Schlieren Whitepaper
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High-Speed Imaging Uncovers The Invisible With Schlieren Techniques By Phil Taylor, field applications engineer, Vision Research Scientists use schlieren imaging, a non-invasive testing method, to visualize density gradients within otherwise invisible flows. Schlieren imaging is a practical method of visualizing air movement around an airfoil in a wind tunnel or gas interactions within a combustion chamber. Over the past decade, significant improvements in the speed and sensitivity of cameras have greatly increased the quality of schlieren images and the speed at which images can be acquired. Although this advanced imaging technique can now deliver detailed images of highly dynamic processes, obtaining high-quality data requires choosing the best high-speed camera for the application and careful optimization of the optical setup. CAPTURING CHANGES IN TRANSPARENT MEDIA Schlieren imaging comes in many forms, all of which capture normally invisible density gradients, or “schliere,” in transparent media such as air, water, and glass. Schlieren imaging is typically used for laboratory-based, in-depth studies while a related technique known as shadowgraphy is more commonly used for field studies because its simpler optical setup is easier to transport and less likely to get damaged. The density gradient, or spatial variation in density over an area, of a medium such as air or gas is determined by environmental factors such as pressure or temperature. As rays of light hit the medium, variations in this density gradient cause the light to change direction in a way that can be imaged. The most common setup used for schlieren imaging is the z-type system. This setup includes two parabolic mirrors, a point light source, a camera, and a knife edge (Figure 1). It provides the highest level of image quality and can be quickly transformed to a shadowgraph Figure 1: In a z-type schlieren imaging system, the first mirror (right) collimates the light rays onto to the second mirror (left), which in turn directs light to the camera. To the camera, the field of view is evenly illuminated and variations in the density gradient of the subject, between the two mirrors, will change the refractive index of the light and appear in the image. Adding a knife edge at the focal point of the second mirror uniformly blocks some of the light traveling to the camera. This creates a high-contrast image that makes the variations in the density gradient more visible. Components at the source and the cut-off can be altered to change various characteristics in the image, such as color

High-Speed Imaging Uncovers The Invisible With Schlieren Imaging system or hybrid shadowgraph/schlieren system. Schlieren imaging can be used for examining aerodynamics, fluid mechanics, thermal exchange, and other processes. For example, Schlieren imaging can be used to examine a thermal plume from a candle (Figure 2), helium gas flow (Figure 3), and shockwave formations (Figure 4). GETTING HIGH-QUALITY DATA Because schlieren imaging captures extremely fast processes with an approach that requires a small light source, it’s important to use high-speed cameras that are very sensitive. A...

High-Speed Imaging Uncovers The Invisible With Schlieren Imaging to blur the motion of the shock and pressure waves. To reduce the capture of unwanted three-dimensional flow structures, the researchers created a system with a narrower depth of field. The experimental setup diffused continuous light to illuminate a source grid of multiple alternating dark bands and clear apertures that created a twodimensional light source array. A Fresnel lens, a special lens with a large aperture and short focal length, was placed in front of the source grid to increase the light collection efficiency. A...

High-Speed Imaging Uncovers The Invisible With Schlieren Imaging to quantitatively measure the growth of the interaction field produced by a jet exhausting in a cross flow, a flow scenario that scientists study to better understand a variety of natural and industrial processes (Figure 6). Figure 6: As the lateral jet opened, a weak Mach wave (a pressure wave traveling at the speed of sound) was generated from the exit of the nozzle. Ten seconds later a weak shock wave was seen upstream of the jet flow and then the interaction field developed and the separation shock was observed. Finally,...