Evolution of the vorticity-area density during the formation of coherent structures in two-dimensional flows

It is shown: (1) that in two-dimensional, incompressible, viscous flows the vorticity-area distribution evolves according to an advection-diffusion eq uation with a negative, time dependent diffusion coefficient and (2) how to use the vorticity-stream function relations, i.e., the so-called scatter-p lots, of the quasi-stationary coherent structures in order to quantify the experimentally observed changes of the vorticity distribution moments leadi ng to the formation of these structures. (C) 2000 American Institute of Phy sics. [S1070-6631(00)50210-4].