Using a Laser To Visualize Chaotic Turbulent Micro-Flow

Version 1.4 of 6/5/2010-4:16 a.m.

Chaotic turbulent air micro-flow occurs continuously around objects which move in air. Usually such turbulence is not visible, because air is transparent, but can be made visible with lasers. On this page the author managed to capture such turbulence using smoke and a green 532nm diode-pumped solid-state laser. The laser beam slices through the smoke column and the turbulence becomes visible as a result of Rayleigh Scattering.

Rayleigh scattering on smoke 1

Rayleigh scattering on smoke 2

Rayleigh scattering on smoke 3

Rayleigh scattering on smoke 4

A variant of the above procedure is often used to create the famous "Laser-Tunnel" effect. This effect was used in several movies, such as the movies The Final Countdown in connection with time travel[1] and Alien in connection with movement surveillance. Here's the operational diagram for this effect:

laser-tunnel effect
Operational diagram for "Laser-Tunnel" effect.

The effect can be reproduced by mounting a small mirror (red) on the tip of a fan's rotor, at an angle with respect to the rotor's axis. The green laser is then mounted so that it aims directly at this mirror. When the rotor rotates, the laser beam sweeps a cone of apical angle 2* in space. When one feeds the arrangement with smoke from below, the laser produces images of smoke segments which subtend solid angles with respect to the viewer. These images can then be photographed, provided the camera is positioned inside the cone and aims directly at the rotor's tip at an angle <2*; with respect to the rotor's axis, as in the diagram, above. Here are some photos of the effect:

circular Rayleigh scattering on smoke 1

circular Rayleigh scattering on smoke 2

circular Rayleigh scattering on smoke 3

Notes

  1. For an explanation of why this setup was used in connection with time-travel, see this article.

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