Schlieren & Shadowgraph Methods 311L Aerospace Engineering 311L Aerospace Engineering
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Schlieren & Shadowgraph Methods Aerospace Engineering 311L Aerospace Engineering Iowa State University 311L Models of Light Physical (wave) model Wave Peaks V c o 3x10 8 m/s Geometric model Aerospace Engineering Iowa State University Light Rays 311L Light From a Point Source Aerospace Engineering Iowa State University 311L Speed of Light in a Medium Index of Refraction: n Typical values: Medium n = co/c 1.0003 Water 1.33 Crown glass 1.52 Plexiglas 1.51 Diamond 2.42 Iowa State University c = speed of light in the medium For gases: Air Aerospace Engineering co c n 1 kρ ρ = gas density k = 0.23 cm3/g for air 311L Light Refraction Snell’s Law: θ1 n1sin θ1 n 2sin θ 2 n1 n 2 Medium 1 Medium 2 θ2 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 y εy εy or x z Aerospace Engineering Iowa State University L n n y εy kL ρ n y 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 Schlieren Shadowgraph • • • • Displays a mere shadow Shows light ray displacement 2n Illuminance level responds to 2 x No knife edge used Aerospace Engineering Iowa State University • • • • Displays a focused image Shows ray refraction angle, Illuminance level responds to nx Knife edge used for cutoff 311L Mirrors Aerospace Engineering Iowa State University 311L ISU’s Z-type Schlieren System Light Source 2nd Field Mirror 1st Field Mirror Test Section Aerospace Engineering Iowa State University Screen/Instrument Panel Knife Edge 311L Light Source Condenser Lens Lamp A-A Section 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 Light Source 1st Field Mirror Test Section Aerospace Engineering Iowa State University 311L Setting Up The Schlieren System Step 3: Set up the second field mirror Light Source 2nd Field Mirror 1st Field Mirror Test Section Aerospace Engineering Iowa State University Screen/Instrument Panel 311L Setting Up The Schlieren System Step 4: Set up the knife edge Focus the source image on the knife Aerospace Engineering Iowa State University Adjust the cutoff Obtain a uniform darkening of the image 311L Uniform Darkening Knife edge too close to second field mirror Aerospace Engineering Iowa State University Knife edge too far from second field mirror Uniform darkening 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. Aerospace Engineering Iowa State University 311L
