Direct measurement of vorticity by optical probe

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Abstract
An optical method for the direct measurement of vorticity in liquid flows is described. At the present state of development it is capable of responding to vorticity fluctuations with a response time of about 1 msec and a spatial resolution of better than 50 μm. Small spherical particles suspended in the flow rotate with angular velocity accurately equal to half the local vorticity; thus measurements of the rotation rates of such particles indicate the vorticity. Transparent spherical particles of less than 50 μm diameter, each containing embedded planar crystal mirrors, have been developed for this purpose and are suspended in a refractive-index-matched liquid. Measurements of the times required for laser reflections from the mirrors to rotate through the small angle defined by a pair of slits yields the rotation rate, and thus the vorticity. Production and physical properties of the probe particles are reported. Theoretical capabilities and limitations of the method, including accuracy, spatial and temporal resolution, data rate, and background noise are calculated and found to be coupled to the optical geometry and flow field. Analysis yields procedures for selective optimization of each parameter as dictated by the particular application. Measurements of steady-state, laminar, two-dimensional Poiseuille flows demonstrate the effectiveness of the technique and confirm theoretical predictions.

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