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High resolution particle image velocimetry is used to measure the turbulent velocity field for fully developed flow (Re = 2,872) in an enclosed channel. Photographs of particle displacement are obtained in a plane that is parallel to the flow and perpendicular to the walls. These are analyzed to give simultaneous measurements of two components of the velocity at more than 10,000 points. Maps of velocity vectors, spanwise vorticity and Reynolds stress reveal structural aspects of the turbulence. In particular, internal shear layers are observed, in agreement with predictions of direct numerical simulation. Ensemble-averaging of a number of photographs yields statistical properties of the velocity in good agreement with laser-Doppler velocimeter measurements, and with direct numerical simulations.

The direction of fluid motion can be determined in particle image velocimetry (and laser speckle velocimetry) if the image field is spatially shifted between the first and second exposures, so that the most negative fluid velocity still produces a positive particle image displacement. Two new techniques are described which accomplish very fast accurate image shifting using electrooptical hardware. These methods involve regulating the polarization of light scattered from the seeding particles by switching the polarization of the illuminating beam, and placing a birefringent unaxial plate in front of the photographing lens to displace the particle image between illuminating pulses.