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Measurement of hypersonic velocities and turbulence by direct spectral analysis of doppler shifted laser light
Volume 32A, number 2
BY
PHYSICS LETTERS
MEASUREMENT OF DIRECT SPECTRAL
15 June 1970
HYPERSONIC VELOCITIES ANALYSIS OF DOPPLER
AND TURBULENCE SHIFTED LASER LIGHT
D. A. JACKSON and D. M. PAUL
Physics Laboratory, University of Kent, Canterbury, Kent, U.K. Received 10 April 1970
A non-contact anemometer based on a single frequency argon laser and spherical Fabry-Perot has been used to measure hypersonic velocities and turbulence widths in large scale wind tunnels. The sysis capable of measuring directional velocities from 1 to 106 m/sec.
Velocity i n v e s t i g a t i o n s at s u p e r s o n i c speed a r e at p r e s e n t p e r f o r m e d using e i t h e r pitot tubes which a r e l i m i t e d to m e a n v e l o c i t y m e a s u r e m e n t , or d e l i c a t e hot w i r e p r o b e s which give both m e a n v e l o c i t y and v e l o c i t y fluctuations. The i n s e r t i o n of such p r o b e s i n e v i t a b l y m o d i f i e s the flow p a t t e r n . We r e p o r t h e r e p r e l i m i n a r y m e a s u r e m e n t s using a n o n - c o n t a c t l a s e r v e l o c i t y p r o b e capable of m e a s u r i n g v e l o c i t i e s f r o m 1 to 2 × 106 m / s e c , in which the v e l o c i t y s p e c t r u m i s m e a s u r e d by d i r e c t s p e c t r a l a n a l y s i s of the f r e q u e n c y shift of light s c a t t e r e d f r o m a i r b o r n e particles. Th e output b e a m light f r o m a s i n g l e f r e q u e n c y a r g o n l a s e r [1, 2] was f o c u s s e d into the r e g i o n to be i n v e s t i g a t e d . A r e f e r e n c e b e a m which s e r v e d as a z e r o v e l o c i t y m a r k e r ( d e r i v e d f r o m the i n cident signal beam) was f o c u s s e d at the s c a t t e r ing v o l u m e , the an g le b e t w e e n the b e a m s d e f i n ing the s c a t t e r i n g angle. Light s c a t t e r e d f r o m p a r t i c l e s in the f o ca l r e g i o n was c o l l e c t e d by a s e c o n d l e n s and f o c u s s e d into a confocal p i e z o e l e c t r i c a l l y scanned F a b r y - P e r o t [3], the a x i s of the F a b r y - P e r o t coinciding with the r e f e r e n c e b e a m (fig. 1). The C . F . P . was s c a n n e d using the sawtooth r a m p of an o s c i l l o s c o p e and the t r a n s m i t t e d light was d e t e c t e d by a p h o t o m u l t i p l i e r the output of which d r o v e the v e r t i c a l a x i s of the o s c i l l o s c o p e . T h e s p at i al r e s o l u t i o n 1 m m by 80 bt was d e fined by the a c c e p t a n c e angle of the a n a l y z i n g o p t i c s and the s i z e of the d i f f r a c t i o n l i m i t e d c y l i n d e r p r o d u c e d by f o c u s s i n g the incident b e a m . The f r e e s p e c t r a l r a n g e of the F a b r y P e r o t was 2 GHz with a r e s o l u t i o n of 7.5 MHz. Ini t i al e x p e r i m e n t s have been c a r r i e d out at the 20 c m 2 × 23 c m 2 wind tunnel at the Royal A i r c r a f t E s t a b l i s h m e n t , Bedford. T h e tunnel
RGON LASER
. . . . . .
