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Aerospace Engineering

Iowa State University 311L

Schlieren & Shadowgraph Methods

Aerospace Engineering 311L

Aerospace Engineering

Iowa State University 311L

Models of Light

8o 3x10cV m/s

Physical (wave) model Wave Peaks

Geometric model

Light Rays

Aerospace Engineering

Iowa State University 311L

Light From a Point Source

Aerospace Engineering

Iowa State University 311L

Speed of Light in a MediumIndex of Refraction:

c

cn o c = speed of light in the medium

Typical values:

Medium n = co/c

Air 1.0003

Water 1.33

Crown glass 1.52

Plexiglas 1.51

Diamond 2.42

For gases:

kρ1n

= gas densityρk = 0.23 cm3/g for air

Aerospace Engineering

Iowa State University 311L

Light Refraction

Medium 2

Medium 1

21 nn

Snell’s Law:

2211 θsin nθsin n

Aerospace Engineering

Iowa State University 311L

Example, Refraction in Water

Water Surface

Pole

Aerospace Engineering

Iowa State University 311L

Lenses

Focus

Aerospace Engineering

Iowa State University 311L

Lenses

Point Source

Aerospace Engineering

Iowa State University 311L

Direct Shadowgraphy

Point Source

Bubble of high density gas

Aerospace Engineering

Iowa State University 311L

Shadowgraph Image

Aerospace Engineering

Iowa State University 311L

Some Math

x

y

z

yεyn

nL

or

nkL

Aerospace Engineering

Iowa State University 311L

The Basic Schlieren System

Point Source

Aerospace Engineering

Iowa State University 311L

The Real Schlieren System

Extended Source

Aerospace Engineering

Iowa State University 311L

Schlieren Image

Aerospace Engineering

Iowa State University 311L

Schlieren vs. Shadowgraph

• Displays a mere shadow

• Shows light ray displacement

• Illuminance level responds to

• No knife edge used

• Displays a focused image

• Shows ray refraction angle, • Illuminance level responds to

• Knife edge used for cutoff

Shadowgraph Schlieren

xn

2

2

xn

Aerospace Engineering

Iowa State University 311L

Mirrors

Aerospace Engineering

Iowa State University 311L

ISU’s Z-type Schlieren System

1st Field Mirror 2nd Field Mirror

Screen/Instrument Panel

Light Source

Test SectionKnife Edge

Aerospace Engineering

Iowa State University 311L

Light SourceLamp

Condenser Lens

Section A-A

A-A

Aerospace Engineering

Iowa State University 311L

Setting Up The Schlieren System

Step 1: Find the focal length of the field mirrors

Focal Length

Aerospace Engineering

Iowa State University 311L

Setting Up The Schlieren System

Step 2: Set up the first field mirror

1st Field Mirror

Light Source

Test Section

Aerospace Engineering

Iowa State University 311L

Setting Up The Schlieren System

Step 3: Set up the second field mirror

1st Field Mirror

Light Source

Test Section

2nd Field Mirror

Screen/Instrument Panel

Aerospace Engineering

Iowa State University 311L

Setting Up The Schlieren System

Step 4: Set up the knife edge

Focus the source image on the knife

Adjust the cutoff Obtain a uniform darkening of the image

Aerospace Engineering

Iowa State University 311L

Uniform Darkening

Knife edge too close to second field mirror

Knife edge too far from second field mirror

Uniform darkening

Aerospace Engineering

Iowa State University 311L

Some Notes

• The angle between the illuminator axis and the collimated beam should be kept to a minimum– Coma: Smearing of the mirror focus into a comet shape (cancelled out by

tilting the mirrors in opposite directions, i.e. z-type system)

– Astigmatism: Changes the focus into two short lines perpendicular to each other (limited by orienting the knife edge the same as the light source)

• A sheet of white paper is a good alignment tool

• A threaded bolt works well as a focusing tool

Aerospace Engineering

Iowa State University 311L

References

• G.S. Settles, Schlieren & Shadowgraph Techniques, Springer-Verlag, 2001.

• D.C. Giancoli, Physics for Scientists & Engineers, Prentice Hall, 1988.

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