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AirSpeed Calibration Facility by using
LDV and A Wind Tunnel at CMS
Cheng-Tsair Yang
Center for Measurement Standards
Industrial Technology Research Institute/Taiwan
Content
Selection of Wind Tunnel
Calibration of A LDV Measuring Probe
Implementation and Characteristics of A
Wind Tunnel for Air-Speed Standard
Estimation of Uncertainty for
Anemometry Calibration
Type of Wind Tunnel
Closed Circuit Wind Tunnel (CCWT)
 directly re-circulate
(low Power).
 Noise is significantly lower.
Open Circuit Wind Tunnels (OCWT)
 Relatively small footprint
 Low construction cost
Concerns of a wind tunnel
Diffuser:
Fan
drive/Blower:
provide
a pressure increase
flow, to
to reach
overcome
Contraction
to accelerate
the flowof
speed
the the
Test
section: Nozzle:
to
lower
the
flow
speed,
consequently
reduce
theanemometer
pressure
loss
pressure
lossair
inlevel
the tunnel
Steady
driving
force
is mostly
desirable
in
the circuit.
test
the
turbulence
provide
desirable
flow
condition
andsection,
space
forreduce
calibration
required.
due
to friction
intensity.
Required flow fields in Wind tunnel test
sections
A uniform-flow, steady and low turbulence
Restriction due to blockage effects


Size effect that induce flow disturbance.
Hard to estimate the uncertainty
(a) ideal uniform flow
(b) symmetric flow resulting fromwall friction
Timeline of air speed standard
at CMS
’98-’99: Small wind tunnel + a TSI LDV
2000: LDV Probe adjustment & calibration*
Traceability of air-speed measurement
Anemometry calibration Service
2003: Design a new wind tunnel with
expanded LDV probe
2004: Implementation of new system
*Optics and Lasers in Engineering, 38, 2002
Measurement Facilities-1
Wind tunnel :
1. Open-loop with expanded test section
2. Outlet of contraction nozzle: 200 mm
3. Motor controlled by a frequency converter
4. Designed air speed: ~30 m/s
5. Flow patterns simulated at test section
CFD simulation vs.LDV measurement
Suction section
Outlet of contraction nozzle
Air
LDV measurement velocity at 10 mm downstream
LDV measurement velocity at 100 mm
Measurement Facilities-2
LDV :
1. TSI Probe 9832 with beam expander
2. Focal length: 450 mm
3. Nominal fringe spacing: 1.921 m for
beam of 514.5 nm, 1.822 m for 488nm
Measurement Traceability of
Anemometry Calibration
Length Standard
Vernier Caliper
Frequency Standard
Universal Counter
Spinning Disc
Check
Anemometry
Laser Doppler Anemometry
+ Wind tunnel
Anemometry under Test
(Time)
Spinning disc traceability
Disc
Pedestal
-Disc eccentricity
adjustable
-Horizontal disc
-rotary encoder for
rotational speed
- Local fringe
spacing detectable
Obtain of Fringe Spacing
Basics of calibration
Time
Particle on
moving surface
X
Rear
region
Spinning Disc
Z
Front
region
β
Vldv= df  fD
Vdisc= r   cos
df = r ×  / fD× cos
 <1 degree
Obtain local fringe spacing
Obtain of Fringe Spacing
Comparison between (a) favorable and (b) improper Doppler bursts. Both the sequences
show the change of burst signals when slightly moves the traverse stage.
Calibration of LDV Probe-1
Why is adjustment and calibration of LDV probe necessary?
Measurement
Volume
df
 
(( x1 z1 z12  z R21 )  ( x2 z 2 z 22  z R2 2 )) 
df 
1

  2 tan  ( x1 z1 z12  z R21 )  ( x2 z 2 z 22  z R2 2 )) 
2 sin
2
*
*Miles, P. C., ”Geometry of the Fringe Field Formed in the Intersection of
Two Gaussian Beams,” Applied Optics1996;35:5887–5895.
Calibration of LDV Probe-2
Step 1. Steer beams to optimize beam crossing
Z=0
Z=1850
m
Z=1000
m
Change state of beam crossing
Minimal variation of fringe spacing
in measuring volume
Calibration of LDV Probe-3
Step 2. Determination of coefficient of air speed
(coefficient of speed: mean fringe spacing)
1.Repeat measurement of fringe spacing at different rotation rates
2.Determined mean fringe spacing: 1.9225 m with U= 0.1%.
(Nominal value: 1.921)
Calibration of LDV Probe-4
Step 3. Evaluation of uncertainty of air-speed coefficient
df = r ×  / fD× cos + ε
Sources of uncertainty
Type
xi
LDV output frequency
fD (MHz)
A
3.2896
0.00066
B
A
B
B
0.09997
63.2498
1
0
0.00001
0.00302
0.00504
0.00070
Diameter of disc r (m)
Rotation speed ω (rad /sec)
Incident angle β (degree)
Correction factor ε (μm)
Combined standard
Uncertainty (μm)
Mean fringe spacing (μm)
Relative standard
uncertainty (%)
Coverage factor
Relative expanded
uncertainty (%)
Standard
Coeff. of
uncertainty (xi) sensitivity c(xi)
u(xi)
×c(xi)
Degrees of
freedomνx
0.58424
0.00038
4
19.225
0.03039
0.03355
1
0.00019
0.00009
0.00017
0.0007
32805
4
∞
5
0.00084
9
1.92280
0.044%
9
2.26
0.10%
Available region for installation
of anemometry-probe
Potential-core Region is considered for locating
anemometry probe and was examined of its
flow characterestics.
Air flow
d
D
X
X = 100 mm
D = 200 mm
d = 80 mm (region I)
140 mm (region II)
: region of interest
: anemometry probe
Characteristics of Wind Tunnel
Considered uncertainty sources of air speed measured
by LDV in wind tunnel:
1. Characteristics of flow*
A. Particle Lag
B. Turbulence
C. Velocity Bias (Sampling bias)
D. Fringe Bias
2. Characteristics of wind tunnel
A. steadiness & stability
B. uniformity of velocity profiles
C. variations of axial velocity
*Ref. Fry, DTNSRDC, 1985
Uncertainty analysis of air speed
in interested region
Vtunnel  Vldv    
Vtunnel : Air speed in wind tunnel
Vldv : Measured air speed by LDV
δ : correction factor due to flow characteristics
ε : correction factor due to tunnel characteristics
u c 2 (Vtunnel )  c1u (Vldv )2  c 2 u ( )2  c3 u ( )2
u(Vtunnel )
u(V )
u( ) 2 u( ) 2 1 / 2
 (( ldv ) 2  (
) (
) )
Vtunnel
Vldv


