Lecture Seeding Particles for PIV

8/18/2019 Lecture Seeding Particles for PIV

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Tracer Particles and Seeding for PIV


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Seeding particles for PIV

Proper tracer must be small enough to follow (trace)fluid motion and should not alter fluid or flow properties.

Proper tracer must be large enough to be visible bythe camera.

Uniform seeding is critical to the success ofobtaining velocity field. o seed particles! no data.

The seeding source must be placed cleverly so that the particles mi" with the flow well.

Particles with finite inertia are #nown to disperse non$uniformly in a turbulent flow! preferential concentration


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Seeding particles for PIV (cont%d)

The tracing ability and the dispersioncharacteristics depends on theaerodynamical characteristics of particles

and the continuous medium&

The visibility depends on the scatteringcharacteristics of particles.

The choice of optimal diameter for seeding particles is a compromise between twoaspects.


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Scattering characteristics of particles

'aser sheet leads to a low energy density

particle scattering efficiency is important&

'ight scattering capability $ scattering crosssection C s is defined as the ratio of the total

scattered power P s! to the laser intensity I 0

incident on the particle

I P

C s s =


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*"ample of scattering cross section (+)

The scattering cross section as a function of

the particle si,e (refractive inde" m-+.).


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*"ample of scattering cross section (/)

Diameter d p

Scattering cross section C s

Molecule

≈10-33m2

1µm C s≈(d

p /λ )4 ≈10-12m2

10µm C s

≈( d p

/λ )2 ≈10-9m2

Scattering cross section as a function of the

particle si,e


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0ie scattering of small particle (/)

'ight scattering by a + µm!

+ µm! and 4 µm glass

particle in water.

5efractive inde" m - +.6/


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Summary of particle light scattering

for PIV The ratio I s907 I s0 decreases with increasing si,e parameterd p7λ ! with values roughly in the range +$+$+$4 for scattering

particles useful in PIV.

The resulting intensity of the scattered light for a given lightsheet intensity will depend on the combined influences of C s

and I s907 I s0! which e"hibit opposing tendencies with

increasing particle si,e. In general! larger particles will still

give stronger signals.

The ratio I s907 I s0 increases with increasing refractive inde" m.

8ence particles in air gives stronger 9o scattering than in

water.


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Trac#ing characteristics of particles

The trac#ing ability depends on• Particle shape assumed spherical

aerodynamically e:uivalent diameter $ d p

• Particle density ρ p

• ;luid density ρ f and fluid dynamic viscosity µ or

#inematic viscosity ν - µ 7 ρ f

ewton%s 'aw governing the motion of a

single particle3 ∑=i

i

p p

p F dt

U d d

.

4π ρ


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added mass?)

V. @asset history integral resistance caused by theunsteadiness of the flow field.

∫ −+−−+−= t

t f p

p

f

f p

f p

p p

pt

d

d

V d d

dt

V d d

dt

U d d V d

dt

U d d

/+

)(/

4

./

+

.4

.

/

444

ξ

ξ

ξ πµρ

π ρ

π ρ πµ

π ρ


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Sto#es% drag law

The Sto#es% drag law is considered to apply when the

particle Reynolds number Re p is smaller than unity!

where Re p is defined as

In a typical PIV e"periment with +µm particles and

/ cm7s mean velocity!

Re p-+"+$ " ./ 7 +.2"+$6 - .+4 (air)&

Re p-+"+$ " ./7+."+$ - / (water).

ν µ

ρ p p f p

Vd Vd ==5e


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Particle parameter$ the particle response time τ p

Velocity lag of a particle in a continuously accelerating

fluid3

The particle velocity response to the fluid velocity if heavy

particles ( ρ pAA ρ f ) in a continuously accelerating flow is3

Particle response time3

dt

dU d U U V

f f p

p f p µ

ρ ρ

+B

)(/

−

=−=

−−=

p

f p t U t U τ

e"p+)(

µ

ρ τ

+B

/ p

p p d =


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Particle parameter

$ the Sto#es number St Sto#es number St as the ratio of the particle responsetime to the Colmogorov time scale3

St 3 the degree of coupling between the particle phase

and the fluid.

St → the particles behave li#e tracers

St →∞ the particles are completely unresponsive to the fluid

flow.

k pSt τ τ 7=


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Particle parameter

$ the characteristic fre:uency C In the case of gas flow where ρ pAA ρ f !

characteristic fre:uency of the particle

motion

Tracing ability in turbulence! ω c-/π f c

/+B p pd C ρ µ =

)7+(

+/

/

C u

u

c f

p

ω +=


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;igure of characteristic fre:uency

The response of particles in turbulence flow. (;rom 8aetig D!

Introductory on particle behavior IS'7=


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Particle si,e vs. Turbulence scale

Seeding particles need to be smaller than the

smallest turbulence scale if one wants to

identify all the structures in the vicinity of

the flow. The smallest fluid length scale is

called the Colmogorov length scale! and it is

related to the si,e of the smallest eddy.


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=dditional Gonsiderations

Particle seeding uniformity


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=dditional Gonsiderations (cont%d)

• Secure sufficient spatial detail in the flow field a higherconcentration of particles is generally needed with PIV thanwith 'EV! with which it is possible to wait indefinitely forthe arrival of a scattering particle in the probe volume.

• = uniform particle size is desirable in order to avoide"cessive intensity from larger particles and bac#groundnoise! decreasing the accuracy! from small particles.

• Particles that naturally e"ist in the flow seldom meet the

above re:uirements. 8ence! in PIV applications! it is oftennecessary to seed the flow with a chosen tracer particle. The particles are either premi"ed with the whole fluid (e.g.!

stirred ) or released in situ by a seeding source.


