Measurement of the rate-of-strain tensor in a turbulent flow using light scattering from asymmetric particles

A method of directly measuring velocity gradients across a small volume in a turbulent flow was developed. Small asymmetric particles suspended in a moving fluid align with the major axis of the local rate-of-strain tensor. This tendency is opposed by rotary Brownian motion. The average number of oriented particles is a linear function of the rate-of-strain components when the rate of rotational diffusion is large compared to the major component of the rate-of-strain. The intensity of polarized light scattered by the suspended particles depends upon their average orientation. Velocity gradients in turbulent flow, and thus the intensity of the scattered light, will fluctuate. Intensity fluctuation spectroscopy may be used to determine the statistical properties of these fluctuations. A particular component of the rate-of-strain tensor may be studied by choosing the appropriate scattering geometry and direction of polarization.