Lecture # 05: Velocimetry Techniques and Instrumentationhuhui/teaching/2014Fx/class...Title: Microsoft PowerPoint - AerE344-Lecture-05-Velocimetry-Pressure-Cylinder.ppt Author: huhui

Copyright Copyright ©© by Dr. Hui Hu @ Iowa State University. All Rights Reserved!by Dr. Hui Hu @ Iowa State University. All Rights Reserved!

Dr. HDr. Hui Huui Hu

Department of Aerospace EngineeringDepartment of Aerospace EngineeringIowa State University Iowa State University

Ames, Iowa 50011, U.S.AAmes, Iowa 50011, U.S.A

Lecture # 05: Velocimetry Techniques and Instrumentation

AerEAerE 3344 44 Lecture NotesLecture Notes


Methods to Measure Local Flow Velocity Methods to Measure Local Flow Velocity -- 11

•• Mechanical methods:Mechanical methods:•• Taking advantage of force and moments that a moving stream appliTaking advantage of force and moments that a moving stream applies on immersed objects.es on immersed objects.•• Vane anemometersVane anemometers•• Propeller anemometersPropeller anemometers


Methods to Measure Local Flow Velocity Methods to Measure Local Flow Velocity --22

b. Flat plateb. Flat plate

a. streamlined airfoila. streamlined airfoil

•• Pressure difference methods:Pressure difference methods:•• Utilize analytical relationship between the local Utilize analytical relationship between the local

velocity and the static and total pressuresvelocity and the static and total pressures• The tubes sensing static and stagnation pressures are

usually combined into one instrument known as Pitot-static tube.

• Pressure taps sensing static pressure (also the reference pressure for this measurement) are placed radially on the probe stem and then combined into one tube leading to the differential manometer (pstat).

• The pressure tap located at the probe tip senses the stagnation pressure (p0).

• Use of the two measured pressures in the Bernoulli equation allows to determine one component of the flow velocity at the probe location.

• Special arrangements of the pressure taps (Three-hole, Five-hole, seven-hole Pitot) in conjunction with special calibrations are used two measure all velocity components.

• It is difficult to measure stagnation pressure in real, due to friction. The measured stagnation pressure is always less than the actual one. This is taken care of by an empirical factor C.

ρ

ρ

ρ

/)(2

/)(2

)(,21

0

0

20

stat

stat

stat

ppCV

ppV

BernoulliVpp

−=

−=

+=


Methods to Measure Local Flow Velocity Methods to Measure Local Flow Velocity --33

•• Thermal methods:Thermal methods:•• Compute flow velocity from its relationship between local flow vCompute flow velocity from its relationship between local flow velocity and the convective heat elocity and the convective heat

transfer from heated elements.transfer from heated elements.

Hot wire anemometersHot wire anemometers Hot film anemometersHot film anemometers


ParticleParticle--based Flow Diagnostic Techniquesbased Flow Diagnostic Techniques

• Seeded the flow with small particles (~ µm in size)

• Assumption: the particle tracers move with the same velocity as local flow velocity!

Flow velocityFlow velocityVVff

Particle velocityParticle velocityVVpp

=

Measurement of particle velocity



•• FrequencyFrequency--shift methods:shift methods:•• Based on the Doppler phenomenon, namely the shift of the frequenBased on the Doppler phenomenon, namely the shift of the frequency of waves scattered by moving cy of waves scattered by moving

particles. particles. •• Laser Doppler Laser Doppler Velocimetry(LDVVelocimetry(LDV) or Laser Doppler Anemometry (LDV)) or Laser Doppler Anemometry (LDV)•• Planar Doppler Planar Doppler VelocimetryVelocimetry (PDV) or Planar Doppler Anemometry(PDV) or Planar Doppler Anemometry (PDA)

Interference fringesInterference fringes


Doppler ShiftDoppler Shift•• The Doppler effect, named after The Doppler effect, named after Christian Doppler (an Austrian mathematician and Christian Doppler (an Austrian mathematician and

physicist )physicist ),, is the change in frequency and wavelength of a wave that is peris the change in frequency and wavelength of a wave that is perceived by an ceived by an observer moving relative to the source of the waves.observer moving relative to the source of the waves.

•• Light from moving objects will appear to have different wavelengLight from moving objects will appear to have different wavelengths depending on the ths depending on the relative motion of the source and the observer.relative motion of the source and the observer.

•• Observers looking at an object that is moving away from them seeObservers looking at an object that is moving away from them see light that has a longer light that has a longer wavelength than it had when it was emitted (a red shift), while wavelength than it had when it was emitted (a red shift), while observers looking at an observers looking at an approaching source see light that is shifted to shorter wavelengapproaching source see light that is shifted to shorter wavelength (a blue shift).th (a blue shift).