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Imaging of small particles

5elation between real particles and particle image recorded

in the camera can be analy,ed by the diffraction limited

imaging of a small particle

;or a given aperturediameter Da and wavelength λ !

the =iry spot si,e

adiff D f d I

x I 722./)(

ma"

λ =⇒=


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Imaging of small particles (cont%s)

Hith an imaging lens! the

diffraction$limited si,e3

*stimate of the particle

image diameter3//)( diff p d Md d +=τ

λ )+(22./ += M f d diff

f Z z

+++

=+

Z z M =

d p3 original particle diameter


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Seeding particles for PIV (li:uid flow)

Type 0aterial 0ean diameter in µmPolystyrene +$+

=luminum / $ F


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Seeding particles for PIV (gas)

Type 0aterial 0ean diameter in µmPolystyrene 0.5- 10

=luminum / $ F0agnesium / $ 6


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Gommercial seeding particles $ TSI

(http377www.tsi.com) Silicon Carbide: Suitable for measurements in li:uids andgases! silicon carbide particles have a narrow particle si,edistribution (mean diameter of +.6µm). Their high refractiveinde" is useful for obtaining good signals in water! even in

bac#scatter operation. They can also be used in hightemperature flows. Supplied as a dry powder! they can bemi"ed in li:uid to form a suspension before dispersing.

Titanium Dioxide: Titanium dio"ide particles (mean

diameter of ./µm) are usually dispersed as a dry powderfor gas flow measurement applications. The smaller particlesi,e ma#es titanium dio"ide attractive for high$speed flows.It can also be used for high temperature flows.


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Gommercial seeding particles $ TSI (

http377www.tsi.com) (cont%d) Polystyrene Latex3 Hith an e"tremely narrow si,edistribution (nominal diameter of +.µm)! polystyrene late"

(PS') particles are useful in many different measurements.

Supplied in water! they are not recommended for high

temperature applications.

Metallic coated: 0etallic coated particles (mean diameter

of 9.µm) are normally used to seed water flows for 'EV

measurements due to their lower density and higher

reflectivity. They cannot be used where salt is present. Saltreacts with the metal coating! causing the particles to

agglomerate and drop out of the flow.

http://www.tsi.com/http://www.tsi.com/


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Gommercial seeding particles $ TSI (

http377www.tsi.com) (cont%d)Particle

Type

MeanDia.(µm)

SizeRange(µm) Shape

Density(g/cc)

RefractiveInde(real)

RefractiveInde

(imag.)

Silicon carbide 1.5 Std. dev.= 1.4 Irregular 3.2 2.65 ---

Silicon dioxide 2.7 --- Irregular 2.3 1.47 ---

Nlon 4 Std. dev.= 1.5 S!"erical 1.14 1.53 ---

#S$ 0.54 Std. dev.= 1.05 S!"erical 1.05 1.55-1.6 ---

%itanium dioxide 3-5 --- Irregular 4.2 2.6 ---

&etallic coated ' 4-12 S!"erical 2.6 0.21 2.62

(ollo) gla**

*!"ere*+-12

10, 3-5

'0, 14-17S!"erical 1.05-1.15 1.5 ---

&etallic coated

(ollo) gla**

*!"ere*

1410, 7

'0, 21S!"erical 1.65 .21 2.62

http://www.tsi.com/http://www.tsi.com/http://www.tsi.com/http://www.tsi.com/


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Gommercial seeding particles $ Eantec

(http377www.dantecmt.com) Polyamide seeding particles PSP!: These are produced by polymerisation processes and therefore have a round but not e"actlyspherical shape. They are microporous and strongly recommended forwater flow applications.

"ollo# glass spheres and sil$er%coated hollo# glass spheres "&S' S%"&S!: Intended primarily for li:uid flow applications! these are borosilicate glass particles with a spherical shape and a smooth surface. =thin silver coating further increases reflectivity.

(luorescent polymer particles (PP!: These particles are based onmelamine resin. ;luorescent dye (5hodamine @3) is homogeneously

distributed over the entire particle volume. In applications with a high bac#ground light level! fluorescent seeding particles can significantlyimprove the :uality of vector maps from PIV and 'EV measurements. Thereceiving optics must be e:uipped with a filter cantered on the emissionwavelength (e"citation ma".3 66 nm& emission ma".3 69 nm).


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Gommercial seeding particles $ Eantec

(http377www.dantecmt.com) (cont%d) PSP

P!lyamide

seeding

particles

"#S

"!ll!$ glass

spheres

S%"#S

Silver%c!ated

h!ll!$ glass

spheres

&PP

&l'!rescent

p!lymer particles

&ean !article *i/e m5 20 50 10 10 10 30 75

Si/e di*tribution 1 - 10 m

5 - 35 m

30 - 70 m

2 - 20 m 2 - 20 m 1 - 20 m

20 - 40 m

50 - 100 m

#article *"a!e non-*!"erical but

round

*!"erical *!"erical *!"erical

en*it gcm3 1.03 1.1 1.4 1.5

&elting !oint 175 740 740 250

e8ractive index 1.5 1.52 9 1.6+

&aterial #olamide 12 :oro*ilicate gla** :oro*ilicate gla** &elamine re*in

ba*ed !olmer

http://www.dantecmt.com/http://www.dantecmt.com/


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Particle generation

'i:uid flow Simple! select proper powder then mi" w7 li:uid


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'i:uid droplets

=dvantage Steady production rate& Inherently spherical shape& Cnown refractive inde"

Problem ;orm non uniform li:uid films‑ on window

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Lecture Seeding Particles for PIV

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