Using laser beams to measure turbulence
A laser beam passing through turbulent fluid is refracted and
scattered, creating a broadened, shimmering beam (like the twinkling
of a star). It should be possible to measure statistical properties
of the beam and infer key turbulence properties such as Chi (the
dissipation rate of thermal fluctuations) and Epsilon (the rate of
dissipation of turbulent kinetic energy). The goal is to develop via
existing theories, and laboratory-test optical measurement techniques
that allow us to measure key turbulence properties.
(Right) Figure from Madieh (2008). Simulation of beam scattering in
turbulence.
Optical measurement of turbulence, achievable with present-day video
and computer technology. will be superior to present techniques using
specialized and delicate microsensors. Optical techniques could be
adapted to ocean-going instrumentation, with possible application to
towed bio-sensing systems to deduce upper ocean mixing rates and
consequent nutrient fluxes. These data would be relevant to the Boyce
et al (2010) observations of phytoplankton decline.
I plan to perform experiments in which a laser beam is passed through
a laboratory tank of turbulent fluid and projected onto a screen and
digitally imaged. Statistical analysis will be based on theories
outlined in two Russian books from the '60's (Chernov, 1960; Tatarskii,
1961), to describe the statistical behaviour of light passing
through a specified field or spectrum of refractive index variations
generated by turbulence. This theory will allow beam behaviour to be
specified in terms of the refractive index "Batchelor
spectrum" generated by turbulence, and specifically in terms of
Epsilon and Chi, the key parameters that are used to characterize
turbulent flows and resulting mixing rates (c.f. Oakey, 1982).