Aerospace MAE309 Engineering Laboratory II
Experiment 2 Schlieren technique
Name: Supichaya Sujariyakul ID: 20091317 Email: [e-mail], [e-mail]
Introduction
The Schlieren Technique and shadowgraph technique are flow visualization technique widely used to photograph flow of fluid with different density.
Objective
The objective of the experiment is to study the phenomenon when light is refracted differently at different angles when it passes through objects of varying density and thus, varying refractive indexes. Another objective is to familiarize with the Schlieren technique which makes use of this phenomenon to visualize flow that is difficult to capture using the naked eye.
Method
The experiment uses a light source, two converging lenses, three diaphragms (two round-shaped iris entrance diaphragms and a knife edge exit diaphragm), two mirrors and a candle flame to impose an image on a white screen.
The path of the light starts at the light source and is refracted through a converging lens. The first iris diaphragm is placed at the focal point of the converging lens and is used to filter noises (frayed rays) and improve image sensitivity. The light then travels through a second converging lens to a second iris diaphragm repeating a similar process as the first set of converging lens and diaphragm. The remaining light rays are then reflected on a mirror as a point source of light to the object, which is in the case of the experiment, the candle flame. Light refracted through the object is then reflected again on a second mirror which leads it to the last diaphragm. It should be noted that since the candle flame has ever changing density, the extent of light refraction is also varying and thus, the light that passes through it is not stable with time. The last diaphragm consists of a knife edge. The knife edge blocks out some of the distorted light rays. This results in lighter and darker patches forming on the screen following the knife edge diaphragm.
Results
Figure 1: Schlieren image
Figure 2: Shadowgraph
Schlieren visualization is a visual process that is used to visualize the flow of fluids of varying density. In this experiment Schlieren system is used to observe the flow of air around the candle. After setting up the Schlieren system there is an image of candle and the flow around the candle appears in dark and light patches. The patches in
Schlieren image actually show the corresponding positive and negative density gradient of the fluid. The image of the flow around the candle varies through time. The varying density gradient gives an ever-changing index, refractive distorting the initially parallel light beam at different angles resulting in an aberration. Some distorted light rays end up being blocked by the knife, causing dark patches to appear on the screen. Shadowgraph is another method of flow visualization. It can be obtained by removing the knife-edge diaphragm. In this case, all the refracted light is reflected on the screen. The shadowgraph method works on a similar optical principle, but only reveals shadows of the singularities in the subjected transparent media.
Discussion
The results of the experiment may not be as good as it was expected to be. This may be due to the aberration phenomenon. Aberration is generally an optical effect that takes place as the light rays hit the edges of the lens or the mirrors. The rays then do not converge or diverge from a single point and thus detrimentally affects the sharpness of the images produced in the experiment. There are several types of aberration that might have taken place during this experiment. They are spherical and chromatic aberrations. Shadowgraph and Schlieren visualization are the methods to reveal flow patterns by way of changes in their optical refractive index. The difference between a Schlieren image and a shadowgraph is that a Schlieren image shows dark image darker than the dark image of a Shadowgraph and a Schlieren image shows light image a little lighter than the light image of Shadowgraph, results in the more difference between the light intensity range. As such, the shadowgraph and the Schlieren image show the same object and surrounding fluid flow with different complementary details. A major drawback in the Schlieren technique is the sensitivity of the experimental results. This optical experiment involves a fair bit of precision and is prone to huge experimental error should there be even a small disorientation of the apparatus. Through advances in flow visualisation techniques, Computational Fluid Dynamic is another method that can be employed to calculate and visualise fluid flow. Calculations can be conveniently done on computers. However, they are only accurate up to a certain extent and have to be verified. On the other hand, the Schlieren Technique can directly produce a more tangible picture of the fluid flow, which doesn't require anymore interpretations.
References
1. Wikipedia. (2011). Schlieren Photography, Website: http://en.wikipedia.org/wiki/Schlieren_photography 2. Wikipedia. (2011). Shadowgraph, Website: http://en.wikipedia.org/wiki/Shadowgraph 3. Spherical Aberration Website : http://toothwalker.org/optics/spherical.html