  Vorticity describes the spinning motion of a fluid, i.e., the tendency to
rotate, at every point in a flow. The interest in performing accurate and
localized measurements of vorticity reflects the fact that many of the
quantities that characterize the dynamics of fluids are intimately bound
together in the vorticity field, being an efficient descriptor of the velocity
statistics in many flow regimes. It describes the coherent structures and
vortex interactions that are at the leading edge of laminar, transitional, and
turbulent flows in nature. The measurement of vorticity is of paramount
importance in many research fields as diverse as biology microfluidics, complex
motions in the oceanic and atmospheric boundary layers, and wake turbulence on
fluid aerodynamics. However, the precise measurement of flow vorticity is
difficult. Here we put forward an optical sensing technique to obtain a direct
measurement of vorticity in fluids using Laguerre-Gauss (LG) beams, optical
beams which show an azimuthal phase variation that is the origin of its
characteristic non-zero orbital angular momentum. The key point is to make use
of the transversal Doppler effect of the returned signal that depends only on
the azimuthal component of the flow velocity along the ring-shaped observation
beam. We found from a detailed analysis of the experimental method that probing
the fluid with LG beams is an effective and simple sensing technique capable to
produce accurate estimates of flow vorticity.
