Laser Doppler Velocimetry (LDV) Part 3 · 2013-06-17 · Microsoft PowerPoint - Lecture#23-LDV-Pat3.ppt Author: huhui Created Date: 10/12/2009 9:40:14 AM

Copyright Copyright ©© by Dr. by Dr. HuiHui HuHu @ Iowa State University. All Rights Reserved!@ Iowa State University. All Rights Reserved!

HHui Huui HuDepartment of Aerospace Engineering, Iowa State University Department of Aerospace Engineering, Iowa State University

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

Laser Doppler Laser Doppler VelocimetryVelocimetry (LDV)(LDV)

Part 3Part 3

AerEAerE 545X class notes #23545X class notes #23


Fundamentals of LDVFundamentals of LDV

•• By using a laser bean of wavelength By using a laser bean of wavelength λλ=488nm =488nm (Argon(Argon--Ion laser), the maximum Ion laser), the maximum Doppler shift Doppler shift from a particle moving with a velocity of V would from a particle moving with a velocity of V would be:be:

–– V=1.0m/sV=1.0m/s ΔΔf f ≈≈ 4.1 MHz4.1 MHz–– V=10.0m/sV=10.0m/s ΔΔf f ≈≈ 41 MHz41 MHz–– V=100.0m/sV=100.0m/s ΔΔf f ≈≈ 410 MHz410 MHz–– V=1000m/sV=1000m/s ΔΔf f ≈≈ 4100 MHz4100 MHz

•• However, since C However, since C = 2.998= 2.998××10108 8 m/sm/s, , λλ=488nm, =488nm, then, f=c/ then, f=c/ λλ= 1.4 = 1.4 ××10 10 9 9 MHz. the Doppler shift in MHz. the Doppler shift in frequency is very small compared with the frequency is very small compared with the frequency of the source laser light.frequency of the source laser light.

•• In practice, it is always quit difficult to measure In practice, it is always quit difficult to measure the the Doppler shift of frequency accurately for lowDoppler shift of frequency accurately for low--speed flows by measuring the received total speed flows by measuring the received total frequency directly.frequency directly.

•• DualDual--beam LDV technique was developed to beam LDV technique was developed to measure the relative frequency change due to the measure the relative frequency change due to the Doppler shift other than the total frequency.Doppler shift other than the total frequency.

λ

φ

λ

φφφ )

2sin(2)

2sin(2 ⋅

=⋅

=−=ΔV

f

fVfff r


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


•• 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)•• Purple (476.4nm)Purple (476.4nm)

33--component LDV systemscomponent LDV systems⊥V

VT

33--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


Advantages and disadvantages of LDV techniqueAdvantages and disadvantages of LDV technique

•• Advantages:Advantages:–– NonNon--intrusiveintrusive–– High resolutionHigh resolution–– High accuracyHigh accuracy–– Wide dynamic range for velocity measurementsWide dynamic range for velocity measurements

•• DisadvantagesDisadvantages–– Single point measurementsSingle point measurements–– Expansive in instrumentationExpansive in instrumentation


Planar Doppler Planar Doppler VelocimetryVelocimetry (PDV)(PDV)or or Doppler Global Doppler Global VelocimetryVelocimetry (DGV)(DGV) techniquetechnique

•• Planar Doppler Planar Doppler VelocimetryVelocimetry (PDV) or (PDV) or Doppler Global Doppler Global VelocimetryVelocimetry (DGV) (DGV) is is capable of determining instantaneous, threecapable of determining instantaneous, three--dimensional velocity vectors of dimensional velocity vectors of moving particles or solid material in a laser light sheet everywmoving particles or solid material in a laser light sheet everywhere in the field of here in the field of view. view.


PDV or PDV or DGVDGV techniquetechnique

•• PDV or DGV relies on light scattering (single exposure) by clouPDV or DGV relies on light scattering (single exposure) by clouds of ds of Particles (high concentration) carried by the flow, rather than Particles (high concentration) carried by the flow, rather than the imaging the imaging of each individual particle.of each individual particle.

•• it has the ability to generate velocity vector field informationit has the ability to generate velocity vector field information more quickly more quickly than LDV, and over a plane of larger area. Each pixel within thethan LDV, and over a plane of larger area. Each pixel within the sensor sensor array could yield a potential velocity vector, therefore, a megaarray could yield a potential velocity vector, therefore, a mega pixels pixels imaging camera would potentially output a million velocity vectoimaging camera would potentially output a million velocity vectors at video rs at video rate.rate.

