Schlieren

A Review on Schlieren Flow Visualization Technique

Maryam KordbachehUndergraduate Student

Supervisor:Dr. Esfehanian

8/4/1389

Faculty of Mechanical Engineering University of Tehran

2/ 1689/ 4/ 8 Maryam Kordbacheh, University of Tehran

Outline

• A Review of Flow Visualization

• Flow Visualization Methods

• Schlieren Technique

A Brief History

Technical Considerations

Specialized Schlieren Technique

Comparison to Other Methods

Applications

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Why Flow Visualization?

• Why? Because the human mind is designed to rapidly process an incredible amount of visual information; as they say, a picture is worth a thousand words.

• Much can be learned from flow visualization; the visual examination of flow field features.

– Understanding flow phenomenon– Verifying model or theory results– Easier measurements for designing– To get a priori knowledge of solution

• There are many types of flow patterns that can be visualized, both physically (experimentally) and/or computationally.

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Flow Visualization Methods

• Compressible flows– Smoke visualization– Particle– Shadowgraphs and Schlieren

• Incompressible flows– Dye– Shadowgraphs and Schlieren– Particles– Bubbles– Fluorescence, phosphorescence

Drop falling on liquid surface

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Incompressible Flows

Normal laminar flow chain

Insects walking on water

dye visualization

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Compressible Flows

laminar smoke jet

Schlieren flow visualization

Shadowgraph

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• Schlieren photography (from German word for “streaks”)

• First observed by “Robert Hooke” in 1665 using a large convex lens and two candles• The conventional schlieren system is credited mostly to German physicist “August Toepler” which was designed to detect schlieren in glass used to make lenses

Schlieren Method

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• The Method Use some Basic Principles:

Schlieren Method

• Different speeds of light for different densities• the refractive index : n = c0/c•For air - index has a linear relation with density: The higher the density, the slower the light moves• So, any compressible flow will cause the wave front of light to “turn”.• A movie

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Schlieren Method

LightSource Test

Section

Imageofsource

ImageofTestSection

Mask(knife edge)

a

b

c

d

a’

b’

c’

d’

Knifeedge

ImageDisplaced

Image

• Knife edge intercepts part of source image• Density gradient causes displacementof source image

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Schlieren Method

Z-folded Schlieren Using Mirrors Camera

Knifeedge

LightSource

Test Section

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Schlieren Method

Colored or Grayscale

• Color Schlieren uses acolor filter in place of knifeedge• The extra dimension ofcolor helps to have vectordirections in addition toscalars• Extra complexity and cost

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Comparison to Other Methods

• Shadow Graph– 2nd derivative of density

– Simplest method

– Lower contrast

• Schlieren– 1st derivative of density

– Small increase in complexity

– Increase in contrast

• Interferometer– 2D quantitative measurement of n, T,

Density

– Sum of path differences < λ /10th

– Least familiarity

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Dr. Leonard Weinstein of NASALangley Research Centerdeveloped first Schlieren camera,which he calls SAF (Schlieren forAircraft in Flight), that canphotograph shock waves of a fullsized aircraft in flight.

Photograph of a T-38 at Mach 1.1,altitude 13,700 feet, taken at NASAWallops in 1993.

Schlieren Technique in Application

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Schlieren Technique in Application

Schlieren for Flight Test

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Thanks for your Attention