t h o f
1.6 t i m e s t h e i n l e t d i a m e t e r . I n o r d e r t o o b t a i n a p p r o p r i a t e
j e t s e e d i n g , smoke p a r t i c l e s i n t h e m i c r o n r a n g e a r e
p r o d u c e d . T h e smoke a n d t h e a m b i e n t a i r a r e m i x e d i n a l a r g e
m i x i n g a n d s e t t l i n g c h a m b e r . T h e a i r smoke m i x t u r e i s t h e n
s u p p l i e d t o t h e s e t t l i n g c h a m b e r o f t h e j e t u s i n g a s m a l l
a x i a l f a n . A s e c o n d smoke g e n e r a t o r o f t h e s a m e t y p e i s u s e d
t o s e e d t h e o u t s i d e a i r s u r r o u n d i n g t h e j e t .
T h e l a s e r s h e e t i l l u m i n a t e s t h e c e n t r a l p l a n e o f t h e
j e t t h r o u g h t h e s m a l l d i m e n s i o n o f t h e n o z z l e . Two l a s e r
p u l s e s w i t h a d u r a t i o n o f 10 n s e c a n d s e p a r a t i o n o f 50
m i c r o s e c o n d s a r e u s e d f o r t h e d o u b l e e x p o s u r e r e c o r d i n g . T h e
p a r t i c l e i m a g e d o u b l e t s a r e r e c o r d e d u s i n g t h e 35 mm c a m e r a
w i t h t h e 105 mm M a c r o l e n s . T h e c a m e r a i s i n s t a l l e d i n a n
a s s e m b l y c o n t a i n i n g t h e p o l a r i z i n g c u b e a n d 1/4 w a v e p l a t e s
t o o b t a i n a s h i f t d u r i n g t h e r e c o r d i n g . P h o t o g r a p h s o f t h e
f l o w f i e l d w e r e t a k e n a t r a n d o m t i m e s . A f t e r d e v e l o p m e n t t h e
p h o t o g r a p h s w e r e a n a l y z e d u s i n g t h e Y o u n g ' s f r i n g e m e t h o d ,
a n d t h e v e l o c i t y v e c t o r s w e r e o b t a i n e d o n a C a r t e s i a n g r i d
6T y p i c a l d o u b l e e x p o s u r e p h o t o g r a p h s o f t h e j e t , f o r two
d i f f e r e n t t i m e s , a r e s h o w n i n f i g u r e 6. T h e p i c t u r e s d i s p l a y
t h e f l o w f i e l d f r o m t h e n o z z l e e x i t t o a d o w n s t r e a m l o c a t i o n
o f 8 w i d t h s . F r o m a n u m b e r ( a b o u t 5 0 0 ) o f s i m i l a r
p h o t o g r a p h s t h e n e a r f i e l d (X/W<20, w h e r e X i s t h e
d o w n s t r e a m c o o r d i n a t e ) o f t h e j e t c a n be c h a r a c t e r i z e d b y
t h r e e d i s t i n c t i v e r e g i o n s . T h e f i r s t r e g i o n i n w h i c h t h e
i n i t i a l s h e a r l a y e r i s u n s t a b l e a n d r o l l s up i n t o d i s c r e t e
v o r t i c e s ; a n i n t e r a c t i o n r e g i o n i n w h i c h two o r m o r e
v o r t i c e s i n t e r a c t w i t h e a c h o t h e r , a n d a r e g i o n i n w h i c h
v o r t i c e s b r e a k - u p i n t o t h r e e - d i m e n s i o n a l m o t i o n .
F i g u r e 7 s h o w s a t y p i c a l i n s t a n t a n e o u s v e l o c i t y f i e l d
c o r r e s p o n d i n g t o o n e o f t h e d o u b l e e x p o s u r e p h o t o g r a p h s i n
f i g u r e 6. T h e r e s u l t s a r e s h o w n a f t e r r e m o v a l o f t h e
v e l o c i t y b i a s . U s i n g a c e n t r a l f i n i t e d i f f e r e n c e s c h e m e t h e
i n s t a n t a n e o u s v o r t i c i t y a n d s t r a i n r a t e s a r e c a l c u l a t e d a n d
shown i n f i g u r e 7. A l s o i n c l u d e d a r e t h e i n s t a n t a n e o u s
R e y n o l d s s t r e s s e s . T h e r e s u l t s r e p r e s e n t , w i t h g r e a t
f i d e l i t y , t h e a f o r e m e n t i o n e d r e g i o n s o f t h e f l o w f i e l d .
