1. Measurement of Flow Velocity
Prof. Václav Uruba
IT ASCR, CTU Praha, UWB Plzeň

2. Resolution Time Space Velocity components Mean value
Instantaneous values
Independent
Time Resolved
Space
0D (point)
1D (line)
2D (plane)
3D (volumetric)
Velocity components 1 2 3
TIME CORRELATION
SPACE CORRELATION
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3. Methods Pressure measurement (M or TR, 0D, 1-3c)
Thermal anemometry (TR, 0D, 1-3c)
Optical methods
LDA (TR, 0D, 1-3c)
PIV (I or TR, 2D or 3D, 2-3c)
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4. Velocity
Pressure measurement
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5. Pressure Probes Total pressure – Pitot Static pressure
Dynamic pressure – Prandtl (Pitot-static) probe
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6. Incompressible Flow air upto 50 (100) m/s Bernoulli equation
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7. Subsonic Compressible Flow
isentropic
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8. Supersonic Compressible Flow
nonisentropic
isentropic
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9. Multihole probes - direction
Evaluated quantity
Total pressure
Static pressure
2-3 velocity comp.
3-6holes
A.a °
7-12 holes sphere
A.a. upto 180°
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10. Multihole probes - direction
Φ 3mm
5 holes
Omnidirectional
Φ 9.5mm
12 holes
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11. Fast response Φ 6.3mm 5 holes Fast response Φ 1.6mm 5 holes
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12. Thermal anemometry
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13. Thermal Anemometry Hot Wire or Film
Measures any fluid quantity depending on heat transfer (velocity, temperature, concentration, …)
Measuring “point”:
The only method for more then 10kHz (upto 200kHz)
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14. Constant Temperature Anemometry
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15. Frequency response
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16. Directional sensitivity
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17. Directional ambiguity
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18. Sensor
Wire
Film
Φ μm
Nickel th. less 1μm
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19. Probes – wires
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20. Probes - films
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21. Calibration
Cooling law
Velocity set using pressures
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22. Thermal anemometry Small measuring point Good sensitivity
High precision (depending on calibration)
High frequency
Range of velocities (air: 0.1m/s – 5M)
Sensitivity to other quantities (T, p, concentration)
Intrusive method
Fragile probe
Problems in harsh environment
Velocity orientation ambiguity
Sensitivity to other quantities (T, p, concentration)
Calibration necessary
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23. Velocity
Optical methods
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24. Optical Methods Laser Doppler Anemometry (LDA, PDA)
Particle Image Velocimetry (PIV)
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25. Laser Doppler Anemometry
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26. LDA - Fringe model
Focused laser beams intersect and form the measurement volume
Plane wave fronts: beam waist in the plane of intersection
Interference in the plane of intersection
Pattern of bright and dark stripes/planes
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27. LDA Velocity = distance/time measuring volume backscattered light
Flow with particles
t (measured)
Signal
Time
Processor
d (known)
Detector
measuring volume
Bragg
Cell
Laser
backscattered light
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28. Measurement volume Length: Width: Height: Fringe separation:δz
Fringe separation:  Z
No. of fringes: δx δf X
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29. LDA system
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30. Application examples
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31. Particle Dynamics Analyzer
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32. LDA High precision No calibration Nonintrusive Up to 3 components
Small measuring point
Velocity orientation
Particles necessary
Unevent sampling
Expensive
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33. Particle Image Velocimetry
Velocity vector fields - space correlation
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34. PIV f = 1 – 100 Hz TR: f = 500 – 2000 Hz Δt = 0.2 – 1000 μs time
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35. Velocity evaluation
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36. PIV evaluation
Correlation
Particle tracking
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37. Image A
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38. Image B
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39. Image B
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40. Vector field
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41. Vectors + vorticity
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42. PIV variants Classical PIV Time Resolved PIV (high frequency)
Plane
2 velocity components
Low frequency
Time Resolved PIV (high frequency)
Stereo PIV (3 comp.)
Tomographic PIV, 3D PIV (volume, 3 comp.)
Micro PIV (<1mm)
Mega PIV, Large Scale PIV (LSPIV) (1-10m)
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43. Stereo PIV True displacement Displacement seen from left
seen from right
Focal plane =
Centre of
light sheet
Left
camera
Right
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44. Volumetric PIV
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45. PIV Spatial correlation No calibration Nonintrusive 2 to 3 components
Velocity orientation
Particles necessary
Lower precission
Expensive
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46. Seeding particles
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47. Seeding: ability to follow flow
Particle
Fluid
Diameter (m)
f = 1 kHz
f = 10 kHz
Silicone oil atmospheric air
TiO2 atmospheric air
TiO2 oxygen plasma
(2800 K)
MgO methane-air flame
(1800 K)
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48. Particles Dynamics Important parameters in particle motion
trajectory
Fluid
pathline
Important parameters in particle motion
Particle shape
Particle size
Relative density of particle and fluid
Concentration of particles in the fluid
Body forces
Acc.
Drag
Pressure
Added mass
History
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49. Repetition rate
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