Schlieren photos



A schlieren photo shows the density gradients in a two-dimensional flow field. The schlieren technique utilizes the feature that the density of a gas is intrinsically tied to the refractive index of light. The schlieren technique transforms density gradients of the gas into changes of brightness or colour in a photo or video (see explanation of method below). Effects of compressible flow as shocks, wakes, vortices and flow separation are revealed, but also thermal effects can be visualized.

    Supersonic turbine flow: flow acceleration, shocks and wakes

Turbine profile with rear flow separation

Flow separation at profile front

Supersonic exit flow in an HP turbine cascade

Detail: separation bubble
by shock boundary layer interaction

Supersonic exit flow in an HP turbine cascade: on the left with separation on the suction side, 
on the right with rear total separation at still higher exit Mach numbers

     
     

 Flow separation in steam turbine cascade at Mach numbers higher than design

     
     
     

Turbine cascade flow with Flat Plate profiles: Mach number increasing from top left to bottom right ; visualization of density gradients, especially zones of flow acceleration, compression shocks, wakes and von Karman vortex streets; light source: flash of 20 ns duration

   
   
   
Turbine cascade flow; Mach number increasing from top left to bottom right ; visualization of density gradients: pressure waves, von Karman vortices, wakes and shocks; light source: flash of 20 ns duration

 

Model of VW beetle in the windtunnel Schlierenphoto at Mach 0.8 (around 900 km/h or 560 mph)

Schlieren optics arrangement at the wind tunnel

Sketch of schlieren optics at the Straight Cascade Tunnel

Schlieren technique (Strioscopie)
Optical setup with spherical mirrors in Z-arrangement: A point light source has to be placed in the focus of the first spherical mirror (on the right side in the figure above). Accordingly parallel light traverses the test section of the wind tunnel. Without flow (in the absence of density gradients) these light rays remain parallel up to the second mirror (left side of figure). In the focus of the second mirror a knife edge (a slit in case of colour schlieren) is placed. In the camera an area of even light intensity is seen with the blades (turbine profiles) as black shades. With flow light rays which pass a flow region with a density gradient are slightly deflected. These deflected light rays produce a shifted focus in the plane of the knife edge and are eventually not able to pass the knife edge thus producing a dark area in the camera picture at the location of the density gradient (e.g. a shock or vortex centre). When applying colour schlieren technique the displacement of the focus in the knife edge plane is converted to colours.

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The Straight Cascade Tunnel Göttingen (EGG) (http://www.dlr.de/at/en/desktopdefault.aspx/tabid-1565/2433_read-3789/usetemplate-print/)