4. D i g i t a l o r " O n - l i n e " P I V
As d e m o n s t r a t e d b y t h e p r e v i o u s e x a m p l e s P I V i s a
s u p e r i o r t o o l f o r t h e d i a g n o s t i c s o f f l o w s t h a t e v o l v e b o t h
i n s p a c e a n d t i m e , a s i t i s t h e o n l y m e t h o d c a p a b l e o f
p r o v i d i n g t h e r e s e a r c h e r w i t h f l o w f i e l d s t r a i n a n d
v o r t i c i t y i n f o r m a t i o n . H o w e v e r , t h e c u r r e n t b o t t l e n e c k a n d
d i f f i c u l t y i n t h e u t i l i z a t i o n o f t h e t e c h n i q u e s t i l l r e m a i n s
t h e p h o t o g r a p h i c s t e p . T h i s d i f f i c u l t y s t e m s f r o m t h e f a c t
t h a t b e f o r e t h e d o u b l e e x p o s u r e f r a m e s c a n b e a n a l y z e d t h e r e
i s one o r two w e t p h o t o g r a p h i c p r o c e s s e s i n v o l v e d , d e p e n d i n g
o n w h e t h e r o r n o t c o n t a c t p r i n t s o f t h e o r i g i n a l n e g a t i v e s
a r e r e q u i r e d t o i m p r o v e t h e d a t a q u a l i t y . T h i s d o e s n o t
a c c o u n t f o r t h e f a c t t h a t f o r p r o p e r a n d o p t i m a l o p e r a t i o n a l
u s e o f t h e t e c h n i q u e , q u i t e a few p h o t o g r a p h i c p a r a m e t e r s
( e . g . t i m e b e t w e e n e x p o s u r e s , v e l o c i t y b i a s a m o u n t ) a n d
s e e d i n g r a t e s m u s t be a d j u s t e d b y t r i a l a n d e r r o r . I t i s ,
t h e r e f o r e , d e s i r a b l e t o d e v e l o p a n a l t e r n a t i v e a p p r o a c h t h a t
f a c i l i t a t e s t h i s s e t - u p s t a g e b y m a k i n g t h e i m a g e
a c q u i s i t i o n s t e p l e s s l a b o r i o u s a n d t i m e c o n s u m i n g .
S e v e r a l i n v e s t i g a t o r s , h a v i n g r e c o g n i z e d t h e a b o v e
m e n t i o n e d s h o r t c o m i n g s , h a v e p r o p o s e d v a r i o u s m e t h o d s o f
s o l u t i o n . O k a d a e t a l ( 1 9 9 0 ) p r o p o s e d t h e r e p l a c e m e n t o f t h e
p h o t o g r a p h i c f i l m b y a l i q u i d c r y s t a l t e l e v i s o n ( L C - T V ) . T h e
d o u b l e e x p o s e d p h o t o g r a p h s a r e r e c o r d e d u s i n g a v i d e o c a m e r a
a n d s t o r e d i n a f r a m e b u f f e r f o r l a t e r d i s p l a y on t h e L C - T V .
T h e v e l o c i t y i n f o r m a t i o n i s t h e n r e t r i e v e d u s i n g a p o i n t b y
p o i n t a n a l y s i s m e t h o d b a s e d on t h e Y o u n g ' s f r i n g e s a p p r o a c h .