Doppler ShiftDoppler Shift

a. Stationary Sound Source a. Stationary Sound Source b. Source moving with b. Source moving with VsourceVsource < < VsoundVsound

c. Source moving with c. Source moving with VsourceVsource = = VsoundVsound( Mach 1 ( Mach 1 -- breaking the sound barrier ) breaking the sound barrier )

d. Source moving with d. Source moving with VsourceVsource > > VsoundVsound(Mach 1.4 (Mach 1.4 -- supersonic) supersonic)


Fundamentals of LDVFundamentals of LDV•• Take the coordinate system to be at rest with respect to Take the coordinate system to be at rest with respect to

the medium, whose speed of light wave is c. There is a the medium, whose speed of light wave is c. There is a source s moving with velocity Vsource s moving with velocity Vss and emitting light and emitting light waves with a frequency waves with a frequency ffss. .

•• There is a detector r moving with velocity There is a detector r moving with velocity VVrr , and the , and the unit vector from s to r is n i.e. . unit vector from s to r is n i.e. .

•• Then the frequency Then the frequency ffrr at the detector is found from at the detector is found from

•• If c>>VIf c>>Vs s , then the change in frequency depends mostly , then the change in frequency depends mostly on the relative velocity of the source and detector.on the relative velocity of the source and detector.

λ

φ

λ

φ

φ

φφ

φ

)2

sin(2)2

sin(2

)2

sin(2)ˆˆ(0

ˆˆˆ

ˆ

⋅=

⋅=Δ⇒

⋅=

−⋅=

Δ⇒

⎭⎬⎫

=−=

−⋅−=

−=

Δ

V

f

fVf

c

V

ceeV

ff

Veen

cVVn

fff

ff

irs

sr

ir

sr

s

sr

s

r

rr


DualDual--Beam LDVBeam LDV

⊥

⊥⊥

=

==

=

=

=

Vf

VVf

fDdDN

TT

e

T

λθ

λθ

δ

θπ

θλδ

)2/sin(2

:ryshift theoDoppler toaccordingshift Frequency

)2/sin(2

:light scattering theofFrequency

)2/sin(2

;4 :number Fring

)2/sin(2 :spacing Fring

⊥V

VT


•• DualDual--beam laser setup only can measure one beam laser setup only can measure one component of the velocity with its direction component of the velocity with its direction normal to the fringe planes.normal to the fringe planes.

•• TwoTwo--color LDV system can be used for 2color LDV system can be used for 2--components of flow velocity measurements.components of flow velocity measurements.

•• ArAr--ion Laser beamsion Laser beams•• Blue (488nm)Blue (488nm)•• Yellow (514.5 nm)Yellow (514.5 nm)

22--component LDV systemscomponent LDV systems

⊥V

VT

22--component LDVcomponent LDV


•• As particles pass through the probe volume, they As particles pass through the probe volume, they scatter light from the beams and create an scatter light from the beams and create an interference fringe pattern. interference fringe pattern.

•• A receiving lens at an offA receiving lens at an off--axis collection angle axis collection angle projects part of this fringe pattern onto detectors, projects part of this fringe pattern onto detectors, which produce a Doppler burst signal with a which produce a Doppler burst signal with a frequency proportional to the particle velocity.frequency proportional to the particle velocity.

•• The phase shift between the Doppler burst The phase shift between the Doppler burst signals from the different detectors is signals from the different detectors is proportional to the size of the spherical particles.proportional to the size of the spherical particles.

Phase Doppler Particle Analyzers/PDPA SystemsPhase Doppler Particle Analyzers/PDPA Systems

PDPA systemPDPA system


ParticleParticle--based techniques: Particle Image based techniques: Particle Image VelocimetryVelocimetry (PIV)(PIV)

• To seed fluid flows with small tracer particles (~µm), and assume the tracer particles moving with the same velocity as the low fluid flows.

• To measure the displacements (ΔL) of the tracer particles between known time interval (Δt). The local velocity of fluid flow is calculated by U= Δ L/Δt .

A. t=tA. t=t00 B. t=tB. t=t00+10 +10 μμssClassic 2Classic 2--D PIV measurementD PIV measurement

C. Derived Velocity fieldC. Derived Velocity fieldX (mm)

Y(m

m)

-50 0 50 100 150

-60

-40

-20

0

20

40

60

80

100

-0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.95.0 m/sspanwise

vorticity (1/s)

shadow region

GA(W)-1 airfoil

t=tt=t00 tLUΔΔ

=

t= tt= t00++ΔΔttΔΔLL



•• Marker tracing methods:Marker tracing methods:•• Trace the motion of suitable flow makers, Trace the motion of suitable flow makers,

optically or by other means to derive local optically or by other means to derive local flow velocity. flow velocity.

