Molecular Tagging techniques Part - 1huhui/teaching/2019-08Fx/AerE545x/...• Molecule-based techniques: • Flow tracking issues can be solved. • Molecular tracers can usually be

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

Molecular Tagging techniquesMolecular Tagging techniquesPart Part -- 11

AerEAerE 545 class notes #4545 class notes #400


•• ParticleParticle--basedbased techniquestechniques ( LDV, PIV, PDV and Particle Image Thermometry):( LDV, PIV, PDV and Particle Image Thermometry):•• The The particleparticle--basedbased techniques measure the velocity or temperature of techniques measure the velocity or temperature of tracer particlestracer particles, ,

other than the velocity or temperature of other than the velocity or temperature of working fluid working fluid directly. directly. •• Flow tracking issues (particle size, density mismatch, Flow tracking issues (particle size, density mismatch, ……))•• Seeding issues (particles donSeeding issues (particles don’’t always go where you need them)t always go where you need them)•• Thermal response of the tracer particles for temperature measureThermal response of the tracer particles for temperature measurements.ments.

•• MoleculeMolecule--basedbased techniques:techniques:•• Flow Flow tracking issuestracking issues can be solved. can be solved.

•• Molecular tracers can usually beMolecular tracers can usually be dissolved dissolved in the working fluids, which move in the working fluids, which move exactly with the exactly with the same velocitysame velocity as as the localthe local fluid moleculesfluid molecules..

•• Thermal response Thermal response for fluid temperature measurement can be significantly mitigatedfor fluid temperature measurement can be significantly mitigated, , and perhaps even eliminated.and perhaps even eliminated.

•• The The sizessizes of the molecular tracers are usually of the molecular tracers are usually much smaller much smaller than particle tracers.than particle tracers.

•• Simultaneous measurements of multiple flow parameters.Simultaneous measurements of multiple flow parameters.•• In addition to In addition to velocityvelocity field measurement, simultaneous mapping of scalar field measurement, simultaneous mapping of scalar

parameters such asparameters such as temperature temperature and and concentration concentration of fluid spicesof fluid spices can also be can also be achieved easily.achieved easily.

MoleculeMolecule--based flow diagnostic techniques !? Why!!?based flow diagnostic techniques !? Why!!?


Molecular Tagging Velocimetry (MTV): General DescriptionMolecular Tagging Velocimetry (MTV): General Description

Tagged regions imagedright after the laser pulse

The velocity field derived usinga spatial correlation procedure

Tagged regionsimaged 8 ms later

• Whole field, non-intrusive, optical diagnostic technique• Use special chemical molecules premixed in fluid flows as long lifetime tracers• A pulsed laser is used to "tag" small regions of interest• Tagged regions are imaged twice with pre-set time delay• The displacement vectors of the tagged regions provide the estimate of the

velocity vectors of the fluid flow. • Line (1-d), Grid (2-d), Stereoscopic (3-d) MTV• Flow visualization and quantitative measurements• Scalar field measurements, simultaneous velocity/scalar field measurements


Comparison between MTV and PIVComparison between MTV and PIV

PIVPIV MTVMTVTracer:Tracer: particles (~particles (~μμmm)) molecules (~nm)molecules (~nm)

Image photon source:Image photon source: scattering lightscattering light emissionemission (fluorescence/phosphorescence)(fluorescence/phosphorescence)

Laser source:Laser source: conventional laserconventional laser UV laserUV lasersignal intensity:signal intensity: strongstrong weakweak

Advantage of MTVAdvantage of MTV•• Flows are hard to seed by particlesFlows are hard to seed by particles

•• boundary layers or vortex coresboundary layers or vortex cores•• Particles may change flowsParticles may change flows

•• micromicro--flowsflows•• Particles do not follow flowsParticles do not follow flows

•• high speed flows, shock waveshigh speed flows, shock waves•• Particles may change the physics of phenomenaParticles may change the physics of phenomena

•• solidification solidification •• Simultaneous vector/scalar distribution Simultaneous vector/scalar distribution

measurements. measurements.

Disadvantage of MTVDisadvantage of MTV•• Less light compared to PIVLess light compared to PIV

•• Emission rather than scatteringEmission rather than scattering•• May require intensified camerasMay require intensified cameras

•• ChemicalsChemicals•• Photon SourcePhoton Source

•• UV and/or deep UVUV and/or deep UV•• High EnergyHigh Energy•• Some require multiple photon Some require multiple photon

sourcessources•• Equipment CostEquipment Cost


Components of Molecular Tagging Visualization / VelocimetryComponents of Molecular Tagging Visualization / Velocimetry

• Long Lifetime Molecular TracersThe tracer photophysics dictates thetype and number of lasers needed

• Tagging MethodsSingle-line, Multi-lineGrid, Laser sheet

• DetectionStandard CCD CamerasGated Image-Intensified CamerasDual Detector for Increased AccuracyMulti-Camera for Stereo MTV

• ProcessingLine Center MethodsDirect Spatial Correlation

Liquid Phase

Photochromic Molecules (Hummel, Falco)

Caged Laser Dyes (Lempert)

Phosphorescent Supramolecules

Gas Phase

Vibrationally Excited O2 (Miles)

Ozone (O3) (Pitz)

OH (dissociation of H2O) (Boedeker, Pitz)

NO (dissociation of Tert-butyl nitrite) (Grünefeld)

NO (air photolysis) (Sijtsema et al.)

Ionic Strontium Fluorescence (Rubinsztein-Dunlop et al.)

Phosphorescent Molecules(Biacetyl, Acetone, etc.)