T h e c u r r e n t l i m i t a t i o n i f t h i s a p p r o a c h i s t h e l i m i t e d
7t h e t h e o r y a s s o c i a t e d w i t h t h e d i g i t a l c o u n t e r p a r t o f t h e
Y o u n g ' s f r i n g e a n a l y s i s m e t h o d . I n h i s a p p r o a c h t h e d o u b l e
e x p o s e d d i g i t a l i m a g e f r a m e s w e r e o b t a i n e d b y d i g i t i z i n g
s i n g l e e x p o s e d v i d e o i m a g e s a n d t h e n a d d i n g s u c c e s s i v e
f r a m e s . T h i s m e t h o d h a s a l s o b e e n i m p l e m e n t e d b y W e s t e r w e l l
a n d N i e u w s t a d t ( 1 9 9 0 ) who u s e d s u c h a t e c h n i q u e t o s t u d y t h e
f l o w b e h i n d a c i r c u l a r c y l i n d e r . T h e m a j o r s h o r t c o m i n g o f
t h e s e m e t h o d s i s t h a t t h e y c a n o n l y be a p p l i e d t o t h e
m e a s u r e m e n t o f s l o w m o v i n g f l o w s a n d l a c k t h e r e s o l u t i o n a n d
a c c u r a c y o f t h e i r a n a l o g c o u n t e r p a r t s . T h e l i m i t a t i o n i n
v e l o c i t y r a n g e i s d u e t o t h e f a c t t h a t t h e i m a g e a c q u i s i t i o n
i s t i e d t o t h e t i m i n g o f t h e v i d e o c a m e r a s ( 2 5 o r 30 H z ) ; a n
a d d i t i o n a l t i m i n g p r o b l e m i s t h a t n o t a l l t h e p i x e l e l e m e n t s
a r e a c q u i r e d a t t h e same t i m e , d u e t o t h e v i d e o i m a g e
s c a n n i n g . A n o t h e r i m p l e m e n t a t i o n o f t h e same i d e a was p u t i n
p r a c t i c e b y W i l l e r t a n d G h a r i b ( 1 9 9 1 ) . T h e m e r i t o f t h e i r
a p p r o a c h i s t h a t i n s t e a d o f a d d i n g t o g h e t e r t h e i n d i v i d u a l
f r a m e s t o g e n e r a t e a d o u b l e e x p o s u r e i m a g e f o r p r o c e s s i n g
u s i n g a u t o c o r r e l a t i o n a l g o r i t h m s , t h e c r o s s - c o r r e l a t i o n o f
s u c c e s s i v e i m a g e s i s c a r r i e d o u t t o o b t a i n t h e d i s p l a c e m e n t .
A s s u c h , b e c a u s e t h e f r a m e s p h a s e i n f o r m a t i o n i s n o t l o s t ,
t h e t e c h n i q u e p r o v i d e s d i s p l a c e m e n t s w i t h o u t d i r e c t i o n a l
a m b i g u i t y w i t h o u t t h e n e e d t o i m p l e m e n t a n y o t h e r m e t h o d
l i k e t h e v e l o c i t y b i a s m e t h o d .
T h e e m p h a s i s on t h e c u r r e n t P I V r e s e a r c h a t t h e F l u i d
M e c h a n i c s R e s e a r c h L a b o r a t o r y a t t h e F l o r i d a S t a t e
U n i v e r s i t y i s m a i n l y i n t h e a r e o f s o - c a l l e d " o n - l i n e "
s y s t e m s . T h e s e s y s t e m s u s e n o n - s t a n d a r d v i d e o c a m e r a s
t o g h e t e r w i t h h i g h c a p a c i t y f r a m e d i g i t i z e r s a n d b u f f e r s f o r
t h e a c q u i s i t i o n a n d s t o r a g e m u l t i p l e e x p o s e d f r a m e s .
P r e s e n t c a p a b i l i t y a l l o w s t h e d i g i t i z a t i o n o f i m a g e s i n
v a r i o u s l e v e l s o f s p a t i a l r e s o l u t i o n a n d a r e a c o v e r a g e .
A v a i l a b l e s y s t e m s a r e t h e s t a n d a r d r e s o l u t i o n ( 5 1 2 x 5 1 2
p i x e l s ) , t h e h i g h r e s o l u t i o n ( 1 2 8 0 x 1 0 2 4 p i x e l s ) a n d t h e v e r y
h i g h r e s o l u t i o n ( 2 0 4 8 x 2 0 4 8 p i x e l s ) . P r o c e s s i n g o f t h e
d i g i t a l f r a m e s i s c a r r i e d o u t u s i n g a u t o a n d c r o s s
-c o r r e l a t i o n a l g o r i t h m s . R e s u l t s a r e v e r y e n -c o u r a g i n g a n d
s e e n t o i n d i c a t e t h a t a t t h e p r e s e n t t i m e some a p p l i c a t i o n s
c a n b e f u l l y met u s i n g t h e s e e a s i e r t o u s e s y s t e m s . A b r i e f
p r e s e n t a t i o n o f o n e o f s u c h s y s t e m s b a s e d on t h e s t a n d a r d
r e s o l u t i o n f o r m a t ( 5 1 2 x 5 1 2 p i x e l ) C I D c a m e r a f o l l o w s .