Uncertainty due to Flow characteristics
in interested region
1. Turbulence intensity
Tu = 1.73 % - 0.81 %
for V=0.5 m/s – 25 m/s
u  Tumax / N 100%  0.055%
Measured at 100 mm downstream centerline of nozzle
2. Velocity bias (sampling effect)
by comparing weighted (residence-time) with un-weighted velocities
(Fry, 1985):
u
Vw  Vuw
4 V
, estimated to be < 0.01%
Effects due to velocity distribution
in interested region
1. Flow profiles along vertical radius (Z-axis flow uniformity)
r (mm)
V1 (m/s)
V2
V3
V4
For region I： r = (-40 ~ 40) mm, X = (0~100)mm
Standard Uncertainty = 0.14 %
Mean
Std. Dev. (m/s)
Std. Dev. (m/s)
Rel. Std. Dev. %
Rel. Std. Dev. %
For region II： r = (-70 ~ 70) mm, X = (0~100)mm
Standard Uncertainty = 0.22 %
Effects due to velocity distribution
in the interested region
2. Flow profiles along horizontal radius (Y-axis flow uniformity,
the same process as 1.)
3. Variation of velocities along flow direction (Xaxis flow uniformity)
Std. dev. = 0.129%
(obtained from repeat measurement at different air speed)
Uncertainty analysis
item
[
[
[
Sources
u (Vldv )
]
Vldv
u ( )

u( )

]
U (Vtunnel )
]
Vtunnel
νx
9.5
Flow Property
Particle lag
Velocity bias
0.0557
0
0.01
1066.7
3
4
Turbulence int.
Fringe bias
Wind-tunnel
Property
1
Velocity Profile
(vertical axis)
0.0548
0
Case A : 0.2304
Case B : 0.3474
Case A : 0.1412
∞
∞
999
∞
23.9
26.5
8
Case B : 0.2245
Case A : 0.1290
Case B : 0.2319
0.1284
Case A : 0.2422
Case B : 0.3554
Case A : 2.074
Case B : 2.059
Case A : 0.50
14
4
7
9
23
25.5
23
25.5
23
Case B : 0.73
25.5
1
2
Velocity Profile
(horizonal)
3
V along x-axis
u (V
) Air speed in
[ c tunnel ]
interested region
Vtunnel
[
%
0.050
LDV facility
]
k
u(xi)/xi
2
Concluding Remarks
Beams in LDV probe can be steered to reach
optimal crossing
Fringe spacing in measuring volume could be
calibrated.

Expanded Uncertainty of df = 0.1%
Characteristics of wind tunnel dominate the
measurement uncertainty.


ie. Good wind tunnel is the key to calibration and
measurement capability.
For the present system, U = 0.5% for air speed in
region I, and U = 0.73% in region II.
Comparison with other Lab.
Lab.
Country
Reference
Min. m/s
Max. m/s
Uncertainty
CMS
R.O.C.
LDV
0.5
25
0.5~0.73 %
PTB
Germany
LDV
0.2
60
0.01~0.05 m/s
NMIJ
Japan
LDV
1.3
40
0.29%~0.67%
NMIJ
Japan
carriage
0.05
1
0.008m/s
UCL
Belgium
LDV
0.3
60
1%
CETIAT
France
LDV
0.15
40
2%
NEL
UK
Pitot tube
4
25
0.077m/s
NIST
USA
LDV
0.2
5
0.006 m/s
NIST
USA
Pitot tube
1.5
20
0.28%~2.6%
KRISS
Korea
Pitot tube
1.5
16
0.83%
Operation principle of RPTM module
The spinning disc outputs a reference signal for tuning time delay. By
superimposing the RPTM signal on the Doppler signals, a favorable burst
can be frozen. Then, tuning the time delay and gate width to enclose
interested region.