schlieren diagrams

Aerospace Engineering

Iowa State University 311L

Schlieren & Shadowgraph Methods

Aerospace Engineering 311L



Models of Light

8o 3x10cV m/s

Physical (wave) model Wave Peaks

Geometric model

Light Rays



Light From a Point Source



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



Light Refraction

Medium 2

Medium 1

1θ

2θ

21 nn

Snell’s Law:

2211 θsin nθsin n



Example, Refraction in Water

Water Surface

Pole



Lenses

Focus



Lenses

Point Source



Direct Shadowgraphy

Point Source

Bubble of high density gas



Shadowgraph Image



Some Math

x

y

z

yεyn

nL

yε

or

yρ

nkL

yε



The Basic Schlieren System

Point Source



The Real Schlieren System

Extended Source



Schlieren Image



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



Mirrors



ISU’s Z-type Schlieren System

1st Field Mirror 2nd Field Mirror

Screen/Instrument Panel

Light Source

Test SectionKnife Edge



Light SourceLamp

Condenser 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



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



References

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

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

schlieren diagrams

Documents

source image

schlieren systemstep

field mirrorlight source

field mirror uniform

speed of light

uniform darkening knife

light ray displacement

xn n xy x zy