5.1 H a r d w a r e c o n f i g u r a t i o n
T h e m u l t i p l e e x p o s e d f r a m e s a r e a c q u i r e d b y a C I D v i d e o
c a m e r a w i t h a 5 1 2 x 5 1 2 p i x e l s e n s o r . T h e s q u a r e p i x e l s a r e 15
m i c r o n s on e a c h s i d e . U n l i k e c o m m o n l y a v a i l a b l e R S - 1 7 0
f o r m a t c o m m e r c i a l c a m e r a s w h i c h " s c a n " t h e i m a g e s a t a r a t e
o f 30 Hz t h i s c a m e r a h a s t h e c a p a b i l i t y o f g r a b b i n g
e x t r e m e l y f a s t ( s t r o b e d ) s c e n e s d u e t o t h e f a c t t h a t a l l t h e
ec a n be a c q u i r e d . T h e t i m i n g f o r i m a g e a c q u i s i t i o n a n d r e a d
-o u t c -o r r e s p -o n d s , a p p r -o x i m a t e l y , a maximum i m a g e r e f r e s h
r a t e o f 30 Hz.
T h e n o n - i n t e r l a c e d C I D c a m e r a o u t p u t c a n be v i e w e d
d i r e c t l y b y i n p u t i n g t h i s a n a l o g s i g n a l i n t o a m o n i t o r
e q u i p p e d t o h a n d l e t h e n o n - s t a n d a r d p r o g r e s s i v e s c a n f o r m a t ,
o r a s s h o w n i n f i g u r e 8, a s t h e i n p u t f o r a f r a m e
g r a b b e r / b u f f e r c o m b i n a t i o n b o a r d r e s i d e n t on a PC-AT t y p e
m i c r o c o m p u t e r . T h e f r a m e g r a b b e r w h i c h a l l o w s f o r t h e i n p u t
a n d d i g i t z a t i o n o f n o n - s t a n d a r d v i d e o f o r m a t s , d i g i t i z e s t h e
i m a g e r e t a i n i n g i t s 5 1 2 x 5 1 2 r e s o l u t i o n , s q u a r e a s p e c t r a t i o ,
a n d w i t h 8 b i t s r e s o l u t i o n o f g r a y s c a l e . T h e f r a m e g r a b b e r
b o a r d a l s o h a s t h e c a p a b i l i t y t o c o n v e r t t h e d i g i t a l f r a m e
i n f o r m a t i o n on one o f i t s memory b u f f e r s i n t o a n a n a l o g
v i d e o s i g n a l ( R S - 1 7 0 f o r m a t ) w h i c h i s d i s p l a y e d i n a
c o n v e n t i o n a l RGB v i d e o m o n i t o r . T h e s y s t e m s y n c h r o n i z a t i o n
i s a c h e i v e d b y m e a n s o f an e s p e c i a l l y d e v e l o p e d i n t e r f a c e
w h i c h e n s u r e s t h a t t h e c a m e r a s t a y s i n t h e a c q u i s i t i o n mode
d u r i n g t h e t i m e t h e l a s e r i l l u m i n a t i o n p u l s e s a r e f i r e d , a n d
i n t h e r e a d - o u t mode d u r i n g t h e i m a g e a c q u i s i t i o n p h a s e b y
t h e f r a m e g r a b b e r . A g a i n w i t h t h i s a r r a n g e m e n t a maximum
n u m b e r o f 30 f r a m e s p e r s e c o n d c a n be d i g i t i z e d a n d s t o r e d .
T h e p r e s e n t o n - b o a r d memory o f t h e f r a m e b u f f e r a l l o w s f o r
t h e s t o r a g e o f a maximum o f 16 i m a g e s . H o w e v e r , i m a g e s c a n
b e t r a n s f e r r e d t o t h e PC-AT h a r d d i s k f o r p e r m a n e n t s t o r a g e
a n d / o r l a t e r p r o c e s s i n g . A n o t h e r m e t h o d a v a i l a b l e t o a c h e i v e
l o n g r e c o r d l e n g t h s o f f l o w e v e n t s i s t o r e c o r d on a h i g h
q u a l i t y s u p e r VHS t y p e t a p e t h e a n a l o g v i d e o o u t p u t o f t h e
f r a m e g r a b b e r , f o r l a t e r p l a y b a c k a n d r e - d i g i t i z a t i o n . T h e
i n c o v i n i e n c e o f t h i s a p p r o a c h i s t h a t some i m a g e d e g r a d a t i o n
w i l l o c c u r .
