1. AirSpeed Calibration Facility by using LDV and A Wind Tunnel at CMS
Cheng-Tsair Yang
Center for Measurement Standards
Industrial Technology Research Institute/Taiwan

2. 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

3. 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

4. Concerns of a wind tunnel
Diffuser:
to lower the air flow speed, consequently reduce the pressure loss due to friction
Fan drive/Blower: provide a pressure increase of flow, to overcome the pressure loss in the tunnel circuit. Steady driving force is mostly required.
Test section:
provide desirable flow condition and space for calibration anemometer
Contraction Nozzle: to accelerate the flow speed to reach the desirable level in the test section, reduce the turbulence intensity.

5. 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

6. 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

7. Measurement Facilities-1
Wind tunnel :
Open-loop with expanded test section
Outlet of contraction nozzle: 200 mm
Motor controlled by a frequency converter
Designed air speed: ~30 m/s
Flow patterns simulated at test section

8. Characteristics of axial fan volume flow rate and pressure rise
Fan selection is based on the volume flow rate and pressure rise.
the size (diameter=0.56 m).
power (energy loss of flow in the tunnel) is proportional to the cube of the air speed.

9. CFD simulation vs.LDV measurement
Outlet of contraction nozzle
Suction section
Air
LDV measurement velocity at 10 mm downstream
LDV measurement velocity at 100 mm

10. Measurement Facilities-2
LDV :
TSI Probe 9832 with beam expander
2. Focal length: 450 mm
3. Nominal fringe spacing: m for beam of nm, m for 488nm

11. Measurement Traceability of Anemometry Calibration
Length Standard
Frequency Standard
(Time)
Vernier Caliper
Universal Counter
Spinning Disc
Laser Doppler Anemometry
+ Wind tunnel
Check
Anemometry
Anemometry under Test

12. Spinning disc traceability
Pedestal
-Disc eccentricity adjustable
-Horizontal disc
-rotary encoder for rotational speed
- Local fringe spacing detectable

13. Obtain of Fringe Spacing
Basics of calibration β
Vldv= df  fD
Vdisc= r   cos
Obtain local fringe spacing
df = r ×  / fD× cos
 <1 degree 13

14. 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. 14

15. Calibration of LDV Probe-1
Why is adjustment and calibration of LDV probe necessary?
Measurement
Volume df *
*Miles, P. C., ”Geometry of the Fringe Field Formed in the Intersection of Two Gaussian Beams,” Applied Optics1996;35:5887–5895.

16. Calibration of LDV Probe-2
Step 1. Steer beams to optimize beam crossing
Z=1850 m
Z=1000 m
Z=0
Minimal variation of fringe spacing
in measuring volume
Change state of beam crossing 16

17. 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: m with U= 0.1%.
(Nominal value: 1.921)

18. Calibration of LDV Probe-4
Step 3. Evaluation of uncertainty of air-speed coefficient
df = r ×  / fD× cos + ε

19. 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

20. 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

21. Uncertainty analysis of air speed in interested region
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

22. Uncertainty due to Flow characteristics in interested region
1. Turbulence intensity
Tu = 1.73 % %
for V=0.5 m/s – 25 m/s
Measured at 100 mm downstream centerline of nozzle
2. Velocity bias (sampling effect)
by comparing weighted (residence-time) with un-weighted velocities
(Fry, 1985):
, estimated to be < 0.01%

23. Effects due to velocity distribution in interested region
1. Flow profiles along vertical radius (Z-axis flow uniformity)
r (mm) V1 (m/s) V V V4
Mean
Std. Dev. (m/s)
Rel. Std. Dev. %
For region I： r = (-40 ~ 40) mm, X = (0~100)mm
Standard Uncertainty = 0.14 %
For region II： r = (-70 ~ 70) mm, X = (0~100)mm
Standard Uncertainty = 0.22 %

24. 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 (X-axis flow uniformity)
Std. dev. = 0.129%
(obtained from repeat measurement at different air speed)

25. Uncertainty analysis item Sources u(x )/x % ν LDV facility 0.050 9.5
Flow Property
0.0557
1066.7 1
Particle lag ∞ 2
Velocity bias
0.01 3
Turbulence int.
0.0548
999 4
Fringe bias
Case A :
23.9
Wind-tunnel
Property
Case B :
26.5
Case A : 8
Velocity Profile
(vertical axis)
Case B : 14
Case A :
(horizonal)
Case B : 7
V along x-axis
0.1284 9
Case A : 23
Air speed in
interested region
Case B :
25.5
Case A : 2.074 k
Case B : 2.059
Case A : 0.50
Case B : 0.73

26. 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.

27. Comparison with other Lab.

28. 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.