WAVE NUMBER SPACE DYNAMICS OF ENSTROPHY CASCADE IN A FORCED 2-DIMENSIONAL TURBULENCE

Enstrophy cascade in two-dimensional turbulence is studied numerically together with dynamics of a passive scalar and the first Lyapunov vector. (i) The vorticity field is decomposed into elliptic (e) and hyperbolic (h) regions by using Weiss' conditional sampling method. The k-1 law of the enstrophy spectrum associated with the h region extends with the dissipation wave number, while the humplike spectrum associated with the e region does not. Asymptotic recovery of the k-1 law is thereby suggested in the inviscid limit. Weiss' decomposition is also applied to the enstrophy spectral flux to elucidate the interaction between e and h regions. (ii) Temporally intermittent nature of the enstrophy cascade is revealed on the (k-t) plane by tracing the wave number with active transfer. Active (inactive) periods are related with a higher (lower) enstrophy dissipation rate. (iii) The wave number characteristic to the enstrophy spectrum of the first Lyapunov vector is also traced on the (k-t) plane. When the peak wave number lies in the inertial subrange, enstrophy dissipation is likely to be large.