U s i n g t h i s a r r a n g e m e n t a v a r i e t y o f l a s e r s o u r c e s c a n
b e u s e d w i t h o u t r e s t r i c t i o n . I n t h e a p p l i c a t i o n d e s c r i b e d
h e r e i n a CW l a s e r w i t h a B r a g g c e l l a s t h e s h u t t e r i s u s e d .
T h e p r o c e s s i n g o f t h e m u l t i p l e e x p o s e d f r a m e s i s
p e r f o r m e d u s i n g t h e PC-AT m i c r o c o m p u t e r a n d a n a r r a y
p r o c e s s o r i n o r d e r t o a c h e i v e c o m p u t a t i o n a l e f f i c i e n c y a n d
s p e e d .
5.2 P r o c e s s i n g o f t h e d i g i t a l f r a m e s a n d t y p i c a l r e s u l t s
U s i n g t h e h a r d w a r e d e s c r i b e d i n t h e p r e v i o u s s e c t i o n a
w h o l e f i e l d v e l o c i t y m e a s u r e m e n t o f t h e f l o w p a s t a c i r c u l a r
c y l i n d e r t o w e d i n a w a t e r t a n k h a s b e e n c a r r i e d o u t . T h e
c y l i n d e r i s 25.4 mm i n d i a m e t e r a n d i s t o w e d w i t h a v e l o c i t y
o f 2.5 c m / s e c c o r r e s p o n d i n g t o a f l o w R e y n o l d s n u m b e r o f
a p p r o x i m a t e l l y 6 0 0 . T h e f l o w i s s e e d e d w i t h 4 m i c r o n i n
d i a m e t e r m e t a l l i c c o a t e d p a r t i c l e s a n d t h e F# n u m b e r f o r t h e
r e c o r d i n g i s 16. C o n s e q u e n t l y t h e p a r t i c l e i m a g e s i z e i s
m o s t l y d o m i n a t e d b y d i f f r a c t i o n e f f e c t s a n d i s e v a l u t e d t o
9. 0 7 . I n o r d e r t o r e s o l v e t h e a m b i g u i t y o f t h e v e l o c i t y
v e c t o r a s c a n n i n g m i r r o r m e t h o d was i m p l e m e n t e d i n t h e
r e c o r d i n g o f t h e f r a m e s .
F i g u r e 9 i s a t y p i c a l e x a m p l e o f a v e l o c i t y f i e l d
o b t a i n e d a f t e r p r o c e s s i n g o f t h e t r i p l y e x p o s e d f r a m e s , a n d
r e m o v a l o f t h e v e l o c i t y b i a s . T h i s v e l o c i t y map c o r r e s p o n d s
t o t h e f l o w a f t e r t h e c y l i n d e r h a s b e e n i m p u l s i v e l e y s t a r t e d
f r o m r e s t a n d moved 3 d i a m e t e r s . A n a l y s i s o f t h i s f i g u r e
r e v e a l s t h a t w i t h t h i s t e c h n i q u e i t i s p o s s i b l e t o map t h e
v e l o c i t y f i e l d w i t h r e a s o n a b l e d e t a i l a n d a c c u r a c y .
P o i n t b y p o i n t p r o c e s s i n g o f t h e m u l t i p l e e x p o s e d
f r a m e s i s a c c o m p l i s h e d u s i n g a n a u t o c o r r e l a t i o n a l g o r i t h m .