•• Particle Imaging Particle Imaging VelocimetryVelocimetry (PIV)(PIV)•• Particle Tracking Particle Tracking VelocimetryVelocimetry (PTV)(PTV)•• Molecular Tagging Molecular Tagging VelocimetryVelocimetry (MTV)(MTV)

t=t0 t=t0+4 ms

PIV image pairPIV image pair

MTV&T image pairMTV&T image pairt=t0 t=t0+5ms

26.5026.4026.3026.2026.1026.0025.9025.8025.7025.6025.5025.4025.3025.2025.1025.0024.9024.8024.7024.6024.50

0.03 m/s

Temperature (OC)

Corresponding flow velocity field


Stereoscopic PIV MeasurementsStereoscopic PIV Measurements

Camera 1Camera 1 Camera 2Camera 2

Laser SheetLaser Sheet

αα11

αα22

ZZ

XX

Displacement vectors in left camera Displacement vectors in left camera Displacement vectors in right camera Displacement vectors in right camera

Stereo PIV techniqueStereo PIV technique

.deg0.5;000,52Re == αC

X/C =4.0

X pixel

Ypi

xel

0 500 1000 1500

0

200

400

600

800

1000

1200

X pixel

Ypi

xel

0 500 1000 1500

0

200

400

600

800

1000

1200


Flow Around a Circular CylinderFlow Around a Circular Cylinder

uniform flow +uniform flow + a 2a 2--D doublet = flow around a circular D doublet = flow around a circular cylindercylinder

Stagnation pointsStagnation points

Pressure coefficient on the surface of cylinderPressure coefficient on the surface of cylinder

)1(sin

)1(cos

)1(cos

)1(sin

2

2

2

2

2

2

2

2

rRVV

rRVV

rRrV

rRrV

r

+−=

−=

+=

−=

∞

∞

∞

∞

θ

θ

θφ

θψ

θ

Lab#04 Measurements of Pressure Distributions around a CircularLab#04 Measurements of Pressure Distributions around a Circular CylinderCylinder



•• At the surface of the cylinder, At the surface of the cylinder,

•• According to BernoulliAccording to Bernoulli’’s equation s equation

•• Pressure coefficient distribution on the surface of the circularPressure coefficient distribution on the surface of the circular cylinder will be:cylinder will be:

⎩⎨⎧

−==

⇒=∞ θθ sin2

0VV

Var r

2

2

2

22 1

212

121

∞∞

∞∞∞ −=

−⇒+=+

V

V

V

PPVPVPρ

ρρ

θθ

ρ

22

2

2

2

2sin41)sin2(11

21 −=

−−=−=

−=

∞

∞

∞∞

∞

VV

VV

V

PPCp

Rθ

P

Incoming flow

X

Y

-3

-2

-1

0

1

0 100 200 300 400

Angle, Deg.

Cp


Numerical results for the velocity magnitude and pressure distributions over a circular cylinder using FlowLab(Vh=35.8 m/s, Re=1.89E+05)

Pressure Distribution around Pressure Distribution around a Circular Cylindera Circular Cylinder



Rθ

P

Incoming flow

X

Y


Cp distributions over a circular cylinder

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7

theta (rad ) -->

cp --

>

EFDCFDAFD



Required Required ResultsResults for for the the Lab ReportLab Report

–– To make a table showing all the timeTo make a table showing all the time--averaged data you obtained averaged data you obtained for all the cases you tested.for all the cases you tested.

–– To show all the calculation steps leading up to the final answerTo show all the calculation steps leading up to the final answer..

–– To plot pressure coefficient (To plot pressure coefficient (CCpp) distributions on the cylinder from ) distributions on the cylinder from for all the cases you tested.for all the cases you tested.

–– To make comments on the characteristics of the pressure To make comments on the characteristics of the pressure distribution compared with the theoretic predictions.distribution compared with the theoretic predictions.

–– To calculate the drag coefficients (To calculate the drag coefficients (CCdd ) of the circular cylinder for ) of the circular cylinder for all the cases you tested.all the cases you tested.

–– To plot the drag coefficients (To plot the drag coefficients (CCdd ) of the circular cylinder as a ) of the circular cylinder as a function of at the Reynolds numbers.function of at the Reynolds numbers.

Documents

Lecture # 05: Velocimetry Techniques and Instrumentationhuhui/teaching/2014Fx/class...Title: Microsoft PowerPoint - AerE344-Lecture-05-Velocimetry-Pressure-Cylinder.ppt Author: huhui