Schlieren optics have been around, in their most primitive forms, since the 17th century. Whilst the most basic types are not really used nowadays, nor are some of the old applications relevant nowadays, there are more advanced systems that are always being developed and this brings new applications—most of which are centred around the engineering and aerospace industries, with a particular focus on applications involving air and transparent media flows. In this article, we look at some of the applications where schlieren optics are now used within these industries.
What are Schlieren Optics?
There are basic and advanced schlieren systems, however, modern day applications use the more advanced systems. Schlieren optics measure the change in the refractive index of either air or another transparent media, and this enables flow patterns around an object to be imaged—it is a technique that is used to not only determine the characteristics of the air flow itself, but to also evaluate how an object behaves in certain airflow environments (e.g. for determining if an object is aerodynamic enough for its application).
For these systems to provide this information, a beam of light is shone through a slit and focused onto a mirror at one end of the test area (i.e. the area where the air flow is being imaged). This light then passes back through the test area, past a light blocker and onto a thin piece of metal. Positioned behind the blocker (and directly in front of the mirror) is a camera which is used to take the image. The air flow can be imaged by detecting any changes in the refractive index within the flow. This occurs when the flow meets an object, and the change in direction (and refraction) causes more light (in one or more directions) to pass by the light blocker and be imaged as streaks of light—where the intensity of the image is dependent upon the degree of refraction and the distance from the mirror where the refraction occurs.
Applications of Schlieren Optics
Aerospace is an area where schlieren optics have seen a lot of use. Often performed in wind tunnels, schlieren optics can be used to see the effects of the air flow around an aircraft. This is often manifested to determine how aerodynamic an aircraft by imaging how the air moves around the aircraft and enables manufacturers to make modifications in their designs using the results—which in turn results in more aerodynamic aircraft. Schlieren optics can also be used to determine the airflow upon take off, in order to see how efficient the aircraft is at lifting from the ground into the air. Other areas of aerospace design that benefit from the use of schlieren optics is the visualization of shock waves in supersonic aircraft and the visualization of heat convection across an aircraft.
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Schlieren optics can be used as a quality control measure to check whether a glass pane is free from defects and other issues that could affect its performance—especially when glass is amorphous and is privy to defects. Schlieren optics can be used to check whether a glass pane has any variations in its thickness at different points across its surface, any surface waviness or ripples, internal strains, blisters or bubbles in the glass, any internal temperature gradients, to see whether any external solid particulates (e.g. stones etc) are present in the glass and if there has been any irregular mixing of the glass melt.
Whilst it is not an obvious application, schlieren optics can also be used to measure various aspects and processes within a combustion engine. The most common sub-applications where they are used within combustion engines is to capture the spray evaporation and spray interference from the fuel injectors, as well as the effects of when fuel and air mix within the engine and the implications that this has on the ignition and the development of the igniting flame within the engine. Another area where schlieren optics are used is to determine the effect that the electrode shape has on the igniting process—where schlieren optics can be used to determine how the arc discharge changes, and in effect how the energy transfer efficiency changes, when the shape of the electrode is altered.
There are various types of schlieren optics, and some have been adapted to utilize the principles of schlieren optics with other techniques, such as interferometry. This has become a technique used to determine whether a material exhibits any inhomogeneities (or unevenness) at its surface. In a similar way to air being refracted by an object, the small bumps at the surface refract the light shone across the surface and shadow patterns are produced that depict these surface deviations.