Fluorescence Phosphorescence

Long Lifetime Phosphorescent Molecular Tracers for LiquidsLong Lifetime Phosphorescent Molecular Tracers for Liquids

• Lifetime decreases in the presence of a quencher

o

o

][1 τττ

QKq+=

• Design molecular complexes withsmall quenching rate Kq

• Minimize quencher concentration [Q]

Emission from the phosphorescent triplex 1-BrNp • Gβ-CD • ROH ;

Lifetime with quencher

Lifetime without quencher

Quenching rate

Quencher concentration

(A) without alcohol, (B) with alcohol.


Molecular Tagging Velocimetry (MTV) techniqueMolecular Tagging Velocimetry (MTV) technique(line(line--typed tagging for onetyped tagging for one--componentcomponent velocityvelocity measurement)measurement)

first image (right after laser firing)

second image (14 ms later)•• Tagged lines are imaged twice with knownTagged lines are imaged twice with known time delaytime delay

•• Intensity profile for each row Intensity profile for each row --> line center locations > line center locations •• Difference between the line centers gives displacementDifference between the line centers gives displacement•• Velocity = displacement / delay timeVelocity = displacement / delay time

Flow visualization of the vortex shedding from the trailing edge of an oscillating airfoil (Bohl et al. 2002)


First image(right after the laser pulse)

Second image(imaged 3.5 ms later)

•• Create a 2Create a 2--D grids with multiple laser beams.D grids with multiple laser beams.•• Take two images with known time delay.Take two images with known time delay.

•• Find the displacement vectors of the grids through a image proceFind the displacement vectors of the grids through a image processing procedure. ssing procedure.

•• Local velocity = displacement/time delay.Local velocity = displacement/time delay.

The velocity field derived usinga spatial correlation procedure (Bohl et al. 2002)

Molecular Tagging Velocimetry (MTV) Molecular Tagging Velocimetry (MTV) (Grid(Grid--typed tagging for twotyped tagging for two--componentcomponent velocityvelocity measurement)measurement)


Planar MTV (twoPlanar MTV (two-- component measurements)component measurements)

Tagged regions imagedright after the laser pulse

The velocity field derived usinga spatial correlation procedure

Tagged regionsimaged 8 ms later

Results taken from (Bohl et al. 2001)


Image Processing for Planar MTVImage Processing for Planar MTV

• Create a 2-D contrast field• Select a region in the first image (green)• Search in the second image for the matching pattern (red)• Pattern is matched via direct correlation technique

undelayed imageat time t=t0

delayed imageat time t=t0 +Δt

Correlation coefficientdistribution



Stereoscopic Molecular Tagging Velocimetry Stereoscopic Molecular Tagging Velocimetry (velocity three(velocity three--component measurements)component measurements)

•• Use 2 cameras viewing from different perspectives.Use 2 cameras viewing from different perspectives.•• Each camera is processed using a planar MTV technique.Each camera is processed using a planar MTV technique.•• Information from the two cameras is combined to Information from the two cameras is combined to

reconstruction three components of velocity vectors.reconstruction three components of velocity vectors.



Applications: Unsteady Flow Separation on AirfoilsApplications: Unsteady Flow Separation on Airfoils

• Airfoils moving to high angles of attack undergodynamic stall, characterized by large excursions in liftand pitching moment.

NACA-0012 airfoil pitching to high angle of attack

hydrogen-bubble visualization

28.1°

37.7°

12°

Boundary layer resolved vorticity measurementshave revealed the flow physics at the onset of theseparation process, and its spatial and temporalscales.


Quantitative MTV MeasurementsQuantitative MTV Measurements

Strong concentrated vortices are formed immediately at the trailing edge.Instantaneous streamlines are highly curved near the trailing edge.Note the location and sign of the vortices formed at the trailing edge.

Using the grid tagging method, the in-plane velocity components and the vorticity field are mapped. Data below are for k = 11.5.

((BohlBohl et al. 2004)et al. 2004)


y

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1 5 m/s

Applications: MTV Measurements in a Motored IC EngineApplications: MTV Measurements in a Motored IC Engine

X (cm)

y(c

m)

0 1 2 3 4 5 6 7 8 9-6

-5

-4

-3

-2

-1

0

1

Spark Plug

Intake valve Exhaust valve

Imaged plane at late compression, 270 CAD

piston

Optically accessible Ford 4-valve, 4.6L engine

Goh, Koochesfahani & Schock (2001)

x

y2 3 4 5 6 7

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1 5 m/s

Two instantaneous realizations showing thelarge cycle-to-cycle variability of the flow field.


Applications: UniApplications: Uni--Directional Solidification ofDirectional Solidification ofOptically Transparent Binary Alloy Analog, NHOptically Transparent Binary Alloy Analog, NH44ClCl

• Uni-directional solidification providesincreased resistance to creeprupture and thermal fatigue in thefinal solidified ingot.

• Solutal and thermal forces produceimperfections in the final solidified ingotin the form of solute-rich channels.

Later stage of solidification

enlarged view of plume

plumes

Mushy Zone

chimney11 mm

Small-scale fingers

Early stage of solidification

((LumLum et al. 2004)et al. 2004)


• Velocity vector of plume (79 min into the solidification process)• Maximum magnitude of velocity is ~7mm/s within the plume

10.41mm

x

y

68 70 72 74 76 78 8034

35

36

37

38

39

40

41

42

43

44

45

46

(mm)

(mm

)

Applications: UniApplications: Uni--Directional Solidification ofDirectional Solidification ofOptically Transparent Binary Alloy Analog, NHOptically Transparent Binary Alloy Analog, NH44ClCl

Reference vector (1mm/s)

((LumLum et al. 2004)et al. 2004)

Documents

Molecular Tagging techniques Part - 1huhui/teaching/2019-08Fx/AerE545x/...• Molecule-based techniques: • Flow tracking issues can be solved. • Molecular tracers can usually be