Fluid vortex mapping using the rotational Doppler effect

The light from a twisted laser beam, scattered at an air/water interface, experiences a rotational Doppler shift. We use a superposition of two beams with different topological charges to measure the beat frequency of the scattered light by a Rankine type vortex at different positions from its center. We show that the angular velocity decreases with the distance in total agreement with a Rankine vortex model. Several extensions are then considered, including the detection of turbulences generated in the wake of airplanes.