Onset of intermittency in two-dimensional decaying turbulence

The statistics of two-dimensional flow decaying from an initially Gaussian velocity field are investigated both theoretically and numerically. During the initial stages of the turbulence decay, small-scale intermittency develops. while the intermediate and large scales remain essentially Gaussian. Corresponding to these early times, an expression describing the evolution of the shape of the probability density P(Omega,S-4) is derived from the Navier-Stokes equations, where Omega = omega/omega(rms) is the normalized vorticity and S-4 is the normalized fourth-order moment of the vorticity. Predictions of the theory, which does not involve adjustable parameters, agree with the results of numerical simulations for the early-time decay.