e
U
--~
FABRY . ~ / _ _ . , \ PEROT,/~'~'~VAR, A B L E APERTURE
MULTIPLI
Fig. 1. Optical arrangement showing the system used to match the reference beam and the signal beam into the analysing optics. The symmetric scattering geometry shown selects only the velocity component U, with frequency shift Au = (2 U/A) sin i was o p e r a t e d at h i g h e r than n o r m a l humidity so that i c e p a r t i c l e s , f o r m e d d u r i n g expansion in the t h r o a t , act ed a s s c a t t e r i n g c e n t r e s . Th e v e l o c i t y along the c e n t r e of the wind tunnel has been m e a s u r e d at s e v e r a l Mach n u m b e r s (dictated by the l i n e r s a v a i l a b l e ) and is corn77
Volume 32A, n u m b e r 2 u¢b
PHYSICS
LETTERS
15 June 1970
600
E
o/
o/
~A
~450
0
300
u,J n
,~ 150
/i
150 PITOT TUBE
Fig. 3. The upper t r a c e shows the zero velocity r e f e r ence m a r k e r . The lower t r a c e is the s c a t t e r e d light s p e c t r u m with a Doppler shift in frequency, D.S. of 700 MHz c o r r e s p o n d i n g to a speed of 456 m / s e c . L
300 450 metre/see
Fig. 2. o - C o m p a r i s o n ot a i r speed m e a s u r e d directly by the F a b r y - P e r o t and indirectly by the Pitot tube f r o m a knowledge of the tunnel operating conditions. [ ] - C o m p a r i s o n of the tangential velocity of a high speed rotating disc with the F a b r y - P e r o t m e a s urement. pared with the pitot tube measurements taken at t h e s a m e t i m e (fig. 2). T h e low v e l o c i t y c a l i bration point was obtained from light back scattered from a high speed disc. A t y p i c a l s p e c t r u m i s s h o w n in fig. 3, t h e x axis defines an absolute velocity scale requiring no calibration under operating conditions as is the case with the pitot tube or hot wireprobes. T h e a c c u r a c y of t h e m e a s u r e m e n t s i s b e t t e r t h a n 0.5%. It i s c l e a r f r o m fig. 3 t h a t t h e r e i s l i t t l e t u r b u l e n c e a l o n g t h e c e n t r e l i n e of t h e p a r t i c u l a r wind tunnel investigated. E x p e r i m e n t s i n b o u n d a r y l a y e r s (to b e p u b l i s h e d ) i n d i c a t e t h a t t h e c h a r a c t e r of t h e d o p p l e r
78
s h i f t e d p e a k c h a n g e s in r e g i o n s of t u r b u l e n c e . Similar turbulent broadening has been observed in t h e e f f l u x of a j e t e n g i n e [4]. T h e o b s e r v e d f r e q u e n c y s h i f t s a r e s e e n to b e - 3 0 0 MHz o r g r e a t e r , w h i c h i s b e y o n d t h e l i m i t of c o n v e n t i o n a l l a s e r h e t e r o d y n e t e c h n i q u e s . T h e Fabry-Perot has the further advantage that the actual velocity direction is unambiguously measured and also the system is not limited by the s h o t n o i s e of t h e l o c a l o s c i l l a t o r a s in t h e heterodyne technique. The authors gladly acknowledge the valuable a s s i s t a n c e of t h e s t a f f of R. A. E . , B e d f o r d , a n d of h e l p f u l d i s c u s s i o n s w i t h P. B o u r k e of A . E . R . E
References [1] M. H e r c h e r , Appl. Optics 8 (1969) 1103. [2] D. A. Jackson and D. M. Paul, J. Sci. Inst. 2 (1969) 1077. [3] M. H e r c h e r , Appl. Optics 7 (1968) 951. [4] R. N. J a m e s , W. R. Babcock and H. R. Siefert, A . I . A . A . J . 6 (1968) 160.
BY
PHYSICS LETTERS
MEASUREMENT OF DIRECT SPECTRAL
15 June 1970
HYPERSONIC VELOCITIES ANALYSIS OF DOPPLER
AND TURBULENCE SHIFTED LASER LIGHT
D. A. JACKSON and D. M. PAUL
Physics Laboratory, University of Kent, Canterbury, Kent, U.K. Received 10 April 1970
A non-contact anemometer based on a single frequency argon laser and spherical Fabry-Perot has been used to measure hypersonic velocities and turbulence widths in large scale wind tunnels. The sysis capable of measuring directional velocities from 1 to 106 m/sec.
Velocity i n v e s t i g a t i o n s at s u p e r s o n i c speed a r e at p r e s e n t p e r f o r m e d using e i t h e r pitot tubes which a r e l i m i t e d to m e a n v e l o c i t y m e a s u r e m e n t , or d e l i c a t e hot w i r e p r o b e s which give both m e a n v e l o c i t y and v e l o c i t y fluctuations. The i n s e r t i o n of such p r o b e s i n e v i t a b l y m o d i f i e s the flow p a t t e r n . We r e p o r t h e r e p r e l i m i n a r y m e a s u r e m e n t s using a n o n - c o n t a c t l a s e r v e l o c i t y p r o b e capable of m e a s u r i n g v e l o c i t i e s f r o m 1 to 2 × 106 m / s e c , in which the v e l o c i t y s p e c t r u m i s m e a s u r e d by d i r e c t s p e c t r a l a n a l y s i s of the f r e q u e n c y shift of light s c a t t e r e d f r o m a i r b o r n e particles. Th e output b e a m light f r o m a s i n g l e f r e q u e n c y a r g o n l a s e r [1, 2] was f o c u s s e d into the r e g i o n to be i n v e s t i g a t e d . A r e f e r e n c e b e a m which s e r v e d as a z e r o v e l o c i t y m a r k e r ( d e r i v e d f r o m the i n cident signal beam) was f o c u s s e d at the s c a t t e r ing v o l u m e , the an g le b e t w e e n the b e a m s d e f i n ing the s c a t t e r i n g angle. Light s c a t t e r e d f r o m p a r t i c l e s in the f o ca l r e g i o n was c o l l e c t e d by a s e c o n d l e n s and f o c u s s e d into a confocal p i e z o e l e c t r i c a l l y scanned F a b r y - P e r o t [3], the a x i s of the F a b r y - P e r o t coinciding with the r e f e r e n c e b e a m (fig. 1). The C . F . P . was s c a n n e d using the sawtooth r a m p of an o s c i l l o s c o p e and the t r a n s m i t t e d light was d e t e c t e d by a p h o t o m u l t i p l i e r the output of which d r o v e the v e r t i c a l a x i s of the o s c i l l o s c o p e . T h e s p at i al r e s o l u t i o n 1 m m by 80 bt was d e fined by the a c c e p t a n c e angle of the a n a l y z i n g o p t i c s and the s i z e of the d i f f r a c t i o n l i m i t e d c y l i n d e r p r o d u c e d by f o c u s s i n g the incident b e a m . The f r e e s p e c t r a l r a n g e of the F a b r y P e r o t was 2 GHz with a r e s o l u t i o n of 7.5 MHz. Ini t i al e x p e r i m e n t s have been c a r r i e d out at the 20 c m 2 × 23 c m 2 wind tunnel at the Royal A i r c r a f t E s t a b l i s h m e n t , Bedford. T h e tunnel
RGON LASER
. . . . . .