⊥V

VT

LDV


PDV or DGV PDV or DGV techniquetechnique

•• In In DGVDGV, the velocity information is obtained by means of an optical & , the velocity information is obtained by means of an optical & spectroscopicspectroscopicfrequency converter (a prefrequency converter (a pre--selected linear spectral line optical transfer function), known selected linear spectral line optical transfer function), known as an as an absorption line filterabsorption line filter (ALF), that transforms the Doppler shifted frequency of light (ALF), that transforms the Doppler shifted frequency of light scattered by the particlesscattered by the particles in the flow to image intensity variations in the imaging plane. in the flow to image intensity variations in the imaging plane.

•• Once this transformation is completed, the converted Once this transformation is completed, the converted DopplerDoppler signal intensity map can signal intensity map can then be processed by light intensity detectors (CCD camera) and then be processed by light intensity detectors (CCD camera) and computers to obtain a computers to obtain a velocity map of the flow of interest. velocity map of the flow of interest.

•• To eliminate the problem of both scattering signal and illuminatTo eliminate the problem of both scattering signal and illumination intensity variations ion intensity variations spatially in the measurement window, the Doppler signal intensitspatially in the measurement window, the Doppler signal intensity map is normalized by a y map is normalized by a reference intensity map from the same view of the flow.reference intensity map from the same view of the flow.

λ

φ

λ

φφφ )

2sin(2)

2sin(2 ⋅

=⋅

=−=ΔV

f

fVfff r

iodine vapor cell: Absorption line filteriodine vapor cell: Absorption line filter

Io Ix



λ

φ

λ

φφφ )

2sin(2)

2sin(2 ⋅

=⋅

=−=ΔV

f

fVfff r

iodine vapor cell: Absorption line filteriodine vapor cell: Absorption line filter

Io Ix



PDV measurements in a supersonic jetPDV measurements in a supersonic jet

λ

φ

λ

φφφ )

2sin(2)

2sin(2 ⋅

=⋅

=−=ΔV

f

fVfff r


Multi component PDV or DGV Multi component PDV or DGV techniquetechnique

λ

φ

λ

φφφ )

2sin(2)

2sin(2 ⋅

=⋅

=−=ΔV

f

fVfff r


Advantages and disadvantages of PDV/GDV techniqueAdvantages and disadvantages of PDV/GDV technique

•• Advantages:Advantages:–– PDV is well suited for highPDV is well suited for high--speed flow measurements where concerns about particle seeding speed flow measurements where concerns about particle seeding

make PIV impractical. make PIV impractical. –– Although PDV requires particles to scatter light, individual parAlthough PDV requires particles to scatter light, individual particles do not need to be ticles do not need to be

imaged thus allowing the use of much smaller seed particles and imaged thus allowing the use of much smaller seed particles and making the measurements making the measurements less sensitive to particle seed density.less sensitive to particle seed density.

–– For example, in some unheated supersonic flow facilities it is pFor example, in some unheated supersonic flow facilities it is possible to use condensation ossible to use condensation of a vapor, such as water, acetone or ethanol, to produce seed pof a vapor, such as water, acetone or ethanol, to produce seed particles in the flow. articles in the flow.

–– In addition, PDV has an inherently higher resolution than PIV asIn addition, PDV has an inherently higher resolution than PIV as smaller image smaller image subregionssubregionscan be used to determine the velocity.can be used to determine the velocity.

•• DisadvantagesDisadvantages–– The main weakness of PDV is the complex optical set up required The main weakness of PDV is the complex optical set up required to get accurate to get accurate

measurements (position registration, etc..).measurements (position registration, etc..).–– For each component of velocity, two images (signal and referenceFor each component of velocity, two images (signal and reference) are required, which ) are required, which

typically necessitates two cameras. To obtain all three componentypically necessitates two cameras. To obtain all three components of velocity, therefore, ts of velocity, therefore, requires the simultaneous use of up to six cameras. requires the simultaneous use of up to six cameras.

–– In addition, the laser used for the measurements must be narrow In addition, the laser used for the measurements must be narrow linewidthlinewidth, which is , which is typically performed by injection seeding of the laser cavity.typically performed by injection seeding of the laser cavity.

–– Even with seeding, the laser frequency can fluctuate with time Even with seeding, the laser frequency can fluctuate with time and must be monitored. and must be monitored. These introduce additional complexity to the experimental setThese introduce additional complexity to the experimental set--up.up.

–– PDV systems, although used in many laboratories, are not yet coPDV systems, although used in many laboratories, are not yet commercially available and mmercially available and can be quite expensive (equipment, data processing, experience, can be quite expensive (equipment, data processing, experience, labor, etc.) if built from labor, etc.) if built from scratch.scratch.

Documents

Laser Doppler Velocimetry (LDV) Part 3 · 2013-06-17 · Microsoft PowerPoint - Lecture#23-LDV-Pat3.ppt Author: huhui Created Date: 10/12/2009 9:40:14 AM