T h e f r a m e s a r e d i v i d e d i n t o i n t e r r o g a t i o n c e l l s 1 6 , o r
m u l t i p l e s o f 16, i n t e r r o g a t i o n c e l l s i n b o t h t h e v e r t i c a l
a n d h o r i z o n t a l a x i s . T h e a s p e c t r a t i o o f t h i s p i x e l s n e e d
n o t b e s q u a r e a n d may b e c h a n g e d t o a c c o m o d a t e f l o w s w i t h a
d o m i n a n t v e l o c i t y c o m p o n e n t . T h e d i g i t a l a u t o c o r r e l a t i o n i n
t w o d i m e n s i o n s o f t h e p a r t i c l e i m a g e d o u b l e t s i s p e r f o r m e d
u s i n g a F F T a l g o r i t h m . The a l g o r i t h m i s i m p l e m e n t e d u s i n g
t h e a r r a y p r o c e s s o r a n d a l l o w s f o r t h e e n t i r e p r o c e s s i n g o f
a f u l l f r a m e i m a g e c o n t a i n i n g a b o u t 1000 v e c t o r s i n j u s t 4
m i n u t e s . F i g u r e 10 i s a u t o c o r r e l a t i o n map f r o m o n e o f s u c h
i n t e r r o g a t i o n c e l l . T h e o f f - c e n t e r p e e k l o c a t i o n o f t h i s
a u t o c o r r e l t i o n a r r a y c o r r e s p o n d s t o t h e a v e r a g e d i s p l a c e m e n t
o f t h e t r a c e r i m a g e s c o n t a i n e d i n t h e i n t e r r o g a t i o n c e l l . An
i n t e r p o l a t i o n a l g o r i t h m i s i m p l e m e n t e d i n o r d e r t o a c h e i v e
s u b - p i x e l r e s o l u t i o n f o r t h e e v a l u a t i o n o f t h e p e e k
l o c a t i o n . D i s c r i m i n a t i o n b e t w e e n a v a l i d m e a s u r e m e n t a n d
s p u r i o u s d a t a ( e . g . , c y l i n d e r w a l l , d r o p - o u t r e g i o n s w i t h o u t
s e e d i n g ) i s p e r f o r m e d b y c o m p a r i n g t h e a m p l i t u d e s o f t h e
t h r e e m o s t i m p o r t a n t p e e k s i n t h e c o r r e l a t i o n a r r a y ,
i n c l u d i n g t h e z e r o o r d e r p e e k .
T h e a c c u r a c y o f t h e t e c h n i q u e was e v a l u a t e d f i r s t b y
m e a n s o f a s i m p l e e x p e r i m e n t . A u n i f o r m f l o w w a s g e n e r a t e d
t o w i n g t h e c a r r i a g e o f t h e t o w i n g t a n k w i t h c o n s t a n t
v e l o c i t y . T h i s f l o w was r e c o r d e d u s i n g t h e " O n - l i n e " P I V
a r r a n g e m e n t , w h i l e c h a n g i n g t h e t i m e b e t w e e n e x p o s u r e s a n d
t h e n u m b e r o f e x p o s u r e s a c c o r d i n g t o t h e s c h e d u l e p r e s e n t e d
i n t a b l e 1. A l s o i n c l u d e d i n t h e t a b l e a r e t h e r e s u l t s o f
t h e a v e r a g e d i s p l a c e m e n t m e a s u r e d i n p i x e l s a n d t h e
c o r r e s p o n d i n g r m s . T h e a v e r a g e d i s p l a c e m e n t a n d rms w e r e
d e t e r m i n e d a v e r a g i n g t h e r e s u l t s o f a b o u t 1 0 0 0
i n t e r r o g a t i o n s p e r f r a m e . A m e a s u r e o f t h e t e c h n i q u e s '
a c c u r a c y i s o b t a i n e d c o m p a r i n g t h e v a r i a t i o n o f c a r r i a g e
v e l o c i t y m e a s u r e d a s t h e r a t i o o f p i x e l d i s p l a c e m e n t d i v i d e d
b y t h e t i m e b e t w e e n e x p o s u r e s . T h i s m e a s u r e m e n t s a r e w i t h i n
1% t o 1.5% o f e a c h o t h e r . A n o t h e r m e a s u r e o f t h e t e c h n i q u e ' s
10d i s c u s s e d i n a f o r t h c o m i n g p u b l i c a t i o n .
