Original author: Ian Smith, By Iansmith6 – Own work, CC BY 3.0
URL link: https://commons.wikimedia.org/w/index.php?curid=7469155
This flow visualization shown uses color schlieren photography to highlight the hot air rising from the tip of a soldering iron. Schlieren photography is able to visualize flow in a transparent material by analyzing the a gradient of refracted light between the high/low pressure zones surrounding the flow. I think this image is especially appealing because of how it visually conveys the flow and heat distribution of something we are all familiar with.
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Hey Jeremy, nice pick. Color Schlieren is a cool flow visualization technique. While strong pressure gradients can be indirectly inferred from Schlieren photos (e.g. shock waves) it is not the pressure gradient but actually the density gradient that makes the Schlieren photo possible. Changes in fluid density alter the refractive index on the fluid or in other words the speed at which light is transmitted through it. Therefore the variation in the intensity of light in a Schlieren photo actually reveals the density gradients in the fluid. In this photo the density gradients in the convective plume are caused by temperature variation. The hot air that comes off the iron tip is lower in density and therefore rises due to buoyancy even though pressures are probably close to uniform throughout the image.