Lecture # 11: Flow Visualization techniques: Shadowgraph ...huhui/teaching/2014Fx/class... · qualitative flow visualization, they can be used to determine pressure, density or temperature

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

Dr. Hui Hu

Department of Aerospace EngineeringIowa State University

Ames, Iowa 50011, U.S.A

Lecture # 11: Flow Visualization techniques: Shadowgraph and Schlieren

AerE 344 Lecture Notes


Shadowgraph and Schlieren technique

• Index of refraction:

• Depend on variation of index of refraction in a transparent medium and the resulting effect on a light beam passing through the test section

• Shadowgraph systems: are used to indicate the variation of the second derivatives (normal to the light beam) of the index of refraction.

• Schlieren Systems: are used to indicate the variation of the first derivative of the index of refraction

1/ 0 vcn

shadowgraph depicting the flow generated by a bullet at supersonic speeds. (by Andrew Davidhazy )

Schlieren images of the muzzle blast and supersonic bullet from firing a .30-06 caliber high-powered rifle (by Gary S. Settles )


• Shadowgraph and Schlieren Systems are often used in shock waves and flame phenomena, in which density gradient is quite big.

• While these techniques are mostly used for qualitative flow visualization, they can be used to determine pressure, density or temperature measurements theoretically.

• These techniques are often used to determine the integrated quantity over the length of light beam.

shadowgraph image of plumes during solidification process (by Lum Chee)

Schlieren image

Shadowgraph and Schlieren technique


Introduction-3

• Index of refraction is a function of thermodynamic state (density) for homogeneous medium:

• Lorenz-Lorentz relationship:

• When n1, for gaseous flow:

• at standard condition, with no and o, :

• When first and second derivative is determined as in Schlieren and shadowgraph apparatus:

constnn

211

2

2

constnconstn 11

11

)1(11

00

000

0

nn

nnconstn

2

2

0

02

2

2

2

2

20

0

11

11

yn

nyyn

consty

yn

nyyn

consty


Introduction-4

• Application of the Schlieren and shadowgraph techniques:

– Compressible flow with shock waves density changes– Natural convective flow density changes– Flame and combustion system: density changes

• Temperature changes inside flows:– For low speed flow with heat transfer:– P = constant

])(2[1

1

11

/

222

2

0

02

2

0

0

0

0

0

0

2

yT

TyT

Tn

yn

yn

nT

yT

yT

Tn

yn

yn

yT

TyT

RTP

yRTP


Fundamentals of Schlieren System

• According to definition of index of refraction, the light velocity will be V=Co/n.

• The slope of the wave front of the light:

• If the angle is quite small.

dynd

dzyd

dzdy

nddzdydn

ndz

dydn

nndz

dyn

dnd

dzdy

Zyn

nyZ

yyn

CZZZn

CZ

yy

)(ln

)(ln)(1][1])1(

['

)/)1(('

)/)1((

2

2

2

2

02

0

y

Z

y

Z

Parallel lights

dzdy

2Z

'

'

'

dzdydnn

dzdydn

n

dzdy

nddzdydn

ndz

dydn

nndz

dyn

dnd

'1

)(1'

)(ln)(1][1])1(

[' 2

1/ 0 vcn


Fundamentals of Schlieren System

• For a gas flow with density change:

Ldydn

af

II

Ldydn

af

IIn

dzdydn

af

II

yn

ny

dzdydn

af

IIdz

dydna

fII

Kk

Kk

Kk

Kk

Kk

0

02

0

02

0

02

0

0

2

2

1

11

11

'

LyT

Tn

af

II

constyT

Tnnif

dzyT

Tn

afdz

dydn

af

II

yT

Tn

dydn

Kk

KKk

2

22

1


Visualization of shock wave in a transonic/supersonic nozzle using Schlieren technique

Before turning on the Supersonic jet

Aftyer turning on the Supersonic jet


Alternative Schlieren system

A. Setup with one converging and one plane mirror

A. Setup with one converging mirror


Shadowgraph technique

dzdy

ndnZ

II

dzdydn

n

dydZ

II

dydZ

ydZ

yy

III

II

dZyy

IyyI

a

sc

a

sc

scsc

sc

sc

sc

scsc

scsc

2

2

0

0

0

0

0

0

1 since

1

• Sensitive is proposal to Zsc


Shadowgraph technique

Experimental setup with one converging mirror

Experimental setup without lens or mirror


Direct Shadowgraph

Point Source

Bubble of high density gas


Schlieren vs. Shadowgraph

Shadowgraph

• Displays a mere shadow• Shows light ray displacement• Contrast level responds to

• No knife edge used

Schlieren

• Displays a focused image• Shows ray refraction angle, • Contrast level responds to

• Knife edge used for cutoffyn

2

2

yn


Examples


Examples: Shlieren Photography

A cough

The firing of an AK-47. A simulated explosion in an airplane cabin.

An invisible gas leak

A hair dryer's effects

A glass of warm water


Examples: Shadowgraph images

.

Shadowgraph Images of Re-entry Vehicles


ISU’s Z-type Schlieren System

1st Field Mirror 2nd Field Mirror

Screen/Instrument Panel

Light Source

Test SectionKnife Edge


Light Source

LampCondenser Lens

Section A-A

A-A


Setting Up The Schlieren System

Step 1: Find the focal length of the field mirrors

Focal Length



Step 2: Set up the first field mirror

1st Field Mirror

Light Source

Test Section



Step 3: Set up the second field mirror

1st Field Mirror

Light Source

Test Section

2nd Field Mirror

Screen/Instrument Panel



Step 4: Set up the knife edge

Focus the source image on the knife

Adjust the cutoff Obtain a uniform darkening of the image


Uniform Darkening

Knife edge too close to second field mirror

Knife edge too far from second field mirror

Uniform darkening


AerE344 Lab#9: Visualization of shockwaves using Schielren technique

• Visualization of shockwaves in an supersonic jet flow using Schielren technique.

• Demonstration experiment only

• Sign-in sheet signature

• No lab report

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Lecture # 11: Flow Visualization techniques: Shadowgraph ...huhui/teaching/2014Fx/class... · qualitative flow visualization, they can be used to determine pressure, density or temperature