We report on the direct measurements of fluid flow vorticity using a spatially shaped beam with a superposition of is estimated from a number of velocity field measurements at several points near the point of interest, which then allow computation of the velocity derivatives over space. These methods provide a measurement of vorticity that is spatially averaged over the (small) spatial resolution area of each method. The first direct measurement of vorticity was attempted more than three decades ago by measuring the rotation rate of planar mirrors embedded in 25μm transparent spherical beads that were suspended in a refractive-index-matched liquid. 4 The implementation of this method is very complex and requirement of index matching significantly limits its use and prohibits its application

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We report on the experimental observation of fluid flow caused by propagation of femtosecond filaments in dry air. We find that the ionization of the medium deposits a non-negligible amount of heat, which creates vortices in a semi-confined glass cylinder. We confirm the influence of thermal gradients on vortex formation by the use of a heated wire in a similar configuration.

A technique is proposed to correct the bias error associated with one-component molecular tagging velocimetry (1c-MTV). 1c-MTV is typically used to capture the velocity component normal to a single or multiple lines of tagged fluid molecules with very high spatial resolution. However, the measurements are affected by an inherent error, which arises from the presence of a velocity component parallel to the lines. In the present study, a Taylor-series-based approach is used to derive a general mathematical expression for this error. The derived expression, which is validated using simulated MTV measurements in laminar channel flow with suction/blowing, is used as the basis for introducing a correction method involving the acquisition of MTV images at multiple time delays. To examine the effectiveness of this ‘multi-time-delay’ approach for correcting 1c-MTV data, an experiment is conducted whereby a known bias error is deliberately imposed on measurements in laminar channel flow. The corrected measurements agree with the true velocity profile to better than 2%, thus validating the correction method.