Investigation has been performed into spatial evolution of energy distribution and local wave-propagation vector of a fluctuating laser beam carrying an optical vortex. In our numerical experiments fluctuations of the beam parameters were due to propagation through a thin (comparing to the whole distance of propagation) atmospheric layer simulated by a phase screen. It was shown that the eddy flow of energy typical for a coherent beam manifests itself in mean characteristics of a vortex beam. Particularly, the mean wave-front tilt can be represented as a sum of vortex and potential components. Circular motion of optical energy is defined mainly by the rotor of the vector field of wave-front local tilts. For a phase screen with a quadratic structure function of distortions the vortex component corresponds to the model of eddy flow of fluid known as Scully vortex. As the distance increases from the screen to the plane of observations the potential component of the field may result in focusing of the beam. Directions of energy current streamlines (or mean diffraction rays) allow one to find similarity between progressions of an optical vortex carried by a laser beam and destruction of eddy flow of fluid.
singular optics, vortex beams, optical vortex, wave front dislocations, phase screen