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Can You Guess The Subject Of This Photo?

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Here's a hint: It's hot, but it moves in waves.

Give up? Flames. The answer is flames. Presented by researcher Christophe Almarcha and his colleagues at the 67th Annual Meeting of the APS Division of Fluid Dynamics, the image above is cropped from a time-lapse photograph of propane air flames propagating downward between two panes of narrowly spaced glass. Here's the full image, and an explanation from FYFD's Nicole Sharp:

Like many flows, flames can be unstable and undergo a transition from orderly laminar flow to chaotic turbulent flow. The timelapse image above shows the propagation of a flame front traveling downward. Each blue line represents the forwardmost position of the flame at a specific time. The flame is essentially two-dimensional, held between two glass plates separated by a 5-mm gap. The V-like points in the flame front are called cusps, and if you look closely, you can see cusps forming and even merging as the flame moves downward.

More details at the always-excellent FYFD. See also this abstract from Almarcha et al.