DEVELOPMENT OF ENSTROPHY AND SPECTRA IN NUMERICAL TURBULENCE

Decaying isotropic turbulence with initial Taylor microscale Reynolds number (R(lambda) less-than-or-equal-to 258) is studied via direct num erical simulations (DNS), with spectral resolution less-than-or-equal- to 256(3). DNS results are compared with two-point closure, in the for m of the direct interaction approximation (DIA) and the test field mod el (TFM). The goals of this study are to understand the time-dependenc e of enstrophy and spectra as they evolve from random initial conditio ns, and to assess and interpret differences between DNS and closure. T wo time scales are identified in the DNS. The first is that for the de velopment of normalized enstrophy production (velocity derivative skew ness) and is independent of R(lambda). The second is that for the satu ration of the enstrophy which follows after a longer period of near ex ponential growth and is strongly R(lambda) dependent. For v not-equal 0, the DIA represents the time development of both integral quantities , such as enstrophy and spectra, with surprising accuracy in spite of its lack of invariance to random large-scale sweeping. The TFM has sig nificant energy range errors, which we attribute to its Markovianizati on assumption.