e
U
--~
FABRY . ~ / _ _ . , \ PEROT,/~'~'~VAR, A B L E APERTURE
MULTIPLI
Fig. 1. Optical arrangement showing the system used to match the reference beam and the signal beam into the analysing optics. The symmetric scattering geometry shown selects only the velocity component U, with frequency shift Au = (2 U/A) sin i was o p e r a t e d at h i g h e r than n o r m a l humidity so that i c e p a r t i c l e s , f o r m e d d u r i n g expansion in the t h r o a t , act ed a s s c a t t e r i n g c e n t r e s . Th e v e l o c i t y along the c e n t r e of the wind tunnel has been m e a s u r e d at s e v e r a l Mach n u m b e r s (dictated by the l i n e r s a v a i l a b l e ) and is corn77
Volume 32A, n u m b e r 2 u¢b
PHYSICS
LETTERS
15 June 1970
600
E
o/
o/
~A
~450
0
300
u,J n
,~ 150
/i
150 PITOT TUBE
Fig. 3. The upper t r a c e shows the zero velocity r e f e r ence m a r k e r . The lower t r a c e is the s c a t t e r e d light s p e c t r u m with a Doppler shift in frequency, D.S. of 700 MHz c o r r e s p o n d i n g to a speed of 456 m / s e c . L
300 450 metre/see
Fig. 2. o - C o m p a r i s o n ot a i r speed m e a s u r e d directly by the F a b r y - P e r o t and indirectly by the Pitot tube f r o m a knowledge of the tunnel operating conditions. [ ] - C o m p a r i s o n of the tangential velocity of a high speed rotating disc with the F a b r y - P e r o t m e a s urement. pared with the pitot tube measurements taken at t h e s a m e t i m e (fig. 2). T h e low v e l o c i t y c a l i bration point was obtained from light back scattered from a high speed disc. A t y p i c a l s p e c t r u m i s s h o w n in fig. 3, t h e x axis defines an absolute velocity scale requiring no calibration under operating conditions as is the case with the pitot tube or hot wireprobes. T h e a c c u r a c y of t h e m e a s u r e m e n t s i s b e t t e r t h a n 0.5%. It i s c l e a r f r o m fig. 3 t h a t t h e r e i s l i t t l e t u r b u l e n c e a l o n g t h e c e n t r e l i n e of t h e p a r t i c u l a r wind tunnel investigated. E x p e r i m e n t s i n b o u n d a r y l a y e r s (to b e p u b l i s h e d ) i n d i c a t e t h a t t h e c h a r a c t e r of t h e d o p p l e r
78
s h i f t e d p e a k c h a n g e s in r e g i o n s of t u r b u l e n c e . Similar turbulent broadening has been observed in t h e e f f l u x of a j e t e n g i n e [4]. T h e o b s e r v e d f r e q u e n c y s h i f t s a r e s e e n to b e - 3 0 0 MHz o r g r e a t e r , w h i c h i s b e y o n d t h e l i m i t of c o n v e n t i o n a l l a s e r h e t e r o d y n e t e c h n i q u e s . T h e Fabry-Perot has the further advantage that the actual velocity direction is unambiguously measured and also the system is not limited by the s h o t n o i s e of t h e l o c a l o s c i l l a t o r a s in t h e heterodyne technique. The authors gladly acknowledge the valuable a s s i s t a n c e of t h e s t a f f of R. A. E . , B e d f o r d , a n d of h e l p f u l d i s c u s s i o n s w i t h P. B o u r k e of A . E . R . E
References [1] M. H e r c h e r , Appl. Optics 8 (1969) 1103. [2] D. A. Jackson and D. M. Paul, J. Sci. Inst. 2 (1969) 1077. [3] M. H e r c h e r , Appl. Optics 7 (1968) 951. [4] R. N. J a m e s , W. R. Babcock and H. R. Siefert, A . I . A . A . J . 6 (1968) 160.