6. C o n c l u s i o n s
W h o l e f i e l d m e a s u r e m e n t t e c h n i q u e s , i n p a r t i c u l a r P I V ,
h a v e d e m o n s t r a t e d t h e a b i l i t y t o r e c o r d t h e i n s t a n t a n e o u s
v e l o c i t y f i e l d w i t h a c c u r a c y c o m p a r a b l e t o t h a t o f LDV w h i l e
r e t a i n i n g i t s n o n - i n v a s i v e n a t u r e . T h e d e v e l o p m e n t o f
s p e c i a l r e c o r d i n g t e c h n i q u e s , s u c h a s t h e v e l o c i t y b i a s make
i t p o s s i b l e t o m e a s u r e c o m p l e x f l o w s a n d w i t h i n w i d e r a n g e s
i n v e l o c i t y . T h e m o s t n o t e w o r t h y d e v e l o p m e n t i s , i n t h e
a u t h o r s ' o p i n i o n , t h e i n t r o d u c t i o n o f v i d e o t e c h n i q u e s a s
m e a n s o f n o t o n l y e l i m i n a t i n g t h e w e t p h o t o g r a p h i c
p r o c e s s i n g s t a g e , b u t a l s o t o p r o v i d e t h e o p e r a t o r w i t h a n
a l m o s t r e a l - t i m e c a p a b i l i t y o f d a t a a c c e s s . Due t o t h e
i n t r o d u c t i o n o f h i g h e r r e s o l u t i o n c a m e r a s , e . g . 2 0 4 8 x 2 0 4 8
p i x e l f o r m a t , t h e s e s y s t e m s a r e u n d e r g o i n g r a p i d
d e v e l o p m e n t , a n d a t t h e p r e s e n t s t a g e c a n r e p l a c e i n q u i t e a
f e w a p p l i c a t i o n s t h e 35 mm f o r m a t c a m e r a w i t h o u t l o s s o f
a c c u r a c y a n d s p a t i a l r e s o l u t i o n .
R e f e r e n c e s
A d r i a n , R . J .
A p p l i e d O p t i c s , v o l 2 3 , 1984
C h o , Y. C .
A p p l i e d O p t i c s , v o l 2 8 , 1 9 8 9
E l k i n s , R . E . e t a l
R e v . S c i . I n s t r . , v o l 4 8 , 1977
G h a r i b , M e t a l
A I A A p a p e r 8 5 - 0 1 7 2
K o b a y a s h i , T. e t a l
P r o c . I n t . Symp. F l o w V i s . , Ann H a r b o r , M i c h i g a n , 1 9 8 3
L a n d r e t h , C. C. a n d A d r i a n R . J .
A p p l i e d O p t i c s , v o l 2 7 , 1988
L o u r e n c o , L .
B u l l e t i n o f A P S , 1 9 8 9
M e y n a r t , R. a n d L o u r e n c o , L .
V K I L e c t u r e S e r i e s , 1984
i iP i c k e r i n g , C. J . a n d H a l l i w e l l , N. A.
A p p l i e d O p t i e s , v o l 2 3 , 1984
U t a m i , T. a n d U e n o , T.
E x p . i n F l u i d s , v o l 2, 1984
W e s t e r w e l l , J . a n d N i e u w s t a d t , F . T .
P r o c . App. L a s e r V e l . F l u i d Mech,, L i s b o n , P o r t u g a l , 1 9 9 0
W i l l e r t , C. E . a n d G h a r i b , M.
E x p . i n F l u i d s , v o l 10, 1991
Y a o , C. S. a n d A d r i a n R. J .
A p p l i e d O p t i e s , v o l 2 3 , 1984
T a b l e 1 : T i m e b e t w e e n e x p o s u r e s v s d i s p l a c e m e n t
T i m e ( m s e c )
Number o f
A v e r a g e
Rms
e x p o s u r e s
d i s p l ( p i x e l s )
d i s p l ( p i x e l s )
10
2
3.21
.089
3
3.36
.078
4
3.24
. 0 7 5
15
2
5.02
.095
3
5. 06
. 074
4
5 . 07
.064
20
2
6 . 99
. 120
3
7.01
.061
4
7.05
.079
30
2
1 0 . 7 7
. 139
3
1 0 . 8 7
. 1 3 3
4
1 0 . 7 6
. 170
40
2
1 2 . 3 2
. 146
3
12 .56
. 127
4
12 .29
. 137
12F i g u r e 1: S c h e m a t i c a r r a n g e m e n t f o r t h e d u a l pulse laser s y s t e m
Monitor
Vax Station II
(c) L U_/C=4.70. a=30.0 deg. ( f ) t U/C=8.56. ct= 30.0 deg.
I P - Polarized • S - Polarized
Imace Plant Toially Reneciive Surface
Panicle J
^ Ami Reflection Surface
I Imace Plane Toially Refleciivc Surface
F i g u r e 5: Velocity Bias: b e a m s p l i t t e r / i s o l a t o r s a r r a n g e m e n t a) p-polarized l i g h t p a t h
b) s-polarized l i g h t p a t h c) o p e r a t i o n f o r velocity bias
