Timelapse of a canyon with a rushing river and clouds passing overhead

Timelapse can be a beautiful way to highlight slow-moving flows like those in the sky. But it can also be valuable in showing differences in speed, as in the latest SKYGLOW Project video, “Colorado Serenade”, which shows the Colorado River and the skies overhead simultaneously. Timelapse highlights the difference in time scales between the fast-moving river and slower-moving clouds.

This mirrors an important phenomenon in fluid dynamics known as “separation of scales”. In a flow, there are often multiple effects at play and they may occur on different time (or length) scales. Which matters most in a given situation will depend on those scales. Consider a rocket engine. Combustion inside the engine ignites fuel and oxidizer, releasing heat. At the same time, the flow in the engine is key to mixing that fuel and oxidizer together so that all of the fuel and oxidizer ignites before it is sent downstream into the rocket nozzle. There are two important time scales here: the time it takes for the flow to mix fuel and oxidizer together and the time it takes for the combustive chemical reaction to take place. In an ideal world, engineers can balance those two time scales to maximize efficiency. But in the (admittedly less ideal) real world, this is not always possible. (Video and image credit: H. Mehmedinovic/SKYGLOW)

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