Visualizing air disturbances

When a flame flickers, the resulting air disturbance is invisible to the naked eye. But it can be unveiled, as shown here, by observing fluctuations in the index of refraction. The use of background-oriented schlieren offers a conceptually simple method to do that. A pattern with high-contrast borders serves as the background. Changes in the index of refraction manifest as apparent displacements of the borders. Jaka Javh, a mechanical engineer and founder of Motion Scope in Slovenia, wrote software to visualize fluctuations in air density. The software converts the relative motion of the background pattern, measured to a resolution of 1 µm, into a color scale to represent the direction and amplitude of the fluctuations. Javh started with a simple checkered background. But to break up the periodicity and thus allow for visualization at different scales, he settled on an irregular pattern.

NASA has previously used the method on a macroscale: Speckles in the Sun or bushes against a desert served as the background to visualize supersonic shock waves and other air density gradients. Other potential applications of background-oriented schlieren include localizing gas leaks, calibrating pressure sensors, and designing face masks to minimize disease transmission. (Image courtesy of Motion Scope.)

This article was originally published online on 5 December 2024.