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The picture below from this answer shows four spacecraft wind tunnel tests from the 1950s. In addition to the test object, the bow shock and turbulence are clearly visible. How is this visualization accomplished? Is the medium air, or a denser fluid such as water? Is there a pigment added to the fluid to help visualization? Is there special lighting, such as polarized light? Any notable camera tricks? Is the 3-D nature of the test object more significant than being a thick 2-D object?
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