Optical Techniques (Qualitative and Quantitaive Flow Visualization)

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Detailed description of the Schlieren technique


Figure at the right shows the schematic diagram of a Z-type schlieren set-up. Lase beam from the laser source (1) is collimated by a mirror system (4). The collimated beam passes via the test section (5) and focussed back using another mirror system (4). A knife-edge (6) is placed at the focus of the beam. The image is formed at the screen (7) which is captured by an CCD camera (8) and the image is transferred to the computer (9) for further processing.

The actual test setup is shown below. This is the experimental set-up present at the Fluid Mechanics Lab in IIT Kanpur.


The schlieren technique works on the principle of deflection of light beam. The knife edge plays a crutial role in defining the contrast of the image. In fact no information can be extracted without the presence of knife edge. Figure below left shows the initial knife edge setting. The knife edge blocks most of the light beam to give a very dark initial image. Slight amount of light (ak) is allowed to pass to give an initial image. Figure below right shows the final position of light beam with respect to the knife edge. Due to variation of refractive index with the presence of thermal disturbance, the light beam gets deflected upwards (or may be downwards) and more (or less) light passes over the knife edge (ak + delta a). This results in change in illumination st the screen giving a different image.



The difference in the contrast of the image gives the first differencial of the refractive index gradient. Integration of the contrast with known boundary condition can provide quantitaive information on the thermal field.
Image below shows a typical schlieren image for hot and cold cup.


The hot cup (left) reduces the density of adjacent air whcih becomes lighter and moves up. The cold cup (right) makes the air dense and heavy which moves down.

Schlieren is a widely used qualitative visualization technique. But it cal also be applied for quantitative data extraction for simple flows with well known boundary conditions. It can then also be advanced to get 3-d data using tomographic techniques.

Page developed by Atanu Phukan. Copy of page materials undesired. Some of the images were collected from the internet and unable to mention the references is regretted.