Optical Fiber Measures Turbulence

Kathleen G. Tatterson

A team of physicists from the University of Pittsburgh and the University of Bordeaux, France, has demonstrated a fiber optic device that measures the two-dimensional turbulence of fluids quickly and inexpensively. This development could simplify the study of fluid mechanics, medicine and manufacturing processes.

Vorticity is a property of fluids in which the liquid swirls around a central point like a whirlpool. In a study published in Physical Review Letters, researchers described how the optical fiber velocimeter measured the vorticity fluctuations in a turbulent soap film flow, measuring speed and direction simultaneously.

Nature's vortices

"The study of two-dimensional turbulence is important if one is concerned with the large-scale velocity variations that nature regularly afflicts on us," said Walter I. Goldburg, physics professor at the Pittsburgh university. "Turbulence in the atmosphere and the oceans is basically two-dimensional." Xiao-Lun Wu at Pittsburgh and Hamad Kellay at Bordeaux also participated in the work.

Measurements of the vorticity fluctuations in rapidly flowing turbulent soap films indicate scaling of the vorticity spectra.

Researchers can measure fluid velocity using commercial laser Doppler velocimetry, particle imaging velocimetry or quasi-elastic light scattering. Devices for these techniques typically cost tens of thousands of dollars; the optical fiber velocimeter can be fabricated in a reasonably equipped laboratory for a small fraction of that cost. The data rate for the optical fiber device is on the order of a few kilohertz, comparable to the other systems' speeds.

According to Goldburg, because the velocimeter is easy to make, researchers can afford to incorporate many such devices in the flow being studied. "This is important in the study of fluid turbulence, since it allows one to explore new physics by measuring the velocity at many points at the same time," he said.

In the study, funded by the National Science Foundation, the scientists etched one end of a single-mode optical fiber to a 50-µm diameter and pushed it through a liquid soap film by about 1 mm. They then coupled the unetched end to a HeNe laser emitting at 633 nm, causing the etched tip to glow. A low-power microscope magnified the glowing tip, and its image fell on a two-dimensional position-sensitive detector. This magnified the displacement of the fiber tip by a factor of 10. Recording the position of the maximum beam intensity gave a direct measurement of the film's velocity.